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WILL you pardon me for asking a question which probably I ought to be able to answer myself? Mr. Corfield, in his interesting comparison of the hygienic performances of the ancients and ourselves, mentions the well-known Cloaca Maxima as one of the great glories of the Romans. Can he tell us how they got the sewage into it? I presume the invention of Bramah was not known in those times, and I was a little disappointed in not finding in his able paper a solution of a mystery which has puzzled me since my childhood. What did the Romans want with a Cloaca Maxima, and how did they use it?

MR. PROCTOR'S interesting paper in your last number reminded me of an essay on “The Nebular Hypothesis,” originally published in 1858, and re-published, along with others, in a volume in 1863 (“Essays: Scientific, Political, and Speculative.” Second Series), in which I had occasion to discuss the question he raises. In that essay I ventured to call in question the inference drawn from the revelations of Lord Rosse's telescope, that nebulae are remote sidereal systems—an inference at that time generally accepted in the scientific world. On referring back to this essay, I find that, besides sundry of the reasons enumerated by Mr. Proctor for rejecting this inference, I have pointed out one which he has omitted.

IN reply to J. H.'s query respecting the flight of the albatross mentioned in a paper of mine on the flight of birds, read at the November meeting of the Norfolk and Norwich Naturalists' Society, I beg to assure him that no bird is able to fly without flapping its wings.

I SHALL feel obliged if, through the medium of your widely-circulated journal, you will allow me to point out an extraordinary mistake into which Mr. Radau has fallen, in a memoir inserted in the Annales Scientifiques de I' Ecole Normale Supereure tom. vi. 1869, in which he criticises certain of my conclusions about the representation of the motion of a free rotating body contained in a paper published by me in the “Philosophical Transactions” for 1866. In his preamble, M. Radau says, speaking of the theory of rotation in connection with the names of Poinsot, Rueb, Jacobi, and Richelot:— “Tout recemment M. Sylvester a essaye d'appliquer au mene sujet des considerations nouvelles qui l'ont conduite a des resultats interessants, a cote d'autres dont l'exactitude peut etre contestee.”

YOUR correspondent G.H. will find in one of Thomson's papers something very like the assertion “that there was a time when the earth rotated too swiftly for the existence of life,” but expressed in a manner at once more precise and less pleonastic. “The existence of life” reminds me of a phrase which I heard a few days ago from a female beggar; she lamented that her husband had “fallen into habits that are habitual.” Well; the required reference is the paper “On Geological Time,” in the Transactions of the Geological Society of Glasgow, vol. iii. Part I. pp. 15 and 16 (§§ 19 and 20). A thousand million years ago, says Thomson, “there must have been more centrifugal force at the equator due to rotation than now, in the proportion of 64 to 49…. If the earth rotated seventeen times faster, bodies would fly off at the equator…. If you go back ten thousand million years ago—which, I believe, will not satisfy some geologists—the earth must have been rotating more than twice as fast as at present—and if it had been solid then [which he thinks improbable], it must be now something totally different from what it was.” Such a state of things he seems to consider inconsistent with any organic life such as we know of. Surely the connection of this question with the argument from retardation by tidal friction is too plain to need exposition.

IN an interesting paper, quoted at p. 661 of NATURE, Prof. Rolleston dwells upon the proportion of short-lived male skeletons, found in Anglo-Saxon interments, as contrasted with the older character of the Romano-British interments, deducing there-from a conclusion as to their respective longevity. The writer appears to have forgotten that the youth of Romano-Britain had for many generations been forcibly expatriated—drafted abroad to feed the armies of Imperial Rome.

DR. INGLEBY is curious to know what Prof. Helmholtz would say on this vexed question. If Dr. Ingleby will turn to page 630 of the “Physiologische Optik,” he will find that Prof. Helmholtz has anticipated his wishes. As others of your readers may be interested in seeing how the matter is treated by one who is facile princeps in this department, I subjoin a translation of the passage. If the curious experiment mentioned by Dr. Ingleby had referred only to the vertical diameter of the disc, it would have seemed to be another illustration of our inveterate tendency to ascribe an exaggerated value to vertical lines or angles, at or near the horizon. It is said that if ten men be required to fix off-hand on a star half way between the zenith and horizon, nine, at least, will choose one very much too low. If an exact square be cut out in paper and pinned against the wall opposite to the eye, the sides will appear longer than the top or bottom. If an equilateral triangle be placed in the same position, the angles at the base will appear larger than the angle at the vertex. If a line be drawn parallel to the bottom of a sheet of paper, and a second line, making with it an angle of 20° or 30°, any one attempting, without moving the paper, to draw a third line through the point of intersection, so as to make an angle with the second line equal to that which the second makes with the first, will make the second angle too large. (This experiment is guaranteed by Helmholtz.) After reading Helmholtz's theory, metaphysicians may be willing to allow that all these illusions are to be derived, after his example, from the clouds. As metaphysicians have, before now, contributed a good deal to the clouds, it is perhaps only fair that the clouds should contribute something to the metaphysicians.

IN the report of the papers read at the Academy of Sciences, Paris, June 13, which appears in NATURE for June 30, it is stated that MM. Jamin and Amaury presented a note on the above subject, in which they point out, apparently as if it were something new, that the specific heat of some of these mixtures rises even above that of water.

