290 EVAPORATION AND CONDENSATION. 



they amounted to ^ of an inch at 47, about -f- 

 of an inch, or -339 at 59, -507, or \ an inch 

 at 80, 1 inch and at 90, 1-360. He also as- 

 certained, that the elastic force of vapour at 

 212 is equal to the pressure of the whole at- 

 mosphere, or 30 inches of mercury.* 



From the above experiments it follows, that 

 if the temperature of the earth were 80, from 

 the equator to the poles, the quantity of vapour 

 would be everywhere the same, and equal to 

 about -5*5- of the average weight of the whole at- 

 mosphere : but that if its temperature were 

 reduced from 80 to 59, one-half of the vapour 

 would be precipitated in the form of rain if to 

 32, f of it would be converted into snow and 

 if reduced to 0, 14 out of 15 parts of the whole 

 would descend to the earth in the form of ice, 

 in obedience to that universal law of attraction 

 by which bodies cohere, and tend towards a 

 common centre. 



But as the temperature of the earth dimi- 



* Dr. Promt observes, that " atmospheric air, under ordinary 

 circumstances, exerts an elastic force equal to the weight of a 

 column of mercury 30 inches high; and that at 212 aqueous 

 vapour obeys precisely the same laws, and exerts the same elastic 

 force as atmospheric air under similar circumstances." (Bridge- 

 water Treatise, Chap. V. Section 2.) But it has been shown that 

 the elastic force of atmospheric air is equal to the pressure of 

 more than 1200 atmospheres, (3000 feet of mercury.) It is 

 therefore obvious that Dr. Prout has mistaken the weight of the 

 atmosphere for its elastic force. 



