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The aim of this paper is to compare two continuous methods for the identification of fire front positions and the rate of spread of linear flame fronts. The first technique is based on the heat fluxes measured by a specific thermal sensor and the use of the inverse method. The second one uses an image processing technique applying the simple direct linear transformation (DLT) algorithm and the fire segmentation algorithm to obtain fire front positions. The fire segmentation method has been selected by benchmarking four recently published studies and involves a supervised evaluation criterion. The two continuous approaches give closely similar results and it is shown that these two techniques can be respectively validated by each other.

Wildfires became a severeecological and financial problem in Israel during the last few years. A significant increase in the incidence of severe wildfires in natural shrublands and planted forests as arson was observed. On 19–20. 9. 89 a large wildfire ignited as an act of terrorism, devastated large parts of Mt. Carmel forests. Weather conditions during a wildfire have an important role in the determination of the rate of spread of the fire. This depends mainly on the availability of dry matter and the wind speed. Two factors that reach their optimum at the end of the summer in September and October during Sharav episodes. The distribution of easterly wind spells associated with Sharav events is considered. The results show that there is no climatic limitation to a spread of a wildfire of the same order of magnitude as the last one, every year.

There is some evidence to the effect that as cities become increasingly congested new housing starts occur at greater distances from urban centers while jobs tend to remain center-concentrated or develop in other suburbs. In either case, mean commute distances tend to increase. Telecommuting is seen as a means of increasing the jobs-housing balance in urban and suburban areas by enhancing the ability to move work to, or closer to, the workers' residences rather than requiring workers to commute to work daily. This has the immediate side effect of decreasing automobile congestion and associated energy consumption and air pollution. There is a possible longer term adverse impact of telecommuting resulting from its ability to decrease constraints on household location, thereby enhancing the rate of spread of suburbia. This paper reviews evidence concerning the possible effects of telecommuting on urban sprawl, as derived from a two-year test of telecommuting in California, and describes two scenarios of urban form development made more feasible by telecommuting.

Describes methods for approximating behavior and size of a wind-driven crown fire in mountainous terrain. Covers estimation of average rate of spread, energy release from tree crowns and surface fuel, fireline intensity, flame length, and unit area power of the fire and ambient wind. Plume-dominated fires, which may produce unexpectedly fact spread rates even with low ambient windspeeds, are covered and supplemental methods suggested for estimating their occurrence. The spread information can be used to estimate and map fire area and perimeter.

The effects of wind and topographic slope are important considerations when determining the rate and direction of spread of wildfires. Accordingly, most models used to predict the direction and rate of spread contain components designed to account for these effects. Over the years, a variety of different approaches have been developed. In the present manuscript, we examine the various mathematical models employed to account for the effects of wind and slope at a formal level, making comparisons where appropriate. The methods reviewed include scalar methods, which ignore the directional nature of wind and slope effects, as well as methods in which the effects of wind and slope are combined in a vectorial manner. Both empirical and physical models for wind–slope correction are considered.

Studies on the occurrence and impact of genetic interactions in the soil microbial community are nowadays considered of great importance, for example to adequately predict the rate of spread in soil of introduced novel genetic combinations produced by genetic engineering.1–3 In addition, these studies may advance our knowledge about the extent to which genetic transfer mechanisms actually play a role in the evolution of microorganisms in soil, for example endowing microorganisms with the capacity for an adequate response to certain (man-imposed) stress conditions.

Fears about transgene escape have focused attention on the potential for hybridization between crops and their wild relatives. Although transgenes will often escape from cultivation ( [1][1] ), their rate of spread will be mainly governed by their fitness effects, not the migration rate ( [2][2] ).

The relative sensitivity and performance of post-hocindices of fire characteristics were examined in a heathland area burnt in1994. Sites burnt at differing times before 1994 were selected andqualitatively allocated to classes of high and low crown damage. Subsequentmeasures were made of height of consumption of crowns, length of the deadportion of Xanthorrhoea resinifera leaves, and minimumtip diameters of burnt branches of the shrubs,Banksia oblongifolia andBanksia ericifolia. Results indicated that significantdifferences in mean, minimum tip diameter corresponded to contrasting classesof crown damage (larger mean tip diameters in high crown damage sites). Meanminimum branch tip diameter in B. oblongifolia was notsignificantly correlated with fuel age but significant correlations were foundwith estimates of rate of spread and Byram fire intensity at the study sites.The minimum branch tip diameter method has potential for further developmentand use as an indicator of fire intensity in heathland vegetation.