PROFESSOR FEE is by many years the oldest amongst living fern-authors. He has held for more than a quarter of a century the chair of botany at Strasburg, and has concentrated his attention principally upon ferns and the other allied higher orders of cryptogamic plants. He published a general treatise upon the classification of the order as long ago as 1844, ancl since then many consecutive years have never passed without producing some memoir upon the subject from his fertile pen. Having recently received a fine collection of ferns from Dr. Glaziou, the superintendent of the Botanic Gardens at Rio Janeiro, he has been stimulated to add one more memoir to the series, and that is the work now before us. All the series of his monographs, several of which are in folio, are illustrated beautifully and copiously, not only with full-sized figures of the plants, but also with careful magnified analytical details; and together they form by far the most extensive and excellent series of fern-plates which anyone upon the Continent has published. The present memoir is quite upon a par with its predecessors in this respect. It is in quarto, and contains seventy-eight quarto plates and a list of all the ferns and fern-allies known to the author as inhabiting Brazil, with a list of special stations, but with descriptions of novelties only. But there is one drawback to the value of Fee's works, and that is a very great one. Living at a distance from the great metropolitan herbaria, our author has apparently worked almost entirely upon his own private collections, and has continually failed to recognise well-known plants, and has made new species in great numbers out of the specimens which his correspondents have sent him, which no one else has been able to understand as such. In none of his works—we have no alternative but to say—has this tendency been carried to a greater excess than in the present one. For Brazil alone he describes and figures in the present memoir upwards of 180 new species, so-called. These are not from tracts of country which the collectors whose gatherings have been already reported upon have not visited, or have left unexplored, but nearly all from the vicinity of the capital, and from the gatherings of Glaziou. Now the neighbourhood of Rio Janeiro is exceedingly rich in ferns; but there is, perhaps, no other part of Tropical America from which herbaria, both in England and on the Continent, have been more bountifully supplied. The consequence is, that out of this 180 we do not think that more than from twelve to twenty species are really newj in any sense in which we understand in this country what is meant by a species. For instance, we have some seven or eight species elaborately characterised and figured from what cannot be called anything else than so many individual fronds of that most cosmopolitan of ferns, our common English Aspidium or Polystichum aculeatum. Or, to take one of the exclusively Brazilian species, Cyathea Gardneri, a very distinct tree-fern, is included in the list under five different names—Gardneri (Dr. Gardner's number on which Hooker described the species quoted), incurvata (a name of Kunze's published in the Linnaea from Regnell's specimens), mamillata, taunaysiana and attenuata, the last three new species here named and figured for the first time; but the figures, beautiful as they are, might, any of them, have been drawn from Gardner's specimens. The author does not seem to have any knowledge of numerous English and German books and papers in which Tropical American ferns are described, as for instance, Grisebach's excellent Flora of the British West Indies; and this leads to further name-crossing. In short, although one cannot but admire the excellence and the copiousness of the illustrations in these memoirs, and ought not to leave out of sight the example of devotedness to science which they show, expenditure of time devoted to one object through a long course of years, and of money, only a very small, proportion of which their sale can possibly repay, yet still the predominant feeling on the mind must needs be that to deal with plants in this way has a direct tendency to bring species-botany at a very rapid rate into a state of utter confusion. Cryptogames Vasculaires du Bresil. Par Prof. A. L. Fee avec le concours de Monsieur le Dr. Glaziou. Pp. 268, 4to., 78 Plates. (Paris: Bailliere.)

THE soul of many an anti-Darwinian will have been cheered by Mr. A. W. Bennett's paper on “The Theory of Natural Selection from a Mathematical Point of View.” It is, in fact, a very admirable piece of special pleading, based on a skilful assumption of premisses which, to a careless or biassed observer, might seem indisputable.

I AM forcibly reminded of Pope's lines, A little knowledge is a dangerous thing: Drink deep, or taste not, the Pierian spring, by the argument used by Mr. Bennett in the P.S. to his letter in NATURE, of the 24th November, in which he says, after quoting a passage from a paper by Mr. Jenner Weir: “Here at least it would seem as if imperfect mimicry was anything but beneficial to the individual; how can the principle of natural selection account for its propagation in these instances?” He considers that a little mimicry is a dangerous thing. I would rather agree with Lord Brougham in his remark on the above lines, that as a little knowledge is better than great ignorance, so a little mimicry is better than great dissemblance.

THE papers read by me before the Entomological Society “On the Relation between the Colour and the Edibility of Lepidoptera and their Larvae” having been noticed and commented upon by Mr. A. W. Bennett and others in NATURE, I have deemed it desirable to offer a few remarks on the subject.

ALLOW me space for a word or two in reply to Mr. Wallace and Mr. Butler's observations on my papers on Mimicry and Hybridisation.

UNDER this heading, in your issue of 5th inst., you report a paper by Prof. Osborne Reynolds, M.A., read at a meeting of the Manchester Literary and Philosophical Society, Nov. 29 last. This paper sets forth that the tails of comets, the solar corona, the aurora, and the Zodiacal lights are due to the ether which “fills” space. Comets' tails, as stated by the Professor, in his paper, are an effect due to the medium through which it passes being heated and illuminated by the comet; and that the other phenomena are also due to the ether.

