336 ON PHYSICAL DATA 



by means of three sources ; the first is, that heat, 

 by being unequally diffused over the earth, will 

 drive the particles of air from the place where it 

 predominates to other places where the heat is 

 not so intense ; by this means the barometrical 

 pressure is disturbed. The second is, that the 

 atmosphere becomes loaded with vapour, which 

 rises from the surface of the earth ; this eva- 

 poration takes place unequally on the earth's 

 surface, and the vapour is carried over it by 

 means of currents of wind ; the atmosphere 

 becomes disburthened from this weight of 

 vapour, by means of its conversion into 

 water, which falls to the ground. The next 

 source is, that the lower part of the atmosphere 

 may be overloaded by having an undue share of 

 oxygen, arising from the escape of that gas, from 

 the various products of vegetation, more freely at 

 one time than another. This will agree with the 

 idea expressed by Dr. Dalton in his Meteorolo- 

 gical Essays, p. 98, where he states, in reference 

 to the variation of the barometer, " that the whole 

 or greater part of the variation is occasioned by a 

 change in the density of the lower regions of the 

 air." This great man, in his exposition of the 

 different theories which have been proposed by 

 various philosophers, to explain the fluctuations 



