23U 



ATMOSPHERE. 



ieyond the ordinary powers of living action to de 

 compose, or even to separate the water from them in 

 its wholesome and limpid state. 



And even if the ocean were as pure as a fountain, 

 or as soft as the rain-drops from a cloud, without the 

 atmosphere, there are no means of bringing it on the 

 land. The law of gravitation which, working in 

 concert, works so powerfully for life, would, working 

 alone, work just as powerfully for death. The spring 

 empties itself into the brook, the brook into the 

 river, and the river into the sea ; and if the sources 

 were not supplied, the whole would run dry in a 

 single season. If it were not for the atmosphere, the 

 tops of the mountain ridges, which now collect the 

 rains, and store up the snows, the melting of which 

 refresh and preserve the valleys in the season of 

 drought, would be the hottest, and for that reason 

 the driest, portions of the whole surface ; and were 

 it not for the atmosphere, the sun, wherever it shone, 

 would be hotter than it now is when its beams fall 

 perpendicularly upon tho plantless karroo, in the sap- 

 less sand, while every night would have the darkness 

 of Erebus, and a degree of cold more intense than 

 that of winter in the extreme north of the American 

 continent ; so that the change every twenty-four 

 hours would be that from a cinder to an icicle, or 

 even greater. Nothing could, in such a state, live 

 or grow ; nay, the very rocks themselves would yield, 

 and the strength of granite would be mouldered to 

 dust. All would speedily sink to one sad level, and 

 from that level there could be no return. These 

 would be necessary results, proceeding from the 

 very nature and constitution of matter, and it is in 

 consequence of the atmosphere preventing them, 

 that the study of it becomes so important to the 

 philosophic naturalist. 



And though at times it kindles the lightning, 

 awakens the thunder, and lets loose the tempest and 

 the hurricane, these are but local as well as tempo- 

 rary ; and its grand and general action is as gentle 

 and silent as it is beneficent. Its simplicity is its 

 strength, and its power of being so very beneficial 

 arises from the readiness with which it yields to even 

 the simplest impressions. 



The constituent parts of the atmosphere, by being 

 in the state of gas, have all their powers, or attrac- 

 tions, of cohesion, completely subdued and neutralised; 

 and they are simply mixed together, so that, as already 

 stated in the article AIR, the portion which it requires 

 can be separated by the weakest creature without 

 any effort. This simplicity of constitution renders 

 the atmosphere all obedience to every cause that acts 

 upon it, and enables action begun at one point to be 

 propagated throughout the whole; as, for instance, 

 the intense heat which is produced by the sun beating 

 strongly on one place, is speedily diffused to other 

 places ; and the clouds which are drank up by eva- 

 poration from humid places, steal their way silently 

 through the obedient atmosphere, till they ultimately 

 descend in refreshing showers upon the thirsty land. 



The cohesion of the atmosphere being perfectly 

 subdued or suspended by the heat which maintains it 

 in the gaseous state, it has a tendency to distribute 

 or diffuse itself equally in all directions, unless it be 

 restrained or prevented by pressure. When in a 

 tranquil state, or equally at rest over the whole 

 surface of the globe, it is subjected to only one 

 pressure, the pressure of gravitation towards the 

 centre of the earth, which is at once the proof of it 



existence and the measure of its quantity as matter. 

 At any point on the surface this pressure acts per- 

 pendicularly ; and at the mean level of the sea it is 

 on the average balanced by, and therefore equal to 

 about 29 inches of mercury, or 33 feet of water, 

 which is nearly 15 pounds on the square inch. But 

 as the atmosphere is a perfect fluid, it yields equally 

 in all directions to pressure, and thus it presses every 

 way, laterally and upwards, with the same force as it 

 gravitates downwards. By this means, it enters and 

 nils the smallest pores and openings in all bodies ; 

 and while its perfect fluidity admits ot the most perfect 

 freedom to all their motions, it bears them up with a 

 very effective support, as if they were equally propped 

 at all points. Upon the surface of a human body of 

 the average dimensions, the pressure of the atmo- 

 sphere is about fourteen tons and a half, but so perfect 

 is the balance on all sides, that this weight, while it 

 renders the whole body stable, instead of rendering 

 progressive motion more laborious, does not impede the 

 pulse of the minutest artery. All animals and plants 

 are supported in a very wonderful manner by this 

 atmospheric pressure, which gives them solidity, and 

 yet hinders none of those actions which are necessary 

 in their economy. 



Its insinuation into the minutest pores and openings 

 also prevents the surfaces of bodies from adhering or 

 sticking together, and the thinnest pellicle answers 

 this purpose. Were it not for this property of the 

 atmosphere, the consequences would be serious : the 

 taking off of a man's clothes would be a much more 

 laborious matter than the flaying of a bullock ; and i. 

 we suppose that the sole of the bare foot, when planted 

 on the ground, and bearing the weight of the body, 

 touches with only 12 inches of surface, then the foot 

 would be held to the ground with a force of 180 

 pounds, or it would lift along with it a stone of about 

 a cubic foot. The stone, however, would, as pre- 

 senting a larger base, adhere to the ground with 

 much more force, and the foot would remain fixed to 

 it. Thus we soe that, without one of the most simple 

 properties of the atmosphere a property which, in 

 our common observation, we overlook, there would 

 literally be no motion in the world. 



The pressure of the atmosphere can be estimated 

 in height of air, as well as in height of mercury or of 

 water. At the ordinary temperature and pressure 

 water is about 820 times heavier than air ; and there- 

 fore 27,470 feet in height of air will balance or be 

 equal in weight to 33 feet of water. This, allowing 

 for the inequalities of the earth's surface, would give 

 to the atmosphere, supposing it equally dense at all 

 elevations, a height of about 1500 or 2000 feet ab'ove 

 the top of the very highest mountain on the globe. 



But the atmosphere cannot have the same density 

 at all elevations, because its density is owing to its 

 own weight only, or rather to the excess of its weight 

 above its expansive tendency or force. And as we 

 ascend above the mean level, the pressure becomes 

 less, not only because the height to the top of the 

 atmosphere diminishes, but because the pressure 

 thereby taken off allows the expansive force to act. 

 It is evident that the expansive force, or the tendency 

 to spread or diffuse, must be greater when the density 

 is greater, and that it must diminish as the density 

 diminishes. It by no means follows, however, that 

 the two should diminish at the same rate. Indeed, 

 it is obvious that the tendency to expand must 

 diminish faster than the density produced by the 