THIS is the first part of a new periodical established primarily for the publication of the results of the observations made at the Botanico-Physiological Institute at Breslau. The part contains five papers on different microscopic algae and fungi, and their pathological effects. In subsequent numbers it is intended to give priority to botanical observations which relate to biological questions, or which are more or less connected with practical natural science, medicine, agriculture, &c. It is hoped that the publication may fill the place formerly occupied by Karsten's “Botanische Untersuchungen.” Beitrage zur Biologie der Pflanzen. Herausgegeben von Dr. Ferdinand Cohn. Erster Heft. Mit sechs zum theil farbigen Tafeln. (Breslau, 1870. London: Williams and Norgate.)

IN NATURE of 29th September, 1870, there is an abstract of a paper read at the British Association by Mr. Laughton, maintaining that the preponderance of west over east winds over the entire globe is such as to point to a cosmical force of some kind, moving the atmosphere round the earth from west to east; and concluding “that the motive force for which we are seeking, is really the disturbing force of the attraction of the heavenly bodies.”

IN NATURE for April 7, 1870 (vol. i. p. 583), Mr. Marcus Hartog stated, as the result of his observations, that the male catkins and female flowers of the hazel are not simultaneously developed on the same twig, and that therefore a kind of quasi-cross-fertilisation necessarily takes place. Although convinced at the time that my observation did not tally with Mr. Hartog's, it was then too late in the season to submit my impression to a practical test. During the past week I have closely observed the hazel bushes in flower, and have found on every bush which has come under my notice, the female and male flowers in a perfect state of development on the same ultimate twigs, in close proximity to one another, the stigmas being frequently loaded with pollen-grains, apparently from the neighbouring catkin; at all events there appears no provision of nature specially to promote fertilisation from other bushes. We see in fact here a confirmation of the general law suggested in my paper in the first number of NATURE, on “The Fertilisation of Winter Flowering Plants,” that when plants flower in the depth of winter, and at a time when no or few insects are about, self-fertilisation is the rule rather than the exception, or in the case of unisexual flowers, as near an approach to self-fertilisation as is possible under the circumstances.

I HAVE presented to the French Institute at its last sitting a copy of NATURE, January 26, with the sketches of a halo seen from Liverpool by two different observers from two stations at a little distance. It is the first time, as far as my knowledge goes, that we have had two different sketches of the same phenomenon. The difference is very considerable indeed, as a para-selene was seen by one of the observers, and not by the other. It would be very important to ascertain what was the exact situation of each of them, and I should be glad if you can take the trouble to ascertain it by an inquiry through your paper. When I return to England I will ascertain the circumstances of the observation, which may lead to some definite conclusions on the form and altitude of floating snow during the night of the 4th January.

As I see you have reprinted at length Dr. Hopkinson's paper with the above title, in which he criticises severely, not to say ungenerously, some papers of mine published in the Quarterly Journal of Science and Chemical News, you will think it only fair to publish my reply, in which I think I shall show that, in the course of his short paper, Dr. Hopkinson has committed mistakes at least as grave and important as any he imputes to me. Let us see if this is not the case.

INVINCIBLE ignorance is said to be excusable. This must be my plea, when I say that I have read over again Sir W. Thomson's paper in the “Philosophical Magazine” of Feb. 1854, and that I cannot see but that it leads to perpetual motion more than anything I have ever written.

