238 



ATMOSPHERE. 



earth rolls on ; but its motion never hurts the most 

 delicate flower, or the most tiny insect, or interferes 

 with the operations which are going on in the remoter 

 and rarer portions of the atmosphere, where, proba- 

 bly, the millionth part of a grain in weight would 

 affect the whole operations. But if, as it careers along, 

 the earth were to meet with an adequate resist- 

 ance, say with another globe of the same size and 

 weight as itself, then though that globe were at rest, 

 and thus merely passive, the collision would be even 

 fearful to think on. What it would positively be we 

 are unable to say, because we know nothing at all 

 analogous with which we can compare it ; but this 

 much is certain, from what we know of the action of 

 comparatively trifling powers when properly resisted, 

 that it could, in an instant, dissipate both globes 

 through the regions of space in viewless air, too thin 

 for being recognised by any sense, or tested by any 

 instrument. Many other instances might be given, 

 but this one may suffice, the more so that any one 

 who reflects, must be convinced that we have no 

 knowledge of power but from the effects which it pro- 

 duces, that is, the resistances which it overcomes. The 

 motion of the earth, the circulation of the blood in 

 our own bodies, and countless other results of power 

 that could be named, never would or could have been 

 found out by any observation of simple facts ; and the 

 pressure of the atmosphere comes so much under the 

 same denomination, that the discovery of it is consi- 

 dered as one of the most brilliant, as well as the most 

 useful, in the whole wide field of modern science. 



Thus we have, in what may be considered as the 

 merely passive resistance of the atmosphere, a very 

 general and very important power, or, to speak more 

 correctly, an evolver of power, in all the actions of 

 living and growing nature, a power to which not a 

 few of the differences which we find between the pro- 

 ductions at different heights in the same latitude are 

 owing. At the base of the mountain we find, for in- 

 stance, the vine and the olive ; on the middle slopes 

 there are luxuriant forests ; to these succeed heaths 

 and other lowly shrubs of slow and creeping growth, 

 then lichens, hardly more vegetable in texture or in 

 taste than the rocks which they incrust ; and, lastly, 

 the never-thawing snow, even where the fire of a vol- 

 cano blazes in the midst, and at times raises its pillar of 

 flame, and volleys its red-hot rocks many hundreds of 

 feet into the air. The variety of animals is as great, 

 from the ox to the buffalo, which grazes in the rich 

 meadow at the base, through all the succession of 

 tribes to the marmot by the margin of the snow, 

 which dozes ou> half its life in a state of hybernating 

 stupor. The differences of pressure in the atmosphere 

 are not the sole causes of these differences of the pro- 

 ductions' of the earth at different altitudes ; but it is 

 one of the causes, and one upon which some of the 

 others depend ; and it is a cause not arising from 

 any action of the atmosphere, either mechanical or 

 chemical, but from its perfectly neuter and quiescent 

 state. 



When the resistance of force, or, which is the same 

 thing, of motion (for we have no notion of physical 

 force unless in so far as it produces, or tends to pro- 

 duce motion), attains to a certain degree, it is always 

 accompanied by the evolution of sensible heat ; and 

 this result is so constant, that we have no knowledge 

 of the evolution of sensible heat but by the resistance 

 of motion of some kind or other ; and conversely there 

 .'s nrobably no resistance of motion without an evolu- 



tion of heat, whether that heat be cognisable by our 

 senses or instruments or not. Of course it is impos- 

 sible for us to have direct evidence in those cases 

 which are too minute for our observation ; but the 

 fact of heat accompanying resistance, and being, un- 

 less in cases where we can easily explain the deviation, 

 so exactly in proportion to the resistance, and in truth 

 the measure of it is so general, so free from a single 

 exception, that to suppose it does not extend to every 

 case, whether that case come within the limits of our 

 observation or not, would be a violation of the funda- 

 mental principle of all philosophic reasoning. 



Here, then, we have another important result of 

 the varying pressure of the atmosphere, as affecting 

 the living and growing productions of nature at dif- 

 ferent heights above the mean level of the sea. The 

 additional resistance which, in places that are situated 

 low, it otters to the action of plants and animals, pro- 

 duces heat as well as more intense action ; and from 

 all that we know of the economy of plants and ani- 

 mals, both of these are essential to their develop- 

 ment, and that development must increase with the 

 increase of both, and diminish with their diminution. 

 Thus we see that, in the quiescent atmosphere itself, 

 as a mere resistance, there are varying causes of fer- 

 tility, which may be considered as having their maxi- 

 mum at the mean level of the sea, and diminishing in 

 the proportion of the density, that is, as the squares 

 of the heights above that level. 



But it is by the action of heat that the cohesion of 

 those substances which form the atmosphere is sub- 

 dued, so that that fluid exists in the state of air ; and 

 therefore, the more rare or attenuated it is, the greater 

 degree of action must be required for maintaining it in 

 that state. But the heat which is necessary for main- 

 taining the state of any substance is not sensible, not 

 felt, or communicated to other substances ; and there- 

 fore, the more rare or attenuated the atmosphere is, 

 the greater must be its tendency to take the heat out 

 of other substances. Consequently, as we ascend 

 above the mean level of the sea, the atmosphere must 

 become gradually colder, till at all places there be a 

 point of elevation, higher than which the temperature 

 shall never be warmer than that at which water 

 freezes. This is called the point of perpetual conge- 

 lation, or frost, and above it there can be no general 

 action either of animal or of vegetable life. From 

 partial data, it is not very difficult to calculate the 

 average elevation of this point above the level of the 

 sea for all latitudes ; but when we come to practice, 

 the calculations thus formed will not apply, because 

 local circumstances have very considerable influence. 

 Thus, on the mountains which rise out of the dry and 

 warm table land of Mexico in America, the summer, 

 or lower elevation of this point, is more than a quar- 

 ter of a mile ; and on the northern side of the Hima- 

 laya mountains, which have the extensive and dry 

 table land of central Asia on the north, it is more than 

 a whole mile higher than theory would lead us to sup- 

 pose. Thus, though the law, that the atmosphere is 

 colder as we ascend above the mean level, be a gene- 

 ral law, which holds true in all parts of the world, yet 

 the law is so much modified by other circumstances, 

 that the precise determination of the decrease of tem- 

 perature for any one place must always be matter of 

 local observation. Here, however, we have another 

 general effect of the diminished pressure and density 

 of the atmosphere upon life and growth, which", 

 though subject to local variations in amount, is uni- 



