264 WRITINGS OF JOSEPH HENRY. [1855- 



would be but 0*043 of an inch ; hence an equiHbrium could 

 not exist, and there would be a continued series of currents 

 from the equator to the poles, a perpetual condensation of 

 vapor into water at the latter, and a constant evaporation 

 of liquid into vapor at the former, for the supply of which 

 a series of ocean currents would be established. A tendency 

 to the same effect must exist in the compound atmosphere 

 of air and vapor which actually surrounds our earth, but 

 the resistance to the permeation of the vapor is so great that 

 a considerable inequality of the elastic force of vapor con- 

 tinually exists in different parts of the earth. 



Though there is a constant tendency to a diffusion of 

 vapor from the equator to the poles, yet the greatest disturb- 

 ance of the equilibrium of our atmosphere results from the 

 diminution of temperature as we ascend in the atmosphere^ 

 and for the establishment of the principle on which this dis- 

 turbance depends, and the consequences which flow from it,. 

 we are indebted to the laborious, persevering, and sagacious 

 investigations of Mr. James P. Espy. 



From observation it is well known that the air diminishes 

 in temperature as we ascend, at the rate of about one degree 

 Fahrenheit for each 100 yards or 300 feet. If therefore a 

 portion of air be transferred from the surface of the earth to 

 a height in the atmosphere, it will be cooled to the temper- 

 ature of the stratum of air at which it arrives ; but it is 

 proper to observe at the beginning of the explanation that 

 this cooling will not be due principally to the coldness of 

 the space to which the mass of air has been elevated, but 

 chiefly to its own expansion. If the air for example ex- 

 pands into double the space by being subjected to half the 

 pressure, it is evident that the amount of heat which it con- 

 tains will be diffused through twice the amount of space; 

 and hence though the absolute quantity of heat remains the 

 same, its intensity of action, or its temperature, will dimin- 

 ish and the substance will become much colder. This is a 

 principle to which we have before alluded, and which will 

 be frequently applied hereafter in the explanation of phe- 

 nomena. 



