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LET the operation M, applied to a graph G, have the effect of removing from G all lines which are not members of at least one K3(a complete graph or sub-graph having three points, that is, a triangle). The new graph MG consists entirely of triangles, and it will be unchanged by a second application of the operator M. M2G = MG.

PREVIOUS estimates of the height of the ozone in the atmosphere, including our own measurements at Arosa1, agreed in giving the centre of gravity of the ozone layer at about 40–50 km. above sea-level. These estimates were based on measurements of the intensity of the spectrum of direct sunlight as the sun was rising or setting, and—as we were careful to point out—it was necessary in deducing the height to assume that there was no regular diurnal variation in the amount of ozone through the day.