THIS work reflects the highest credit upon its author, and will establish his reputation as an Ornithologist. Very few monographs published in England are so entirely satisfactory as this one, for not only have the several parts appeared regularly during the last three years, but the concluding double number just issued contains a copious and well-written introductory chapter on classification, geographical distribution, and literature, which renders the book a model of what such a work should be. The Kingfishers, although represented in our country by only one species, are especially abundant in the Eastern Tropics, where they exhibit a great variety of form and the most exquisite beauty of plumage. A considerable number of them are inhabitants of the forests, and never frequent water, subsisting on insects, small Crustacea and mollusca, and the larger species even on snakes, lizards, and other reptiles, which they capture by darting down upon them from a branch just as our own species pounces upon a fish. Mr. Sharpe has been fortunate in securing the services of a young Dutch artist, Mr. Keulemans, who has himself studied birds in the tropics, and seizes upon their various attitudes with the happiest fidelity. He also surrounds his figures with little bits of appropriate scenery, so that a considerable number of the 120 plates with which the book is illustrated are beautiful pictures, as well as admirable representations of the several species. We do not hesitate to say that many of these plates are equal to the very best that have appeared in any illustrated work of Natural History. The body of the work consists of coloured figures of every known species of kingfisher, with full synonymy, careful description, and record of whatever is known of its habits. In the introduction, the classification of the species is carefully considered, only those generic groups being retained which can be characterised by marked structural differences. The whole family is first divided into two sub-families: the Alcedinidae, or true kingfishers, characterised by a compressed keeled bill; and the Daceloninae, or kinghunters, which have a depressed bill rounded or furrowed above. These are subdivided into nineteen genera, in which are grouped the 125 species of kingfisher now known. The groups are all characterised by modifications of the bill, feet, or tail, and a plate exhibits these generic characters at one view. There is also a tabular key of the species in every genus and of the genera in each sub-family, and the reasons are given for rejecting numerous genera proposed by other authors on insufficient characters. The geographical distribution of the species is then discussed in the same careful manner, an exact account of the known range of every species being given, as well as tables showing at a glance the distribution of all the species of a genus or group of allied genera; after which the results of the examination are ably summed up. Kingfishers present us with some of the most curious anomalies of distribution to be found in the whole class of birds. There is no part of the world so rich in peculiar forms of bird-life as America, more especially the southern half of it, yet it is the poorest of all parts of the world in kingfishers, only eight species being found in the whole continent,—a continent with more rivers and more fish than any other! The single island of Celebes actually contains as many different kinds of kingfisher as all North and South America, while New Guinea contains more than twice as many. It is perhaps even a more extraordinary fact that there is no peculiar type of kingfisher in America, all the eight species belonging to one genus, and that genus found also in Europe, Asia, and Africa. In Africa we have three peculiar genera of kingfisher, and twenty-four peculiar species. In continental India there are only five peculiar species, and not one genus. The western Malay Islands (Indo-Malayan sub-region) have one peculiar genus, and eleven peculiar species; the Philippines, seven peculiar species; but the Australian region has no less than ten peculiar genera and fifty-nine peculiar species, or nearly half those of the whole world. The peculiarities of the island of Celebes are well shown by the kingfishers, for not only has it eight peculiar species and three peculiar genera, but one of the latter has affinities with an African genus. In discussing the general relations of this isolated group of birds to the rest of the order, and the mutual affinities of the genera, the conclusion is arrived at that they are most nearly allied to (although still very remote from) the hornbills; and their relations are expressed by a branching diagram, as well by a map of the genus on the plan of Professor Flower. A copious account of the literature of the family is also given, no less than 135 separate works being enumerated, with references to every species of kingfisher described or noticed in them. An elaborate paper on the anatomy of these birds by Dr. Murie, with a full index, completes this exceedingly valuable work, which will be equally acceptable to the naturalist for its detailed and accurate information, and to all who love nature for its beautiful and artistic illustrations. A Monograph of the Alcedinidae, or Family of Kingfishers. By R. B. Sharpe., &c. Librarian to the Zoological Society of London. 4to. (Published by the Author. 1868—1871.)

IN the faithfully-recording columns of NATURE for March 30, at p. 434, is a much required abstract of Mr. Stone's important paper, recently communicated by him to the Royal Society, London, on the connection between terrestrial temperature and sun-spot phenomena. By comparing the curves of mean annual temperatures during the last thirty years (as observed, ready to his hand, by his indefatigable predecessor Sir Thomas Maclear) with another curve constructed on Wolf's observations of sun-spots, Mr. Stone has been enabled to deduce, almost immediately after arriving on his new scene of labour at the Cape of Good Hope, first, that there is an approximately decennial period of such temperature, and so similar to that of the sun-spots as to indicate more than a mere coincidence; and secondly, that the sun-spots are not to be looked on as the direct ages of thent temperature variation, but that either phenomenon results from some general change of solar energy.

IN a letter with this heading in NATURE for February 16th, Mr. Murphy has very roundly objected to certain views which I have put forward regarding the predominance of westerly winds. In the paper read before the British Association, to the abstract of which he refers, and which was itself little more than a resume of the propositions maintained at greater length in my “Physical Geography,” reviewed by “A. B.” in NATURE for March 16th, my object was not so much to show that westerly winds predominated in volume over easterly winds, as to show that all prevailing winds, not westerly, may be properly considered as deflected or secondary currents of air, and that more especially the trade winds may be so considered. I have supported this view by a detailed examination of the geographical circumstances, habitudes, and characteristics of the principal winds; but to have included every local exception—as “A. B.” seems to consider I ought to have done—would have required more time than even the most industrious can spare, an amount of special topographical knowledge which is practically unattainable, and would have had no important bearing on the main question. I may go even further. I may say that, from a general point of view, isolated local registers have no value at all, unless the method of observing and the position of the vane are distinctly made known. It would be perfectly easy to name a dozen localities in Wales, in the Lake District, or in Scotland, where a vane would show a prevailing wind widely different from the W.S.W., which, however, we have no difficulty in accepting as the prevailing wind of the country; even at Liverpool the prevailing wind has been observed to be W. N. W., and at Valentia there is a marked difference between the wind in the northern and southern entrance. In Mr. Buchan's paper in the Transactions of the Royal Society of Edinburgh, December, 1869, I find that at Irkutsk the wind is almost always due north, or due south, would “A. B.” imply that the Irkutsk observations afford any information as to the prevailing wind of Siberia?

