THE SMITHSONIAN INSTITUTION. 185 



next the bottom, and least so at the top. The air, in the same way, is 

 more rare as we ascend to greater heights, the atoms being farther 

 apart, and their repulsive force diminished. In consequence of the 

 great capabilities of gases for expanding with diminished pressure, 

 the atmosphere, instead of being only about five miles in height, as it 

 would be if of equal density with the lowest stratum, (?) is really up- ,. 

 wards of fifty miles high. 



The atmosphere is not only in a most delicate state of balance in 

 respect to elasticity and pressure, but it is no less so in respect to the 

 amount of heat contained in its different strata. The expansive force 

 of gases, or the repulsion which exists among their atoms, seems, as 

 we have already stated, to be identical w'ith heat. When air, at the 

 temperature of freezing water, is condensed in the hollow globe of an 

 air-gun, an immense amount of heat becomes sensiljlc. Tinder, it is 

 well known, can be lighted with a single stroke of a condensing 

 syringe. In the rarefied gas a large amount of heat is stored away 

 and inappreciable by our instruments or senses, which is again given 

 out*by compression. 



The intimate connexion subsisting between heat and the expansion 

 of gases is most beautifully seen in the atmosphere. As already 

 stated, the atoms of air as w^e ascend are at greater distances from 

 each other. If the distance between any two atoms is diminished, 

 they give out heat or render it sensible ; whereas, if w^e increase the 

 distance between them, they store it away. The upper strata are 

 sensibly colder than the lower, not because the atoms have less heat, 

 but because the heat is diffused through a larger space when the atoms 

 are farther apart. One pound of air at the level of the sea, within the 

 tropics, may be said to contain no more heat than the same weight at 

 the top of the highest mountain perpetually covered with snow. It is 

 for this reason that the same w'infl wliich is w^arm in the valley 

 becomes colder as it ascends the sides of the mountain. The diminish- 

 ing pressure allows the air to expand and store away its heat. It is, 

 therefore, not the snow on the top of mountains which cools the air, 

 but it is the rarity of the air which keeps the snow itself from melting. 

 As a general law, the decrease of temperature amounts to 1° Fahren- 

 heit for every 300 feet in perpendicular height. 



A variation in the amount of heat affects the vtAume of a gas as 

 sensibly as a variation in the pressure. An addition of heat increases 

 thn repulsive force of the atoms, and thus expands the volume. All 

 gases, reckoning from the freezing point of w^ater, expand nearly the 

 480Lh part of their bulk for every degree of temperature ; or, in other 

 words, if one cubic foot of air had its temperature raised 480^ above 

 the freezing point, its elastic force would be doubled, or it would tend 

 to expand to twice its former bulk. 



If a number of atoms of air in the low^er stratum receives a greater 

 amount of heat than those in the vicinity, they will repel each other to 

 a greater distance apart than they were before they were heated, and 

 will have a tendency to ascend, on the same principle that a piece of 

 cork rises in water. 



On these undisputed data Dalton founded his two famous conditions 

 of atmospheric equilibrium, which are now regarded as the true basis 



