THE PHYSICAL NATURE OF METEOR TRAINS.—An interesting discussion of the nature of meteor trains is published in No; 2, vol. xxvi. (p. 95, September), of the Astro-physical Journal by Prof. C. C. Trowbridge. Prof. Trow-bridge, believing that valuable information concerning the upper layers of the earth's atmosphere may be thereby deduced, has compiled a catalogue of observed meteor trains, and for several years has made a comparative study of the data, at the same time making, a study of the phenomena of gas phosphorescence. The discussion of altitudes leads to the conclusion that there is a definite layer of the earth's atmosphere, probably some fifty to sixty miles high, where the conditions are favourable to the production of the peculiar glow constituting a meteor train. Prof. Trowbridge believes that the secondary appearance of duality, so frequently observed in meteor trains, is due to the probable tubular form of the trains. The train itself is probably a tube of gas and particles of meteor dust, rendered phosphorescent by some temperature or electrical effect produced by the meteor's passage. The rate of diffusion and the colour of meteor trains agree with similar phenomena observed in phosphorescent air in the laboratory.