You will see from the proceedings of the Literary and Philosophical Society at Manchester, that, since the discussion there, Dr. Joule has definitely abandoned the reasonings in his famous paper on the mechanical force of electro-magnetism, steam, and horses. I have now had time to test the facts and experiments of this new theory, and find it, as I hope soon to show in detail, as untenable as his former one. Indeed, I am sure that the mechanical equivalence of heat must soon be generally abandoned as inconsistent with facts. You will see that the April numberof the “Review of Popular Science,” has definitely pronounced a decision in my favour; and I am sure you will soon be convinced yourself that your own first reviewer of my article in the Quarterly Journal of Science was more reasonable than your second.

PROF. CLERK MAXWELL in his valuable paper on Colour in NATURE (vol. iv. p. 13) commits himself to the opinion that there must be three distinct sets of retinal nerves, one for each of the three primary sensations of colour. It is obvious that demonstrative proof or disproof of this is unattainable: we can only reason analogically. The analogy of the ear is in favour of such an opinion, so far as it goes; for there appears to be proof, or probability almost amounting to proof, that sounds of different pitch are conveyed to the brain by different nerves. But the ear resembles the other organs of sense less than they resemble each other; and there is surely no reason for thinking that there are distinct nerves of smell for every distinct kind of smell, or distinct nerves of taste for every distinct kind of taste. Nor I believe is there the slightest proof of nerves for the sensation of heat distinct from those of touch.

MR. HIGHTON'S letter in NATURE is almost identical with his communication to the Chemical News. My answer is similar to that which I sent to the latter publication, viz., that the object of my paper in the Proceedings of the Literary and Philosophical Society was simply to place the theory of the electro-magnetic engine in a form which might prove useful to those who had not worked on the subject, and not in any respect to withdraw the reasonings in what Mr. Highton is good enough to term my “famous paper.” Mr. Highton handsomely acknowledged the justice of my note to the Chemical News in a letter addressed to me on the 28th ult.

IN your report of the discussion that followed the reading of my paper on Flint, before the Geologists' Association on June 2nd, Prof. Morris is said to have asserted that the views I suggested were first propounded by Dr. Brown of Edinburgh. I think the Professor must have been slightly misrepresented in this; at all events I must most decidedly decline to be coupled with Dr. Brown, or to allow myself to be associated with his very remarkable statements. These may be found in the Trans. Roy. Soc. Edinb., vol. xv. He asserts that carbon is transmutable into silicon; at p. 229 he says, “Carbon and silicon are isomeric bodies, and that the former element may be converted into a substance presenting all the properties of the latter.” At p. 244, “3.04 grains of silicic acid were extracted from 5 grains of paracyanide of iron;” at p. 245, “5.4 grains of silicic acid were procured from 30 grains of the ferrocyanide of potassium,” and “there were obtained 9,334 grains of silica from 3,240 grains of ferrocyanide, although some of the product was lost in two of the operations.” The view I advocated as explanatory of the formation of flints was the substitution of silicon for carbon, not a transmutation, and I distinctly showed the source from which the silicon was derived. Dr. Brown's statements are so extraordinary that I could scarcely believe them serious. I find, however, in the same volume of the “Transactions” that they were most patiently examined and confuted by Dr. George Wilson and Mr. John Crombie Brown, and they say, “We tried the greater number of Dr. Brown's processes, and rejected them one after another without pursuing their investigation further, on finding they would not yield quantitative proofs of the conversion of carbon into silicon. The limited time, which from various circumstances we could devote to the subject, obliged us to follow this course; and the confident expectation we entertained till a recent period that each new process would supply what the rejected ones had failed to afford, led us to neglect noting many particulars of our early trials which otherwise we should have recorded. . . . In conclusion, we need scarcely say that we have been unable to supply any proof of the transmutability of carbon into silicon.”

A YEAR ago Mr. Crookes, in a paper published in the Quarterly Journal of Science, announced his intention of scientifically investigating a certain class of phenomena, then known as “spiritual,” which he complained had been strangely and unwarrantably neglected by those whose duty it was to investigate them. The results of some of these investigations have at last been published, in the same journal, under the title of “An experimental investigation of a new force.”

I ENCLOSE one or two bonâ fide extracts from elementary examination papers which have during the past few years come under the notice of candidates officially. I do not wish thereby to reflect so much on the regulations drawn up by senates and committees, as upon the way in which those regulations are carried out by examiners. Though cramming is officially denounced, yet there is scarcely anything which is in greater demand; and, so long as this is the case, candidates will of course insist, in spite of their teachers, upon undergoing the operation.

IT was unfortunate that at the recent meeting of the British Association, Prof. Williamson's paper had to be discussed in a very hurried manner, and he is, no doubt, justified in taking care “that there shall be no misunderstanding as to the real point at issue.” I do not think that he has brought it out very plainly in his paper in NATURE, and perhaps, as he mentions me as an opponent of his views, I may be allowed to state precisely in what respects I differ from him.

IN the summary of the proceedings of the late meeting of the British Association, in the issue of NATURE for August 31, is an abstract of a paper by Captain L. Brine, R.N., On the Ruined Cities of Central America. The gallant captain is wrong in stating that the existence of these ruined cities was unknown until within a comparatively recent period. All the early chronicles abound in allusions to them—Remesal. Vasquez, Cogolludo, Villagutierre, Juarrez, and others. Uxmal and Chichen Itza, which Captain Brine speaks of as “discoveries,” were undoubtedly occupied places at the time Grijalva touched the shores of Yucatan. Copan, although then a ruin, was visited and minutely described by Dr. Palacios as long ago as 1576. Captain Brine would lead us to infer that these remains have been “discovered” since the expedition of Del Rio to Palenque in 1787.