The analysis of laboratory fire experiments led to the development of a reaction-diffusion model for the spread of fire across a fuel bed in windless and slopeless conditions. A method for the determination of coefficients in this model based on the dynamic features of a spreading fire is given. The numerical study of the mathematical problem proposed allows us to predict the rate of spread, the fire front perimeter and the temperature distribution for line-ignition and point-ignition fires. These results are compared with success to experimental data. Furthermore, the model allows us to estimate the acceleration of spread for a point-ignition fire in its initial stage and in the steady-state phase. Résumé Une analyse menée sur des expériences de laboratoires nous a permis de proposer un modèle de réaction-diffussion pour la propagation du feu sans vent et sans pente au travers d’une litière. Une méthode d’identification des coefficients du modèle, à partir des caractéristiques dynamiques de la propagation du feu, est donnée. L’étude numérique du problème mathématique nous permet de prédire la vitesse de propagation, le périmètre du front de feu et la température dans le domaine d’étude pour des allumages en ligne et pour des allumages ponctuels. Ces résultats sont comparés avec succés à des données expérimentales. De plus, nous sommes aussi en mesure de décrire l’accélération du front de feu dans les premiers instants suivant un allumage ponctuel.

The use of the terms "growth" and "acceleration" appears to be inconsistent in the literature and we believe this inconsistency has hindered our understanding of behaviour in the early stages of a fire. The development of a fire from a point ignition to some equilibrium state and the associated increase in fire size and intensity has been referred to variously as the fire growth (Pyne 1984); build-up (Luke and McArthur 1978); or acceleration (Chandler et al. 1983) phase of the fire. More specifically the "acceleration phase" has been used to describe the increase in rate of spread from ignition to a quasi-steady rate of spread (Luke and McArthur 1978, McAlpine and Wakimoto 1991). Characteristic curves showing the increase in rate of spread are illustrated for different fuel and weather conditions (Luke and McArthur 1978). Hypothetical models to describe these curves have been proposed by Cheney and Bary (1969), Van Wagner (unpublished) and McAlpine and Wakimoto (1991). They have been called acceleration curves and acceleration models. The terms growth and acceleration, however, represent different concepts that are not interchangeable. We would like to clarify these concepts and discuss the practical implications for fire managers.

Laboratory fire experiments were conducted in both Pinus pinaster and Pinus halepensis litters in order to investigate the effect of slope on fire behaviour for different levels of fuel load. Simulated slopes ranged between -30 degrees and +30 degrees. The results are reported in terms of rate of spread and rate of mass loss when observed fire was quasi-steady. Upslope fires were observed, on the present devices, to be unsteady, and their flame to be three-dimensionnal, when slope and fuel load exceeded certain limits. The heat transfers involved in the explanation of the observed behaviours are discussed, especially on the base of the quite different results obtained in the two tested fuel. beds.

It has been theorized that the amount of fuel involved in a fire front can influence the rate of spread of the fire. Three data sets are examined in an attempt to prove this relationship. The first, a Canadian Forest Service database of over 400 experimental, wild, and prescribed fires showed a weak relationship in some fuel complexes. The second, a series of field experimental fires conducted to isolate the relationship, showed a small effect. The final data set, from a series of 47 small plots (3m x 3m) burned with a variety of fuel loadings, also show a weak relationship. While a relationship was shown to exist, it is debatable whether it should be included in a fire behavior prediction system. Inherent errors associated with predicting fuel consumption can be compounded, causing additional, more critical, errors with the derived fire spread rate.

The results of a number of laboratory tests of wind-driven fires indicate the existence of a characteristic wind speed, U'. The form of the fire spread (V) as a function of mid-flame wind speed (U) differs above and below this characteristic speed. The scatter in field data is so great that it is difficult to confirm this result for field data. However, expressions of the form: V/V0 -1 = α(U/U')0.5 U/U' < 1 and V/V0 -1 = α(U/U')3 U/U' > 1 with U' = 2.5 m s-1 perform in a similar manner to existing models. For many fuel types α = 15. A difficulty with existing fire spread models is the measurement and definition of V0, the no-wind rate of spread. It can hardly ever be measured in the field and has to be inferred from analytical formulae, or by extrapolating measured data. The value of a depends on the method used estimate V0.

