ATMOS 
periment was four times repeated, and the thermometer 
uniformly funk from five to feven degrees. During the 
time of condenfation there was a great difference in the 
heat, as perceived by the hand, at the two ends of the 
condenfing fyringe: that next the air-globe was almoft 
painful to the touch; and the globe itfelf became hotter 
than could have been expefted from its contaft with the 
fyringe. “ Add to this (fays Mr. Darwin), that in explo¬ 
ding an air-gun the ft ream of air always becomes vilible, 
which is owing to the cold then produced precipitating the 
vapour it contained; and, if this ftream of air had been 
previoufly more condenfed, or in greater quantity, fo as not 
inftantly to acquire heat from the common atmofphere in 
its vicinity, it would probably have fallen in fnovv.” 3. 
A thermometer was placed in the receiver of an air-pump, 
and, the air being haftily exhaufted, It funk two or three 
degrees; but after fonie minutes regained its former fta- 
tion. The experiment was repeated with a thermometer 
open at the top, fo that the bulb could not be affefted by 
any diminution of the external preffure; but the refult 
was the fame. Both during exhauftion and re-admiffion 
of the air into the receiver, a fteam was regularly obferv- 
ed to be condenfed on the (ides of the glafs; which, in 
both cafes, was in a few minutes re-abforbed, and which 
appeared to be precipitated by being deprived of its heat 
by the expanded air. 4. A hole, about the fize of a crow- 
quill, was bored into a large air-veflel placed at the com¬ 
mencement of the principal pipe of the water-works of 
Derby. There are four pumps worked by a water-wheel, 
the water of which is firft thrown into the lower-part of 
this air-veflel, and riles from thence to a refervoir about 
thirty-five or forty feet above the level; fo that tire water 
in this veil'd is conftantly in a ftate of compreffion. Two 
thermometers were previoufly fu(pended on the leaden air- 
velfel, that they might a flu me the temperature of it, and, 
as foon as the hole above-mentioned was opened, had their 
bulbs applied to the ftream of air which iflued out; the 
confequence of which was, that the mercury funk fome 
degrees in each. 
There is a curious phenomenon obferved in the foun¬ 
tain of Hiero, conftrufted 011 a very large fcale, in the 
Chemnifcenlian mines in Hungary. In this machine the 
air, in a large veflel, is comprelfed by a column of water 
260 feet high; a flop-cock is then opened ; and as the air 
ifliies with great vehemence, and in confequence of its 
previous condenfation becomes immediately much expand¬ 
ed, the moifture it contains is not only precipitated, as in 
the exhaufted receiver above-mentioned, but falls dow n in 
a fhower of fnow, with ilicles adhering to the nofe of the 
cock. See Phil. Tran!', vol. lii. From this phenomenon, 
as well as the four experiments above related, Mr. Dar¬ 
win thinks “ there is good reafon to conclude, that, in all 
circumftances where air is mechanically expanded, it be¬ 
comes capable of attracting the fluid matter of heat from 
other bodies in contact with it. 
“ Now (continues he), as the vaft region of air which 
furrounds our globe is perpetually moving along its fur- 
face, climbing up the Tides of the mountains, and descend¬ 
ing into the valleys ; as it pafies along it mult be perpetu¬ 
ally varying the degree of heat according to the elevation 
of the country it traverfes: for, in riling to the fummits 
of mountains, it becomes expanded, having fo much of 
the preffure of the fuperincumbent atmofphere taken away; 
and, when thus expanded, it attrafts or abforbs heat from 
the mountains in contiguity witli it; and, when it defeends 
into the valleys and is comprelfed into lei's compafs, it 
again gives out the heat it has acquired to the bodies it 
comes in contaft with. The fame thing mull happen to 
the higher regions of the atmofphere, which are regions 
of perpetual froft, as has lately been dilcovered by the 
aerial navigators. When large diftrifts of air, from the 
lower parts of the atmofphere, are railed two or three 
miles high, they become fo much expanded bv the great 
diminution of the preffure over them, and thence become 
fo cold, that hail or fnow is produced by the precipitation 
P H E R £» 479 
of the vapour: and as there is, in thefe high regions of 
the atmofphere, nothing elfe for the expanded air to-^ic- 
quire heat from after it has parted with its vapour^, the 
fame degree of cold continues till the air, on descending to 
the earth, acquires its former ftate of condenfation and of 
warmth. The Andes, almoft under tine line, refts its bale 
on burning fands : about its middle height is a moll plea- 
fant and temperate climate, covering an extenfive plain, on 
which is built the city of Quito : while its forehead is en¬ 
circled with eternal fnow, perhaps coeval with the moun¬ 
tain. Yet, according to Don Ulloa, thefe three difeord- 
ant climates feldom encroach much on each other’s terri¬ 
tories. The hot winds below, if they afeend, become 
cooled by their expanfion; and hence they cannot afteft 
the fnow upon the fummit; and the cold winds that fweep 
the fummit become condenfed as they defeend, and of 
temperate warmth before they reach the fertile plains of 
Quko.” 
