272 WRITINGS OF JOSEPH HENRY. [1855- 



Washington were to precipitate at once all its moisture, 

 the quantity of water would be indicated by about 3 inches 

 of the gauge. To supply therefore 30 or 40 inches of rain 

 in the course of a year it is necessary that the vapor con- 

 tained in the atmosphere should be very frequently renewed, 

 and that consequently localities which cannot be reached 

 by moist winds must be abnormally dry. 



Effects of vapor on the general currents of the atmosphere. — 

 From what has been previously stated it is evident that the 

 atmosphere which surrounds the globe being composed of 

 two portions, one of permanent elastic gases, and the other 

 of a readily condensable vapor containing a large amount of 

 latent heat, it must frequently be in a state of tottering equi- 

 librium, liable to be overturned by the slightest extraneous 

 forces, and in assuming a more permanent condition to 

 give rise to violent commotions, and currents of destructive 

 energy. 



It has previously been shown that the equilibrium of a dry 

 atmosphere depends upon the fact that each pound from the 

 top to the bottom of an aerial column contains approxi- 

 mately the same amount of heat. If therefore a portion of 

 air be caused to ascend (by mechanical or other means) to a 

 greater elevation, it will expand, and its heat being distrib- 

 uted through a larger space, its temperature will fall to that 

 of the new region to which it has been elevated, and be 

 again in equilibrium. If on the other hand a portion of 

 air be caused to descend, it will be condensed into a smaller 

 space on account of the increased pressure, and its tem- 

 perature will be raised to that of the stratum at which 

 it has arrived. But this is not the case with moist air; for if 

 by any means it be elevated above a given level, the coldness 

 produced by its expansion will, as we have said, condense 

 a portion of the vapor into water, and in this process the 

 vapor will give out its latent heat to the surrounding air, 

 and therefore the column in which this condensation has 

 taken place will not be as cold as the surrounding atmos- 

 phere; consequently an upward force will still exist, the col- 

 umn will rise to a greater height, and a new portion of vapor 