A series of laboratory fire experiments were conducted in fuel beds consisting of either excelsior (wood wool), or 6.35 mm sticks, or a mixture of these. Tests were done both with and without wind. Various characteristics of the fire, including rate of spread and fireline intensity, were compared with predictions from the Rothermel model. The behavior of the mixed fuel fires, compared to that of the fires in the constituent fuels, did not agree well with the predictions of the model. We conclude that, in order to model the behavior of fire in mixed fuels, the Rothermel model needs modification.

A technique for recording time-temperature curves within field-scale range fires was accomplished using a commercially available data logger capable of rapidly reading large numbers of thermocouples. A specially designed fireproof box was utilized to house and protect the data logger within the center of the burned area. Programming features allowed temperatures to be measured and recorded rapidly (each second) during the passage of the fire front and recorded as 1-minute means before and after the combustion interval. Strategic placement of thermocouples provided time-temperature profiles for various heights above ground, rate of spread, and duration of heat above specific temperatures. Additionally, measurement of preheating prior to the actual flame passage was obtained by placement of the recorder and thermocouples well within the burned area. This technique may provide better quantification of fire effects on vegetation, especially woody weeds targeted for control with fire, by documenting temperature extremes and their duration at critical growing points on plants.

400 AIDS (acquired immunodeficiency syndrome) vaccine researchers and policy-makers met 15-19 October 1991 in Marco Island Florida to discuss plans for clinical trials of AIDS vaccines. 3 major health research institutions plan to begin testing vaccines: World Health Organization in Rwanda Uganda Brazil and Thailand; the U.S. Armys medical research division in Thailand and the Division of AIDS (DAIDS) of the U.S. National Institute of Allergy and Infectious Disease (NIAID in Zaire. 9 vaccines have already been tried in Phase I clinical trials for safety and an immune response. Conferees realize that plans for trials should be accelerated because HIV is spreading rapidly but also vaccine experiments are proving more promising than anticipated. A whole killed simian immunodeficiency virus vaccine and several genetically engineered HIV fragments have given hopeful results in primate experiments. Planners also realize that it will take until 1995 at least before any results of actual clinical trials will be available. The existing rate of spread of seropositivity in the test areas must be documented vaccines selected based on the endemic HIV strains in the area pilot trials must be mounted then actual trials completed. The consensus is that existing vaccines should be tested now while research on more perfect vaccines progresses.

PLANS to conduct a random population survey that would allow the Centers for Disease Control (Atlanta, Ga) to better grasp the rate of spread of the human immunodeficiency virus (HIV) in the United States have been abandoned after 5 years. The agency's decision came just 2 weeks before it announced that 100 777 persons in this country had died of the acquired immunodeficiency syndrome (AIDS) between June 1981 and the end of 1990. Nearly one third of these deaths took place in the last year, and the federal agency said that in the next 3 years as many as 215 000 more will die. Beset from the beginning by the problem of nonresponders, the proposed National Household Seroprevalence Study was designed to provide a more precise estimate of the number of Americans infected with HIV than the oft-cited 1 million. That figure, derived from mathematical models, has remained unchanged even

A grassland wildfire burnt 120,000 ha in the western Riverina of New South Wales and caused exten sive losses of sheep and fencing. The rate of spread of the fire was accurately recorded at 6.4 m s-1 (23 km h-1) as it traversed one property and on the basis of conservative estimates of fuel loads measured later on unburnt patches, a fireline intensity was calculated at around 20,000 kW m-1. This rate of spread is one of the highest recorded for grassland wildfires in southern Australia and represents an important datum point for wildfire behaviour models. Quite reasonable predictions of rate of spread were pro vided by the McArthur Mark 4 fire danger model, how ever, the Mark 5 model seriously underestimated rate of spread for this and two other grassland wildfires.

Modelling of the wind effect on the rate of spread of a flame in a forest fire usually employs a wind velocity measured at mid-flame height. An alternative formulation is proposed in this paper, based on the wall shear-stress produced by the wind on the fuelbed in the absence of combustion. Some tests performed on a labo ratory scale are described and the results obtained to gether with those from other sources confirm the possi bility of using wall shear-stress to correlate the rate of spread of a flame front.

