A TMOSPHER E. 



237 



>ressure or weight ; for if such were not the case, no 

 jart of the still atmosphere could be more dense than 

 mother, for the superior density of the lower strata 

 jaii be produced only by the pressure overcoming, to a 

 certain extent, the tendency to expand ; and farther, 

 if the pressure arising from the weight were not the 

 greater, the expansive force would soon dissipate the 

 whole atmosphere through the regions of space. 



But though the pressure or gravitation of the 

 atmosphere is as much greater than its tendency to 

 expand as to prevent any part of it from being thus 

 dissipated, yet the expansive force acts as the pressure 

 diminishes, which it does as the height above the 

 mean level increases, because the quantity of air over 

 head becomes less the higher that we ascend, and the 

 pressure or gravitation, being the measure of that 

 quantity, must be equal to it in all its variations. 

 Farther, as the density is the result of the pressure 

 or weight, it must always be in proportion to that, 

 or the rarity must be inversely as the pressure. The 

 densities at different heights will, therefore, express 

 the quantities of air in the superincumbent portions 

 of the atmosphere above those heights, but not the 

 heights upward to which they extend, or the total 

 range of the atmosphere. That range is not, how- 

 ever, difficult to determine ; for as the expansion 

 must be as the diminution of pressure, and the in- 

 creased action of the expansive force jointly ; and as 

 these are equal to each other, and each of them pro- 

 portional to the height, it follows that the expansions 

 or decreases of density will be as the squares of the 

 heights, or conversely, that the heights will be as the 

 square roots of the expansions. 



In consequence of this increasing expansion with 

 the height, the atmosphere occupies far more space, 

 with the same quantity of matter, than if it every 

 where maintained the density which it has at the 

 mean level of the sea : we have shown that with that 

 density it would have a height of only between five 

 and six miles. But with the decreasing density it 

 has, at the height of about 44 miles, body enough 

 so to act upon the light of the sun as to produce twi- 

 light. Beyond that, its range must be considerable ; 

 but in the extreme of its altitude, our knowledge of 

 it is obscure, or altogether wanting, and the twilight 

 may be taken as the most elevated natural proof we 

 have of its existence ; but, as that is an elevation 

 which commands a horizon of about 2500 miles in 

 diameter, or one at which a meteor over the centre 

 of Europe could be seen from every point of that 

 continent and its islands, we may suppose that it 

 includes all appearances that can, with any thing like 

 knowledge, come within the range of natural history. 

 The short view which has been taken of the 

 mechanical structure of the atmosphere, in its simple 

 state, and without reference to the action of any 

 external agent or cause upon it, is necessary to be 

 borne in mind, in order that we may have a proper 

 understanding of the part which it performs in the 

 operations of nature upon the grand scale. It must 

 be understood to be, in itself, wholly passive ; the 

 ready and efficient medium of action, but not the 

 cause. At all altitudes, the two forces which deter- 

 mine its density, the pressure of gravitation and the 

 reaction of expansion, so exactly balance each other, 

 that the smallest possible action of any other cause, 

 will turn the scale either way; the smallest con- 

 densing agency will produce condensation, or the 

 smallest expansive agency will produce expansion. 



Thus it is, to all the agents in nature, a medium 

 always at hand, and always ready for action. 



In estimating those two opposing actions by which 

 the atmosphere is balanced, we have taken no notice 

 of the centrifugal force arising from the rotation of 

 the atmosphere along with the earth. We omitted 

 that in order not to confuse the statement by the in- 

 troduction of too many principles. The centrifugal 

 force, as far as it goes, is an auxiliary of the exp ansive 

 force ; but in the case of the atmosphere,it diminishes 

 with the height ; for though it increases with the dis- 

 tance from the earth's axis, it is also in proportion to the 

 quantity of matter in a given space, and therefore it 

 diminishes in the same ratio as the density, so that 

 in the higher regions of the atmosphere it must be 

 verv small. 



But, upon the general principle that, under the 

 same or similar circumstances, the resistance which 

 the inertia of matter offers to any force acting upon 

 it, is always in proportion to the quantity of matter 

 contained in an equal space, it follows that the lower 

 strata of the atmosphere must offer more resistance 

 to the active energies of nature, than the strata which 

 are more elevated, and for that reason less dense ; 

 that, for instance, if we suppose the base of a moun- 

 tain to be on the mean level of the sea, and the top 

 to rise to a considerable height, the resistance of the 

 atmosphere to any natural action, as, for example, to 

 that of plants and animals, will be greatest at the base, 

 becoming gradually less and less as the mountain is 

 ascended, and be least of all on the summit. Water 

 will boil sooner, a bud will expand sooner, and, ge- 

 nerally, any operation will be performed in a less time, 

 or with less effort, on the summit of the mountain than 

 at the base ; and, if the height be considerable, the 

 difference will be palpable to common observation. 

 At the Nitee Ghaut, in the Himalaya mountains, 

 the barometer stands at about sixteen inches and a 

 quarter, while, at the level of the sea, it is about 

 twenty-nine and a half ;^and if the pressure upon the 

 surface of a man's body, at the level of the sea, be 

 fourteen tons and a halt, that at the Nitee Ghaut will 

 be barely eight tons, and the pressure on all other 

 surfaces will be diminished in the same proportion. 

 In the elevated situation, there will, in consequence, 

 be a distension of the smaller vessels of the body. Upon 

 the same principle, all expansive forces, in the growth 

 of vegetables, the freezing of water, or whatever else 

 acts by expansion, will act under less restraint of 

 pressure, and, therefore, other circumstances being 

 equal, the same effect will be produced with less ex- 

 ertion. 



It must not thence be inferred, however, that the 

 action of plants, or of animals, is rendered more ener- 

 getic by the removal of atmospheric pressure, for the 

 fact is directly the reverse. The power of action is 

 determined by the resistance which it overcomes, and 

 by that only ; and if there be no resistance, the action 

 of the power produces nothing, and amounts practi- 

 cally to nothing. Considering its mass and the ra- 

 pidity with which it travels in its orbit, the power of 

 the revolving earth is tremendous : so great that all the 

 powers which we see or hear of upon the earth, the 

 descent of waters, the blowing of winds, the action of 

 men and animals, steam, gunpowder, hurricanes, earth- 

 quakes, all combined, do not amount to a measurable 

 fraction of it ; and these occurrences affect it no more 

 than a single dew-drop affects the ocean. Yet this 

 tremendous power is silent and ineffective. The 



