48 WRITINGS OF JOSEPH HENRY. [1855- 



From this table it appears that one-half of the whole 

 atmosphere is found within the upward limit of 3| miles, 

 and one-third of the whole quantity beneath the average 

 height of the Rocky Mountains: this fact has an important 

 bearing on the influence of mountain ranges in modifying 

 the direction of the winds. 



The question occurs at this place, Why does the air grow 

 colder as we ascend? The answer is that a pound of air, 

 at all distances above the earth contains at least an equal 

 amount of heat with the same weight taken at the surface, 

 and that as the pressure is removed this air is expanded in 

 bulk; consequently the heat is diffused through a greater 

 amount of space, and hence the reduction of its intensity or 

 temperature. To illustrate this, take a large ball of sponge 

 and squeeze it into one quarter of the space which it natur- 

 ally occupies; in this condition dip it into water, it will 

 imbibe a certain quantity of the liquid, and when drawn out 

 will be dripping wet; now let it expand to its natural dimen- 

 sions, the water will be distributed through a large amount 

 of space, and the sponge itself will appear comparatively dry. 

 Squeeze it again into its former condensed state, and it will 

 appear wet; suffer it again to expand, and the apparent 

 dryness will be resumed. In a like manner we suppose that 

 while the quantity of heat is the same, its intensity is in- 

 creased by condensation into a smaller space and diminished 

 by the converse process. In the foregoing illustration the 

 amount of water contained in the sponge represents the 

 amount of heat in the air, and the degree of wetness pro- 

 duced by condensation the intensity of the temperature 

 exhibited in diminishing the bulk of air. 



It follows from this that the blowing of a current of air 

 over a high mountain, provided it descends again into the 

 plain, does not necessarily diminish its temperature. When 

 it arrives at the top of the mountain, it will become as cold 

 as the circumambient air, not because it has lost any of its 

 heat, but because that which it contained is now distributed 

 through a greater space; when it descends again to the plain, 

 it will suffer a corresponding diminution of bulk, on account 