A one-dimensional model of fire spread through a nonhomogeneous fuel is developed. The fuel is modelled as a spatial Markov chain. Spatial dependence of fuel type, and lag times when crossing fuel boundaries, are both allowed. Formulae for the average rate of spread and for the variance in rate of spread are derived. Numerical examples of fire spread in sagebrush and grass fuels are developed.

Mosaic disease of cassava is one of the most important factors limiting production in East Africa. The disease can be controlled by the use of healthy planting material. Plots of carefully selected, apparently healthy cassava of several cultivars of differing genetic constitution were established and successfully kept free of disease by inspection and roguing. The rate of spread of mosaic disease into mosaic‐free plots over a five‐year period was consistently very low (less than 2% and most frequently less than 1%) irrespective of cultivar type or size of plot (0.02–1.00 ha), which clearly indicates that mosaic disease can be effectively controlled in Kenya by the use of mosaic‐free propagation material. Comparative yield data indicated that yields of local cassavas are at least equal to those of improved resistant cuitivars and suggest that agronomic evaluation of Kenya cuitivars would be of great value.

The downward rate of spread of laminar flames over sheets of polymethylmethacrylate (PMMA) in high oxygen concentration environments under free convective flow conditions is studied experimentally. Experimental results are compared with exisiting theoretical predictions. The range of applicability of the theory, as well as its limitations, is discussed.

In a series of field trials at Chitedze, Malawi, groundnuts were sown at spacings of 30 cm, 7·5 cm and 2·5 cm within rows 90 cm apart. Contrasting sowing dates were approximately 1 December and 1 January. Immigration of alatae of Aphis craccivora Koch, monitored on observation plots, commenced 5–6 weeks after the emergence of early-sown groundnuts. Numbers of immigrant alatae were significantly greater on late-sown or open-spaced than on early-sown or close-spaced groundnuts, and were correlated with numbers of randomly distributed primary rosette virus infections. During observations, more alatae landed on 56–60-day-old plants than on 85–112-day-old plants. Similar numbers landed on 11–26-day-old plants at 30-cm and 2·5-cm spacing, but significantly more aphids settled on 30-cm spacing. The incubation period of rosette virus was consistently longer in 2·5-cm than in 30-cm spacing, and increased with plant age at inoculation. The virus was transmitted from a low proportion of infected plants before the appearance of rosette symptoms, and from a high proportion of plants snowing symptoms. The rate of spread of virus transmissions from inoculated sources within the crop varied significantly with spacing as 30 cm > 7·5 cm > 2·5 cm. Secondary spread of rosette varied, approximately, with the numbers of aphids per infected plant.

Correlated random walk models provide a framework for making quantitative predictions about an organism's rate of spread. In this paper we present an algorithm that calculates the variance in squared displacement for a correlated random walk model. This can be used to make statistical comparisons of observed displacements to the model and to approximate the expected net displacement of the model. We illustrate the techniques with examples of insect movement and the vegetative spread of clonal plants. See full-text article at JSTOR

The semitransparency of a material, which determines the penetration of external radiation into the inside layers, affects significantly the temperature profile in the body [1, 2]. Since the viscosity of a melt of viscous materials depends strongly on the temperature, deformation of the temperature profile close to the surface leads to considerable change of the rate of spread of the liquid film, which has a significant effect on the rate of distortion of the body. In the present paper, the problem of distortion is formulated taking into account the transfer of radiation inside the body. The dependence of the distortion parameters and the degree of blackness of the body on the fraction of radiation in the external thermal flux and the mean free path of the radiation inside the material is determined. A sufficient condition is also obtained for the presence of a temperature maximum inside the body in a more general case than in [3].

