184 



gland, and the mountains of the south of India, while the observa 

 tions are themselves so thoroughly consistent, that the conclusion is 

 inevitable that the hypothesis is untenable. 



A similar conclusion as to the entire incompatibility of the hypo- 

 thesis of a separate vapour atmosphere with the facts, may be drawn 

 quite independently of any observation of tensions, from a mere con- 

 sideration of the known laws of the diminution of temperature as we 

 ascend. An argument, something to this effect, will be found in 

 BessePs paper on Barometric Heights * ; but its form being too ma- 

 thematical to be generally intelligible, I shall endeavour to place the 

 matter in a rather more popular point of view. 



Let us suppose, then, that we are at a place at the sea-level where 

 the temperature of the air is 80, the tension of vapour being '80, 

 which would make the dew-point 72* 5 a case that must be of con- 

 stant occurrence. If, now, we rose gradually above the earth's sur- 

 face, the temperature of the air would be reduced at the known rate 

 of about 3 for 1000 feet ; while the tensions of vapour, and the cor- 

 responding dew-points, calculated upon the hypothesis of an atmo- 

 sphere of vapour pressing upon itself, would be as follows : 



ft. 

 At , . Tension -80 . . Dew-point 72-5 . . Air 80'0 



1000 .. -78 .. 71-8 .. ,, 77-0 

 2000 .. -77 .. 71-2 .. 74-0 

 3000 .. '75 .. 70-5 .. ,, 71'0 

 4000 .. '74.. 69-9 .. 68'0 

 Hence, up to about 3000 feet, the temperature of the air would 

 be found to be higher than the dew-point, and the supposed tensions 

 might of course exist. But the temperature of the air, it will be 

 seen, diminishes much more rapidly as we ascend than that of the 

 dew-point ; and the former will therefore soon fall below the latter. 

 Thus at 4000 feet, the air being at 68, the theory demands vapour, 

 with a dew-point of 69'9, which is impossible; for any vapour, in 

 excess of that corresponding to the air temperature 68, would be 

 instantly precipitated. In like manner it might be shown that, under 

 all conceivable conditions of heat or cold, and of damp or dryness at 

 the surface of the earth, we could always ascend to a height where 



* Astronomische Nachrichten, Nos. 356,357 ; and Taylor's Scientific Memoirs, 

 vpl. ii. p. 517. 



