Marsh.] J-— ■^ ' [March 6, 



using the above value of x. But, the kiminosity of meteors is usually 

 attributed to atmospheric resistance. Kirkwoocl, in his Meteoric Astro- 

 nomy (Philadelphia, 1867, p. 81), says: "Several hundred detonating 

 meteors have been observed, and their average height at the instant of 

 their first appearance has been found to exceed 90 miles. The great 

 meteor of February 3d, 1856, seen at Brussels, Geneva, Paris, and else- 

 where, was 150 miles high when first seen, and a few apparently well 

 authenticated instances are known of a still greater elevation. We con- 

 clude, therefore, from the evidence afforded by meteoric phenomena, 

 that the height of the atmosphere is certainly not less than 200 miles. 



It might be supposed, however, that the resistance of the air at such 

 altitudes would not develop a sufficient amount of heat to give meteorites 

 their brilliant appearance. This question has been discussed by Joule, 

 Thomson, Haidinger, and Reichenbach, and may now be regarded as 

 definitively settled. When the velocity of a meteorite is known, the 

 quantity of heat produced by its motion through air of a given density 

 is readily determined. The temperature acqviired is the equivalent of the 

 force with which the atmospheric molecules are met by the moving body. 

 This is about one degree F. for a velocity of 100 feet per second, and it 

 varies directly as the square of the velocity. A velocity, therefore, of 

 30 miles in a second would produce a temperature of 2,500,000°. The 

 weight of 5,280 cubic feet of air at the earth's surface is about 2,830,000 

 grains. This, consequently, is the weight of a column one mile in length, 

 and whose base or cross section is one square foot. The weight of a 

 eolumn of the same dimensions at a height of 140 miles would be about 

 a G-as kept at a constant temperature," says, '■ This relation is not a relation of simple 

 mechanical equivalence, as was supposed by Mayer. * * * The first attempt to determine 

 the relation in quesiion for the case of air established an approximate equivalence with- 

 out deciding how close it might be, or the direction of the discrepancy, if any. Thus ex- 

 periments "On the Changes of temperature produced by the Rarefaction and Condensa- 

 tion of Air," (Philosophical Magazine, May, 1845, showed an approximate agreement 

 between the heat evolved by compressing air into a strong copper vessel under water, 

 and the heat generated by an equal expenditure of work in stirring a liquid ; and again 

 conversely an approximate compensation of the cold of expansion when air in expanding 

 spent all its work in stirring its own mass, by rushing through the narrow passage of a 

 slightly opened stop-cock." 



Whilst this language fully confirms my interpretation of Joule's experiment, the 

 inference drawn by the authors from their subsequent experiments upon air forced 

 through a " porous plug," composed of compressed cotton-wool or silk, is incompatible 

 with the theory which I have advanced. They showed that when air was forced through 

 such a plug, its temperature was lowered ; and that the cooling effect was in proportion 

 to the difference in the pressure of the air, on the two sides of the plug. For reasons 

 previously stated by Prof. Thomson, (Transactions of the Royal Society of Edinburgh, 

 vol. XX., 1851) they assumed that this cooling effect represented the amount of heat 

 rendered latent by expansion ; and hence concluded that this, also, varied directly as 

 the difterence of pressure. 



It is, however, by no means self-evident, that the mechanical energy of the condensed 

 air would be exactly balanced — neither more nor less — by the work done in overcoming 

 the friction of the plug, and thus completely isolate the effects of latent heat of expan- 

 sion This being only a theoretical deduction, cught not to have the weight of a direct 

 result from experiment. Hence, perhaps, the omission of Tyndall and of Stewart to 

 allude to it. If standard writers thus fail to recognize it aa conclusive, we may fair'y 

 consider the subject as still open to discussion. 