THE Academy journal of the 12th of February, 1870 (vol. i. p. 128), contained a paper by Prof. Helmholtz upon the Axioms of Geometry in a philosophical point of view. The opinions set forth by him were based upon the latest speculations of German geometers, so that a new light seemed to be thrown upon a subject which has long been a cause of ceaseless controversy. While one party of philosophers, especially Kant and the great German school, have pointed to the certainty of geometrical axioms as a proof that these truths must be derived from the conditions of the thinking mind, another party hold that they are empirical, and derived, like other laws of nature, from observation and induction. Helrnholtz comes to the aid of the latter party by showing that our Axioms of Geometry will not always be necessarily true; that perhaps they are not exactly true even in this world, and that in other conceivable worlds they would be entirely superseded by a new set of geometrical conditions.

IT falls so rarely to the botanical reviewer in this country to notice works on Lichenology, that we gladly avail ourselves of the present opportunity of introducing to our readers a little unpretentious volume which has the excellent object primarily—“of elevating the knowledge of our insular lichens to a level with that of other branches of our country's flora,” and which, moreover, completely vindicates the title of Britain's lichens to at least equal study with the other families of her cryptogamia. Since the publication of Mudd's excellent “Manual” in 1861, the additions made to the lichen-flora of Great Britain and Ireland have been both so numerous and important, that lichenological students have felt the want of some systematic work containing a complete list of the British lichens up to the present date, along with specific diagnoses and other aids to their identification. It was generally felt, moreover, that no fitter authority could undertake so intricate a labour than Mr. Leighton, whose name is identified with lichenological progress in this country by the publication of many important papers of a monographic character, and who is justly regarded, both by home and foreign botanists, as the representative and father of lichenology and lichenologists in Britain. The present work, which we are glad to find is to be followed, in due time, by another which is even more urgently required—a Conspectus of all known lichens throughout the world—is a convenient 12mo volume of about 470 pages, which confines itself mainly to a systematic enumeration, with specific diagnoses, of all the lichens at present known to occur in “Great Britain, Ireland, and the Channel Islands.” The nomenclature and classification followed are those of Dr. Nylander, of Paris, who is described as “the facile princeps of modern microscopic lichenologists.” Succeeding the specific diagnoses, the author cites the leading synonyms; gives references to published plates and fasciculi of dried specimens; narrates the general geographical distribution of species throughout the world, on the one hand, and throughout the three kingdoms on the other; specifies the particular localities of growth in each of these latter kingdoms; and gives, so far as possible, the date of original discovery in Britain, with the name of the discoverer. The Lichen-Flora of Great Britain, Ireland, and the Channel Islands. By the Rev. W. A. Leighton. (Published for the Author. Shrewsbury, 1871.)

YOUR correspondent Mr. Potts in the last number of NATURE furnishes us with a few interesting facts regarding the Kea. In a paper which I read about three years ago to the Dumfries Natural History Society, entitled “The Influence of the Human Period on the Sagacity of Animals,” and subsequently in a letter published in NATURE, vol. i., on the “Mental Progress of Animals,” I endeavoured to show from general considerations, and from the few facts which we possessed on this subject, that the habits and instincts of animals were not so fixed and definite as might be supposed. The general principle for which I contended was that whether we considered the globe to have received its human inhabitants according to the laws of evolution, or in some miraculous manner, the arrival of the human race produced great modifications and changes of surrounding circumstances. These changes were in the direction of increasing the fertility of all vegetable productions capable of sustaining life, and at the same time securing their use entirely for the human family. Hence arose, in the vicinity of man, two new factors; the superior attraction of better food for all kinds of animals, and at the same time the extinction of such animals whose greed was not overruled by sufficient wariness or cunning to become successful thieves. Hence a probable gradual increase in these qualities in the animals maintaining themselves against man.

No exception can be taken against Dr. Aitken's argument in your number for Oct. 12. The colours of the substances he experimented on could not be regarded as simple. But he does not consider how loosely all names of colours must be applied in common language. The colours of most blue pigments, especially in thin washes, no doubt contain a large proportion of green. But let the colour of the blue salvia, or that of the pigment called French blue or ultramarine (often given as the best example of true blue) be tested in conjunction with the purest yellows (even with the almost greenish yellow of the pigment called lemon-yellow) and the two will be found perfectly complementary. This is the colour of Newton's indigo rays, which he himself in his colour circle put opposite to his yellow. In fact, in good English, not only sea-greenish blues, like the colour of Newton's blue part of the spectrum, or that of the pigment called azure or caeruleum, but even the colour of the violet itself, is properly called blue. Witness Milton's “beds of violets blue.” The violet of the spectrum is in truth little more than a pure blue diluted with white by reason of the fluorescence of the retina, as recent researches have shown. (See J. J. Muller's paper in Poggendorff's Annalen, March and April last.) I must, therefore, protest against substituting a fanciful term like violet for the good English blue, as the designation of a simple colour-sensation. It is hard enough to make artists believe that yellow is not a simple colour. To tell them the same of its complementary blue would add to their disgust, and not unreasonably.