Various attempts have been made to afeertain the height 
to which the atmofphere is extended all round the earth. 
Thefe commenced loon after it was difeovered, by means of 
the Torricellian tube, that air is endued with weight and 
preffure. And had not the air an elaftic power, but were 
it every where of the fame denjity, from the furface of the 
earth to the extreme limit of the atmofphere, like water, 
which is equally denfe at all depths, it would be a very 
eafy matter to determine its height from its denfity and 
the column of mercury it would counterbalance in the ba¬ 
rometer-tube: for, it having been obferved, that the 
weight of the atmofphere is equivalent to a column of 
thirty inches or 2^ feet of quicklilver, and the denfity of 
the former to that of the latter, as 1 to 11040; therefore 
the height of the uniform atmofphere would be 11040 
times feet, that is, 27600 feet, or little more than 5^ 
miles. But the air, by its elaftic quality, expands and 
contrafts ; and it being found, by repeated experiments in 
mod nations of Europe, that the (paces it occupies, when 
comprelfed by different weights, are reciprocally propor¬ 
tional to thole weights themfelves; or, that the more the 
air is preffed, fo much the lefs fpace it takes up; it fol¬ 
lows that the air in the upper regions of the atmofphere 
mult grow continually more and more rare, as it attends 
higher; and indeed that, according to that law, it mult nc- 
cetiarily be extended to an indefinite height. Now, if we 
fuppofe the height of the whole divided into innumerable 
equal parts, the quantity of-each part will be as its den¬ 
fity ; and, the weight of the whole incumbent atmofphere 
being alfo as its denfity, it follows, that the weight of the 
incumbent air is every where as the quantity contained in 
the (ubjaeent part; which caufes a difference between the 
w eights ot each two contiguous parts of air. But, by a 
theorem in arithmetic, when a magnitude is continually 
diminiflied by the like part of itfelf, and the remainders 
the lame, thefe will be a feries of continued quantities de- 
creafing in geometrical progreflion: therefore if, according 
to the fuppofition, the altitude of the air, by the addition 
of new parts into which it is divided, do continually in- 
creafe in arithmetical progreflion, its denfity will be dimi- 
nifhed, or, which is the fame thing, its gravity decreafed, 
in continued geometrical proportion. And hence, again, 
it appears that, according to the hypotlielis of the denfity 
being always proportional to the compreffing force, the 
height of the atmofphere mult neceff'arily be extended in¬ 
definitely. And, farther, as an arithmetical feries adapted 
to a geometrical one, is analogous to the logarithms of the 
laid geometrical one; it follows therefore, that the alti¬ 
tudes are proportional to the logarithms of the denfities, 
or weights of air; and that any height taken from the 
earth’s furface, which is the difference of two altitudes t» 
the top of the atmofphere, is proportional to the difference 
of the logarithms of the two denfities there, or to the lo¬ 
garithm of the ratio of thole denfities, or their correfpond- 
ing compreffing forces, as meafured by the two heights of 
the barometer there. This law was firft obferved and de- 
lnuaftrated by Dr. Halley, from the nature of the hyper¬ 
bola } 
