ATMOSPHERE. 



after it has parted with its vapour, the 

 same degree of cold continues till the air, 

 on descending to the earth, acquires its 

 former state of condensation and of 

 warmth. The Andes, almost under the 

 line, i*ests its base on burning sands : 

 about its middle height is a most pleasant 

 and temperate climate, covering an ex- 

 tensive plain, on which is built the city of 

 Quito ; while its forehead is encircled 

 with eternal snow, perhaps coeval with 

 the mountain. Yet, according to the ac- 

 count of Don Ulloa, these three discord- 

 ant climates seldom encroach much on 

 each other's territories. The hot winds 

 below, if they ascend, become cooled by 

 their expansion ; and hence they cannot 

 affect the snow upon the summit ; and 

 the cold winds, that sweep the summit, 

 become condensed as they descend, and 

 of temperate warmth before they reach 

 the fertile plains of Quito. 



Various attempts have been made to 

 ascertain the height to which the atmos- 

 phere is extended all round the earth. 

 These commenced soon after it was dis- 

 covered,by means of the Torricellian tube, 

 that air is endued with weight and pres- 

 sure. And had not the air an elastic pow- 

 er, but were it every where of the same 

 density, from the surface of the earth to 

 the extreme limit of the atmosphere, like 

 water, which is equally dense at all depths, 

 it would be a very easy matter to deter- 

 mine its height, from its density, and the 

 column of mercury which il vould coun- 

 terbalance in the barometer tube ; for, it 

 having been observed that the weight of 

 the atmosphere is equivalent to a column 

 of 30 inches, or 2$ feet of quicksilver, 

 and the density of the former to that of 

 the latter as 1 to 1104-0: therefore the 

 lieigbt of the uniform atmosphere would 

 be 11040 times 2 feet, that is 27600 feet, 

 or little more than five miles and a quar- 

 ter. But the air, by its elastic quality, 

 expands and contracts ; and it being- 

 found, by repeated experiments in most 

 nations of Europe, that the spaces it oc- 

 cupies, when compressed by different 

 weights, are reciprocally proportional to 

 those weights themselves ; or, that the 

 more the air is pressed, so much the less 

 space it takes up ; it follows, that the air 

 in the upper regions of the atmosphere 

 must grow continually more and more 

 rare, as it ascends higher; and indeed 

 that, according to that law, it must neces- 

 sarily be extended to an indefinite height. 

 Now, if we suppose the height of the 

 whole divided into innumerable equal 

 parts, the quantity of each part will be 



as its density ; and the weight of the 

 whole incumbent atmosphere being also 

 as its density ; it follows, that the weight 

 of the incumbent air is every where as 

 the quantity contained in the subjacent 

 part; which causes a difference between 

 the weights of each two contiguous parts 

 of air. But, by a theorem in arithmetic, 

 when a magnitude is continually dimi- 

 nished by the like part of itself, and the 

 remainders the same, there will be a se- 

 ries of continued quantities, decreasing in 

 geometrical progression : therefore, if, 

 according to the supposition, the altitude 

 of the air, by the addition of new parts 

 into which it is divided, do continually 

 increase in arithmetical progression, its 

 density will be diminished, or, which is 

 the same thing, its gravity decreased, in 

 continued geometrical proportion. And 

 hence, again, it appears that, according 

 to the hypothesis of the density being 

 always proportional to the compressing 

 force, the height of the atmosphere must 

 necessarily be extended indefinitely. And, 

 farther, as an arithmetical series adapted 

 to a geometrical one is analogous to the 

 logarithms of the said geometrical one ; it 

 follows, therefore, that the altitudes are 

 proportional to the logarithms of the den- 

 sities, or weights of air : and that any 

 height taken from the earth's surface, 

 which is the difference of two altitudes 

 to the top of the atmosphere, is propor- 

 tional to the difference of the logarithms 

 of the two densities there, or to the loga- 

 rithm of the ratio of those densities, or 

 their corresponding compressing forces, 

 as measured by the two heights of the 

 barometer there. 



It is now easy, from the foregoing pro- 

 perty, and two or three experiments, or 

 barometrical observations, made at known 

 altitudes, to deduce a general rule to de- 

 termine the absolute height answering to 

 anv density, or the density answering to 

 any given altitude above the earth. And, 

 accordingly, calculations were made upon 

 this plan by many philosophers, particu- 

 larly by the French ; but it having been 

 found that the barometrical observations 

 did not correspond with the altitudes, as 

 measured in a geometrical manner, it was 

 suspected that the upper parts of the at- 

 mospherical regions were not subject to 

 the same laws with the lower ones, in re- 

 gard to the density and elasticity. And 

 indeed, when it is considered that the at- 

 mosphere is a heterogeneous mass of par- 

 ticles of all sorts of matter, some elastic, 

 and others not, it is not improbable but 

 this may be the case, at least in the re- 