IN your paper of Nov. 9 I observed a letter about New Zealand Forest-Trees, signed by Mr. John R. Jackson of Kew.

IN the number of NATURE for July 20, 1871, I find a paper by Ferrel, “On the Cause of Low Barometer in the Polar Regions,” &c. The author says that the law which deflects a body to the right in the northern hemisphere and to the left in the southern is not understood by meteorologists, and that it is admitted, only when the movement is north and south.

I AM sure that the Rev. M. J. Berkeley will exonerate me from any deliberate intention to misrepresent him; nor do I think that there is, after all, much difference of opinion between us regarding the present subject, unless, perhaps, that I am more sceptical. I alluded to the paper cited by him from the “Journal of the Horticultural Society,” on propagation of bunt spores, and not to his communications on the hop or vine mildew. I was under the impression that he regarded the “four consecutive forms of reproductive cells in the bunt” as an instance of alternation of generations. On reference to the original paper, I find that he did not go so far then as to indicate four consecutive forms of reproductive cells; but that Tulasne followed on his track in 1854, and in 1857 Mr. Berkeley seemed to have accepted the results of Tulasne's observations, since, in his “Introduction,” he gives figures at page 318, in the description of which the following phrases occur:—“spores of the second order,” “spores of the third order,” “spores of the fourth order.” Here are the “four consecutive forms of reproductive cells” to which I alluded. At page 321 he writes concerning the bunt:—“The spores, however, are not immediate means of propagation; they are, in fact, only a sort of prothallus, from which the mycelium grows, producing at the tips, or on lateral branchlets, bodies of various forms, which are themselves capable of germination, and immediately reproduce the species.” The real issue between us seems to lie in the phrase, “alternation of generations.” If the bunt spores, on germination, produce fusiform bodies, which, after conjugation, produce short cylindrical spores, and thus intermediate reproductive cells unlike the parent cell come between that and the ultimate reproduction of the species, I am induced to call it an “alternation of generations.” It would be waste of time to discuss phrases, or I might take exception to the application of this phrase to the Erysiphei. The conidia and pycnidia of the hop mildew may be developed without sporangial conceptacles, and the parasite reproduced without sporangial fruit, but I cannot recognise alternation of generations in the reproduction of a species by means of conidia, stylospores, or sporidia, or by one of these alone. If such may be construed into an alternation of generations, it must be by permitting greater elasticity to the phrase. Conidia germinating and producing pycnidia, the stylospores of the pycnidia germinating and producing sporangial conceptacles, containing the sporidia which, upon germination, will produce the mycelium and conidia again, returning to the original form after two or three consecutive departures from it, appears to me a perfect type of alternation of generations. I fully admit that “if it is once established that a Puccinia produces an AEcidium, or an AEcidium a Puccinia, we should have a clear case, especially when the third form reverts to the first again.” Without the slightest desire to “depreciate the labours of Oersted and De Bary,” I cannot admit that they have established facts until their observations are confirmed, especially when there is an evident possibility of their having been deceived. I shall have no hesitation in accepting the facts when they are confirmed by independent and equally trustworthy observers, although I may be unable to account for some of the phenomena. At present I must confess that I am not so sanguine as Mr. Berkeley appears to be.

IN the new edition of Jukes's “Manual of Geology,” by Prof. Geikie, at page 592, it is stated, on the authority of Mr. E. Hull, that “in Leitrim, Fermanagh, and Tyrone, there are true representatives of the Yoredale series of England.” I, however (as also the late Mr. Jukes), contend that no comparison can be drawn between the coal bearing rocks of Ireland and those of England. Furthermore, as Mr. Hull is unacquainted with these Irish rocks, having only seen a few isolated patches of them, I protest against his being quoted as an authority on the question, more especially as in the paper to which Mr. Geikie referred, “On the Geology of the Bally castle Coal Fields, &c.,” Mr. Hull states that while in the counties above mentioned there are true coal measures, in the provinces of Leinster and Munsler there are none—a statement quite contrary to facts, as all the sections of these rocks in Leinster, Munster, and Connaught arc identical, and probably, as suggested by the late Mr. Jukes, were once connected, as the lowest bed of coal occurs everywhere at a nearly equal height above the limestone. Furthermore, the intervening strata are nearly identical, there being a certain thickness of argillaceous beds below, next the limestone, and a mixture of arenaceous and argillaceous beds above.

SOME years ago I wrote a paper for the Ethnological Society on some changes of surface affecting Ancient Ethnography. Since this was printed many facts have accumulated. These have led me to a tentative generalisation on the subject, which I should like to have discussed in your pages.

AT a recent meeting of the Astronomical Society a paper was read by Mr. Ranyard, in which some suggestions were put forward concerning the possibility of accounting for the solar prominences on the supposition that they may be caused by the projection of matter from a lower level, and that such an uprush into and through the layers above, emerging into the lighter envelope of the chromosphere, might lift before it a cone of compression of the gaseous matter, producing an elevation on the surface, visible to us as a prominence. And the solid particles or masses thus projected might form meteorites, the shape of the prominence being afterwards modified by other causes.

