B. TERRESTRIAL PHYSICS AND METEOROLOGY. 105 



until the summer sun, heating the surface of the earth, stirs 

 up the atmosphere, very much as water boils in a vessel 

 over the fire. That which takes place in Asia is repeated 

 in a much less intense degree in America. The area of the 

 greatest cold on this continent is not prevented by any range 

 of mountains from extending southward and eastward, but 

 is only hemmed in on the west by the Kocky Mountains. 

 Thus while the Pacific coast is protected from an overflow 

 of very cold air, the whole eastern portion of America be- 

 comes peculiarly subject to it. 1 C, II., 9. 



dalton's law, and the constitution of toe atmosphere. 



It is well known that the famous law of Dalton, with 

 reference to the diffusion and independent existence of aque- 

 ous vapor in the atmosphere has exerted a very strong in- 

 fluence in moulding the views of meteorologists as to the 

 method by which this portion of the atmosphere operates 

 in modifying meteorological phenomena. Of late years, how- 

 ever, very strong suspicions have existed as to tlie propriety 

 of carrying Dalton's views to the extreme that has been 

 maintained by many. In fact, so early as 1840, Espy dis- 

 tinctly controverted them. Of late, Hildebrandsson, Hann, 

 and especially Stefiin,have more correctly explained the lim- 

 itations within which Dalton's Laws may be applied. Ac- 

 cordino" to the latter, these obtain in tlie case of a mixture 

 of Q;ases only under conditions of static equilibrium, and do 

 not hold when these are in relative motion. Since the vapor 

 of water in the air, in consequence of its continual evapora- 

 tion and condensation, is always in movement, it does not 

 conform to Dalton's laws. The permanent gases of the at- 

 mosphere, especially oxygen and nitrogen, which retain in 

 general the same proportions, are in equilibrium, and do 

 form atmospheres independent of each other, according to 

 Dalton's laws. It necessarily follows that the percentage of 

 dense oxygen must be smaller as we ascend in the air ; and, 

 in fact, chemical analysis gives this result. 19 C, VIII., 90. 



THE TEMPERATURE OF THE AIR. 



One of the most diflicult problems in all matters relating 

 to the atmosphere consists in the determination of the true 

 temperature of the air ; and the errors in these determina- 



E 2 



