436 PHYSICAL GEOGRAPHY. 



scribed limit. A column of the whole circumambient air is nearly equal in weight to a 

 similar column of mercury of thirty inches, or of water of thirty-four feet, which would 

 give it an elevation of but 27,000 feet, or rather under five miles, if its density were 

 uniform. But the elasticity of the air causes it to expand with the diminution of its own 

 pressure, which becomes less at every step from the surface of the earth ; and owing to 

 this expansion we must place the limit to its height at a far greater distance than that 

 suggested by the simple barometrical measurement of its weight. A pretty common 

 opinion prevails that its extreme boundary does not exceed forty or fifty miles, and we 

 have sensible evidence on the high lands of the globe, that for all the purposes serviceable 

 to vegetable and animal life, the atmospheric zone is of very contracted elevation. It is 

 a well-known property of the air that the temperature diminishes with its height, a 

 circumstance referable to the general physical law, that as the density of gases decreases 

 they acquire an increased capacity for heat. The higher, therefore, a body ascends in 

 the atmosphere, the greater is the quantity of heat abstracted from it, the surrounding 

 fluid becoming more rare. Hen'ce the perpetual snow, and the piles of glaciers, that 

 crown the summits of mountains, at whose base the orange and the citron bloom, and man 

 pants in the fierce sultriness of a torrid climate. 



But while the atmosphere may be considered generally as an aerial zone of the earth, 

 the companion of the massy spheriod in its annual revolution round the sun, and rotating 

 with it upon its axis, it has independent movements which present very complex pheno- 

 mena, however clear the causes which put them in operation. The particles of air are 

 constantly suffering displacement, and it is easy to conceive of various circumstances 

 disturbing the dilatable and elastic fluid in which we live. A body in movement will 

 communicate its motion to the adjoining particles, which may be sensibly propagated by 

 them to a considerable distance ; but this cause operates so slightly in the production of 

 atmospheric currents that it might be entirely overlooked. It will be sufficient to state 

 that some of the vast oceanic streams are supposed to produce a corresponding flow in the 

 air. The varying attractions of the sun, moon, and planets on the atmosphere, will 

 occasion tides in it analogous to those of the Ocean, or an alteration in the heights of 

 vertical columns of air, winds and currents arising from the resulting inequalities of 

 horizontal pressure ; but La Place has proved the action of this cause to be scarcely 

 appreciable. The atmospheric agitations of which we are sensible, both the more violent 

 and gentle, appear to proceed either from a change in the temperature of a portion of 

 the air, or from a change in the quantity of water which it holds in a state of vapour. 

 In both these cases a temporary destruction of the equilibrium subsisting between 

 different parts of the atmosphere is produced, and its particles are set in motion to restore 

 the balance. The effect of heat upon a volume of air is to rarefy and expand, to increase 

 its bulk and diminish its density. When any portion, therefore, of the earth's surface is 

 more heated than the surrounding districts, the air there ascends and flows over the 

 adjoining cooler and denser strata, causing an upper outward current, while the colder 

 ind denser fluid rushes towards the spot where the balance has been lost by expansion, 

 and a lower inward current is produced. An easy experiment will illustrate this inter- 

 change. In a room warmed by a good fire, if a candle be held at the crevice between the 

 door and the floor, an inward current will be observed from the exterior colder air, but 

 near the ceiling, by the same means, an outward flow will be detected. In the other 

 condition an addition of vapour to the atmosphere gives rise to a wind blowing on all 

 sides away from the district of evaporation, while an abstraction of it by showers creates 

 a partial vacuum, towards which the air rushes from all points of the compass. The 

 diversity of the winds in power is principally owing to the different degrees of vigour 

 with which these causes act. 