Large herds of cattle are ranged on the marsh lands of southwestern Louisiana. These cattle are heavily infected with brucellosis. Cattlemen work their herds twice a year as they come from or go to the marsh ranges. This form of marsh range cattle management is not compatible with the frequent retesting schedule utilized in clearing conventional brucella-infected herds. Producers are further hampered by their inability to purchase replacement cattle to replace the reactor cattle removed from their herds. In response to these concerns, a 5 year field study was initiated and is currently in progress to determine and compare the spread of brucellosis in strain 19 adult-vaccinated herds in which either reactor cows were removed from or allowed to remain within the herds. The study will also attempt to compare the cost effectiveness of these two programs as they relate to both the producers and to regulatory agencies. The rate of spread of brucellosis in the conventional whole herd vaccination program over a period of 17 to 29 months was 4.8% as compared to a 3.6% rate of spread in 24 months in the vaccinated herds with retained reactors. While these differences are slight, the modified program demonstrated that testing twice a year tests was as successful in limiting disease spread as 60 or 90 day tests in the conventional program. It also demonstrates that there is little additional risk in keeping reproductively active reactor cows within the herds. Reactor cow retention thus eliminated the producers need to purchase replacement animals to maintain acceptable stocking rates.

A study has been made of the soil chemistry and atmospheric quality at 9 Sudbury area sites where populations ofDeschampsia cespitosa have invaded the contaminated industrial barrens near the smelters since 1972. The rate of increase in cover within marked quadrats was greater at sites near the discused Coniston smelter than at sites near the two active smelters. Air quality monitoring, using bulk collectors and sulphation plates, indicated substantial drops in phytotoxic air pollutants during the 1978–79 smelter shutdowns. Surface loadings of various metals were evident in soil at sites near the active smelters, whereas this was no longer true at sites near the Coniston smelter disused since 1972. However, water extractable levels of Ni and Cu at these sites remained as high as those found near the active smelters which also have high extractable soil Al. Both comparative studies and multiple regression analysis indicate that the rate of increase in cover ofD. cespitosa in the populations studied correlate best with particulate deposition of Cu and Ni followed by sulphation rate and then by soil extractable Al. These findings are discussed in relation to tolerances of local populations, as well as to the potential effects of airborne particulates on growth.

CELO virus spread rapidly to uninoculated fowls in direct contact with inoculated fowls; keeping them on litter or on wire did not affect the rate of spread of infection. However, the virus only spread under certain circumstances to fowls kept in separate pens in the same room as those inoculated. The results suggest that CELO virus is not highly infectious but that airborne spread is an important method of dissemination.

A series of experimental fires, each 0.4 ha in size, was conducted between 1975 and 1981 in an unthinned stand of immature jack pine (1948 origin) in central Ontario to gather quantitative fire behavior data for forest fire management purposes. Twelve fires were conducted over a broad range of burning conditions. Fire behavior and impact characteristics (i.e., rate of spread, fuel consumption, and frontal fire intensity) were found to be strongly correlated with fire weather severity as expressed through various component codes and indices of the Canadian Forest Fire Weather Index (FWI) System. This type of experimental fire information, along with wildfire data, is being used in the development of guidelines for quantitative prediction of fire behavior in major Canadian forest fuel types.