UNDER the above heading in your issue of January 18, 1872, Dr. Elliot Coues describes the peculiar movements made by the bones of the wing of the bird in flexion and extension. It may interest some of your readers to know that those movements were minutely described and elaborately illustrated in a paper by Dr. J. Bell Pettigrew, communicated to the Linnean Society in June 1867, and published in vol. xxvi. of the Transactions of that body.

IN the list of papers read before the Paris Academy of Sciences, which was given in last week's NATURE, I noticed one on the magnetic perturbations observed at Alencon during the late total eclipse. Now it would at first sight appear reasonable to expect that any effect produced on the magnetic needle at Alengcon by a phenomenon whose maximum phase was as far removed as India or Australia, should have nearly equal effect on the needle in England, and in all countries adjoining France. It has moreover been established by frequent comparisons of carefully measured photographic records, taken at different magnetic observatories, that any disturbance of the earth's magnetic force is felt almost simultaneously at stations differing several hundred miles in both latitude and longitude. I was, therefore, justified in supposing that I should find some indications on our photomagnetic records of a disturbance corresponding to the perturbations of the needle at Alencon, alluded to by M. Lion in his note to the Academy. The result of my examination of the records is, that there is not the slightest trace of a disturbance on either the vertical or horizontal curves, and that the declination magnet has been more than usually quiet, although on the two previous days it happened to have been somewhat disturbed about the hour at which the totality of December 11 occurred.

WILL you permit me to inquire, for the instruction of the many who are familiar with Mr. Herbet Spencer's “Doctrine of Evolution,” and especially in regard to “First Principles,” sec. 58, referred to by Mr. Spencer in his paper in your number for February 1, if the theory of the “Dissipation of Energy” does not upset a very considerable and significant portion of Mr. Spencer's “First Principles”?

I HAD designed sending a note to you, critical of the abstract of my paper on “The Laws of Orgaaic Development,” republished from the American Naturalist in one of your recent issues, before I read the remarks of Mr. Spencer in your number of Febraary 1.

THE chief difficulty in the way of explaining the annual and diurnal variations of the barometer by the heating and cooling of the air, appears to be the existence of a double maximum and minimum. To show how such a double maximum and minimum might result from the changes in the temperature of the dry air alone, is the object of the present paper. I commence with the diurnal variation.

NATURALISTS have long recognised the curious cases oftentimes occurring, of the resemblance between the colour of an animal and its immediate surroundings. It had been supposed that climatic influences, or peculiarities of food, or greater or less access to light, had something to do with these coincidences. Mr. Alfred R. Wallace has shown that the varied phases of these phenomena could not be explained by such agents, and in a paper “On Mimicry and other protective resemblances among Animals,” published in the Westminster Review, July 1867, and since made widely public in his work on “Natural Selection,” he shows that the singular resemblances between the colour of animals and their surroundings are mainly brought about by the protection afforded them through greater concealment. Many very interesting examples are then cited from the Vertebrates and Articulates in support of these views. Briefly may be mentioned, as examples, the almost universal sand colour of those animals inhabiting desert tracts; the white colour of those animals living amid perpetual snows; the resemblance seen again and again between the colour of many insects and the places they frequent. Among the hosts of examples cited by Mr. Wallace as illustrating plainly the views he advances, may be mentioned the many species of Cicindela, or tiger beetle. The common English species, “C. campestris;, frequents grassy banks, and is of a beautiful green colour, while C. maritima, which is found only on sandy sea shores, is of a pale bronzy yellow, so as to be almost invisible.” He then states that a great number of species found by himself in the Malay Archipelago were similarly protected. “The beautiful Cicindela gloriosa, of a deep velvety green colour, was only taken upon wet mossy stones in the bed of a mountain stream, where it was with difficulty detected. A large brown species (C. heros) was found chiefly on dead leaves in forest paths; and one which was never seen except on the wet mud of salt marshes, was of a glossy olive so exactly the colour of the mud as only to be distinguished when the sun shone, by its shadow. Where the sand beach was coralline and nearly white, I found a very pale Cicindela; wherever it was volcanic and black, a dark species of the same genus was sure to be met with.”

IN the extract from his Address to the Entomological Society, given in NATURE, February 29, Mr. Wallace remarks that Mr. Spencer's views have not been so much as once alluded to in the discussion of the Origin of Insects. The general question of the Annulosa obviously includes that of Insects, and I therefore desire to correct this statement, and to refer your readers to a paper by me on Chœtogaster and AEolosoma, published in the “Linnean Transactions,” vol. xxvi. (read Dec. 1867), in which I have more than alluded to Mr. Spencer's views, and have offered some suggestions on the morphology of the head, and as to the unisegmental Annulose ancestor. Mr. Wallace quotes from this paper in reference to Chœtogaster, though from the context it would appear that he is quoting from Professor Owen.

SOME short time ago I observed in one of the daily papers an account of “an odd fish” which had been captured, and described by Prof. Agassiz as a denizen of the Gulf weed, on which it is said to walk with legs, and not to swim as other fishes do.