Summary The infestations of two biotypes of Panicum miliaceum L. were monitored over a 5-year period (1981–1985) as they occurred in two townships in southwestern Ontario, Canada. On average, 16·5% of the cultivated fields in Hullett township were infested with the black-seeded biotype while 12·9% of the fields in North Norwich township were infested with the golden-seeded biotype. Neither population showed clear signs of increasing. The two biotypes were similar in their ability to persist as adult infestations. The black-seeded biotype showed a greater potential for colonizing uninfested fields. The crops grown in this region showed different susceptibilities to infestation by P. miliaceum. Corn (maize) fields were susceptible while most grain fields were resistant to infestation. White beans were susceptible to infestation by the black-seeded biotype. Wheat and oat crops were more resistant to infestation by the golden-seeded biotype than to those of the black-seeded biotype. Resume La propagation du panic faux millet (Panicum miliaceum L.) dans l'Ontario, Canada. I. Niveau de propagation et sensibilite culturale. Les infestations de deux biotypes de Panicum miliaceum L ont fait l'objet d'un suivi pendant 5 ans (1981–1985) dans deux districts du Sud-Ouest de l'Ontario, Canada. En moyenne 16,5% des parcelles cultivees dans le district de Hulett etaient infestees avec le biotype classe noir tandis que 12,9% des parcelles dans le district de Norwich-Nord etaient infestees avec le biotype classe dore. Aucune des populations n'a clairement exprime des signes d'augmentation. Les deux biotypes avaient une aptitude comparable a demeurer infestant a l'etat adulte. Le biotype noir a demontre un plus grand potentiel de colonisation des champs non infestes. Les cultures cultivees dans cette zone ont exprime differentes sensibilites a une infestation par P. miliaceum. Les parcelles de mais etaient sensibles tandis que les parcelles de cultures a petits grains etaient resistantes a l'infestation. Les haricots blancs se sont montres sensibles a l'infestation par le biotype noir. Le ble et l'avoine ont ete plus resistants a l'infestation par le biotype dore qu'a celle par le biotype noir. Zusammenfassung Ausbreitung der Echten Hirse (Panicum miliaceum L.) in Ontario, Kanada. I. Ausbreitungsrate und Bedeutung in Kulturen Der Besatz der Felder in zwei Gemeinden in Sudwest-Ontario mit zwei Biotypen von Panicum miliaceum L. wurde funf Jahre lang (1981–1985) erfasst. Durchschnittlich 16,5% der Acker der Gemarkung Hullett hatten den schwarzsamigen Biotyp, Wahrend auf 12,9% der Acker in North Norwich der gelbsamige auftrat. Keine der Populationen zeigte deutliche Ausbreilung; sie waren in gleicher Weise bestandig, aber der schwarzsamige Biotyp tendierte mehr zur Ausbreitung. Die Konkurrenz von Panicum miliaceum gegenuber den Kulturen war unterschiedlich. Mais war empfindlich, wahrend Getreide ihr meistens widerstand. Bohnen wurden vom schwarzsamigen Biotype beeintrachtigt. Weizen und Hafer waren gegenuber dem schwarzsamigen Biotyp schwacher als gegen den gelbsamigen.

Fire behaviour characteristics were investigated in the montane grasslands of the Natal Drakensberg. The behaviour of experimental fires was measured together with the accompanying biotic and abiotic conditions. Estimates of heat yield, mass of available fuel and forward rate of spread enabled the calculation of intensity of fires in shrublands, woodland and grassland. The results show marked seasonal differences in fire behaviour for given vegetation types, and give an indication of the conditions under which fires can be safely burned.Keywords; Drakensberg; fire; fire behaviour; fire intensity; Fire types; flame height; Fuel availability; grassland; grasslands; KwaZulu/Natal; montane grassland; Montane grasslands; natal; natal drakensberg; rate of spread; south africa; type of fire; vegetation typesJournal of the Grassland Society of Southern Africa.

SUMMARYThe co-ordinates of the dwellings where cases of variola minor (alastrim) occurred during a small epidemic were used in a worked example of contour mapping of disease spread. The contoured variable was the date of onset, relative to an arbitrary base date, of the case introducing the disease into each of twenty-two households. Three contour maps prepared with slightly different computer programmes or dates exhibited similar concentric loops whose centres were close to the first infected household. The average rate of spread of the disease was estimated by regression of the number of days to onset of the first case in the household on the average distance from an arbitrary origin to the relevant contour line. The calculated average rate of spread was 1.22 metres per day. An additional map was contoured using the cumulative number of cases as the contoured variable, relative to the onset of the example epidemic.

Summary A model with nonrandom latent and infectious periods is suggested for epidemics in a large community. This permits a relatively complete statistical analysis of data from the spread of a single epidemic. An attractive feature of such models is the possibility of exploring how the rate of spread of the disease depends on the number of susceptibles and infectives. An application to smallpox data is included.

Human selective adaptation to malaria has been not to a single disease, but to infection by different parasites of different evolutionary antiquity but having closely similar life cycles. The diseases produced vary in severity, as do the effects of the mutations conserved in response to their challenge. The milder parasites have tended to promote the conservation of haematologically relatively innocuous mutations, mostly affecting the cell surface, while P. falciparum is responsible for the prevalences of mutations affecting the erythrocyte interior, such as the major haemoglobinopathies, including the major, but possible not the minor, thalassaemias, as well as some of the more deleterious G6PD variants. Population biological responses have depended both on the rate of spread of the disease and the opportunities for gene flow, with a variety of mutations conserved in regions where the former has tended to happen much faster than the latter. The most recent and most efficient mutations are probably the S-s-U-variant and HPFH, which have not yet had time to spread sufficiently to be effective on a large scale.

SUMMARYEvidence for influenza-associated excess mortality in the three centuries before the 20th has been sought from parish burial registers in Cumbria, Devon, Dyfed, East Anglia, Gloucestershire and Northumbria, compared with inter-epidemic years. Most of the registers showed excess of burials concordant with eight historic influenza epidemics.Comparison of the dates of these epidemics, deduced from the burials data in different areas, showed a rate of spread difficult to reconcile with direct personto-person spread of influenza from the sick. An alternative explanation based on development of latency of the virus in the sick person and subsequent seasonal reactivation is discussed.

We have modelled the spread of vesicular-arbuscular mycorrhizas (VAM) from localized inoculum in a developing root system by assuming that the rate of spread of infection at a particular time is proportional to the product of the length of root already infected and the fraction of the total root which is susceptible to infection at that time. An important parameter of the model is its asymptote n, the maximum fraction of the root system which becomes mycorrhizal. Our model is formally similar to the logistic equation for growth. An earlier model by Tinker (1975), which predicts that rate of spread of infection would be enhanced by increased density of roots L, (cm cm-3), was compared with ours, using results of experiments on wheat and clover sown at three densities. The new model fitted the data for spread of infection much better, especially that of young infections. The relative rates were generally little altered by total length of roots, Lt, but the absolute values for the two hosts differed widely. This suggests that infection spreads primarily along roots, rather than by random contact between roots and hyphae. This type of model may be useful for exploring the mechanisms by which environmental changes affect spread of vesicular-arbuscular mycorrhizas.

Problems associated with current field procedures for measuring the rate of spread of wildland fires are noted. The procedure was used to document an experimental prescribed fire that exhibited a large variability in rate of spread.

Three commercial Indian wheat varieties —‘Sonalika’, ‘Kalyansona’ and ‘Janak’ were used to develop homogeneous and heterogeneous lines for leaf rust resistance, utilizing diverse sources of resistance from fourteen donors. Four kinds of multilines were developed to study coefficients o: infection, areas under disease progress curves (AUDPC), rates of spread and grain yields over their component means. Heterogeneous populations showed minimum coefficients of infection at initial as well as final stages, which were less than or equal to the resistant component even in dirty multilines carrying up to 50 % susceptible populations. Large reductions in coefficients of infection and in AUDPC were also observed. Rate of spread was independent of coefficient of infection and AUDPC. Grain yield advantage over both component means and recurrent parents was higher in clean multilines. The significance of the use of multilines in limiting losses due to disease is discussed.

We examine the necessary conditions for the spread of genes that determine ‘selfish’ and ‘cheating’ behaviors and the rate of spread of these genes through structured populations, in order to address the question of the invadability of altruistic systems by ‘anti-social’ mutations. We find that, although ‘cheaters’ always have a higher relative fitness than altruists within groups, population structures which permit the evolution of altruism also preclude invasion by anti-social mutations. These results are related to a discussion by Hamilton (1971) concerning the limits to the evolution of altruistic and selfish behaviors.

Forest fire danger rating research in Canada was initiated by the federal government in 1925. Five different fire danger rating systems have been developed since that time, each with increasing universal applicability across Canada. The approach has been to build on previous danger rating systems in an evolutionary fashion and to use field experiments and empirical analysis extensively. The current system, the Canadian Forest Fire Danger Rating System (CFFDRS), has been under development by Forestry Canada since 1968. The first major subsystem of the CFFDRS, the Canadian Forest Fire Weather Index (FWI) System, provides numerical ratings of relative fire potential based solely on weather observations, and has been in use throughout Canada since 1970. The second major subsystem, the Canadian Forest Fire Behavior Prediction (FBP) System, accounts for variability in fire behavior among fuel types (predicting rate of spread, fuel consumption, and frontal fire intensity), was issued in interim form in 1984 with final production scheduled for 1990. A third major CFFDRS subsystem, the Canadian Forest Fire Occurrence Prediction (FOP) System, is currently being formulated. This paper briefly outlines the history and philosophy of fire danger rating research in Canada discussing in detail the structure of the current CFFDRS and its application and use by fire management agencies throughout Canada. Key words: fire danger, fire behavior, fire occurrence prediction, fuel moisture, fire danger rating system, fire management.