476 ATMOS 
weight, the weather is commonly fine, and we feel our- 
felves what we call braced, and more alert and afifive; 
but, on the contrary, when the weight of the air diminilh- 
es, the weather is bad, and people fee! a liflleffnefs 
and inactivity about them. And hence it is no wonder, 
that perfons fuller very much in their health, from fuch 
changes in the atmofphere, efpecially when they take place 
very fuddenly, for it is to this circmnltance chiefly that a 
fenfation of uneafinefs and indifpofition is to be attributed ; 
thus, when the variations of the barometer and atmof¬ 
phere are hidden and great, we feel the alteration and ef¬ 
fect on our bodies and fpirits very much ; but, when the 
change takes place by very (low degrees, and by a long 
continuance, we are fcarcely fenlible of it, owing, undoubt¬ 
edly, to the power with which the body is naturally en¬ 
dowed, of accommodating itfelf to this change in the (late 
ot the air, as well as to the change of many other circum- 
ftances of life, the body requiring a certain interval of 
time to effect the alteration in its (late, proper to that of 
the air, &c. Thus, in going up to the tops of mountains, 
where the prellure of the atmofphere is diminifhed two or 
three times more than on the plain below, little or no incon¬ 
venience is felt from the rarity of the air, if it is not mixed 
with other noxious vapours ; becaufe that, in the afcent, 
the body has had fufficient time to accommodate itfelf 
gradually to the flow variation in the date of the atmof¬ 
phere: but, when a perfon afcends with a balloon, very 
rapidly, to a great height in the atmofphere, he feels a 
difficulty in breathing, and an uneafinefs of body ; and the 
fame is loon felt by an animal when inclofed in a receiver, 
and the air fuddenly drawn or pumped out of it. So al- 
fo, on the condenfation of the air, we feel little or no al¬ 
teration in ourlelves, except when the change happens 
fuddenly, as in. very rapid changes in the weather, and in 
defcending to great depths in a diving-bell, &c. The late 
unfortunate Mr. Spalding always fouhd it abfolutely ne- 
celfary to defcend wit!', the bell very llovvly, and that only 
from one depth to another, reding awhile at each depth 
before he began to defcend farther: lie fil'd defcended 
flowly for about five or fix fathom, and then (topped awhile ; 
he felt an uneafinefs in his head and ears, which increafed 
more and more as he defcended, till he was obliged to (top 
at the depth above-mentioned, where the denlity of the 
air was nearly doubled; having remained there awhile, he 
felt his ears give a hidden crack, and after that he was 
foon relieved from any uneafinefs in that part, and it feem- 
ed as if the denlity of the air was not altered. He then 
defcended other five fathoms, or thirty feet more, with 
the fame precaution and 'he fame (dilations as before, be¬ 
ing again relieved, in the fame manner, after remaining 
awhile flationary at the next ftage of his defeent, where 
the denfity of the air was tripled. And thus he continu¬ 
ed proceeding to a great depth, always with the fame cir- 
cumftances repeated at every five or fix fathoms, and add¬ 
ing the prellure of one more atmofphere at every period 
of his progrefs. 
It has been aflerted, that, on the tops of very high 
mountains, the air is fo light as to occafion a great difficul¬ 
ty of refpiration, and even violent retching and vomiting 
of blood. It docs not appear, however, that thefe afler- 
tions are well founded. Mr. Brydone found no inconve¬ 
nience of this kind on the top of mount Etna; nor is any 
fucli thing mentioned by Mr. Houel, who alfo afeended 
this mountain. Sir William Hamilton indeed fays, that 
he did feel a difficulty of refpiration, independent- of any 
fulphureous fleam. But, on the top of a volcano, the ref¬ 
piration may be affetfied by fo many different caufes, that 
it is perhaps impoffible to aflign a true one. The French 
mathematicians, when on the top of a very high peak of 
the Andes, did not make any complaint of this kind, tho’ 
they lived there for lome time. On the contrary, they 
found the wind lbextremely violent, that they were fcarce- 
ly able to withffand its force; which feems an argument 
for at leaft equal denfity of the atmofphere in the fuperior 
as in the inferior regions. Dr. Heberden, who afeended 
P H E 'R E. 
to the top of Teneriffe, a higher mountain than Etna, 
makes no mention of any difficulty of refpiration. M. 
Sauffiure, however, in his journey to the top of mount 
Blanc, the higheft of the Alps, felt a very great uneafi¬ 
nefs in this w ; ay. His refpiration was not only Extremely 
difficult, but his pulfe became quick, and he was feized 
w ith all the fymptoms of a fever. His ffrength was alfo 
exhaufted to fuch a degree, that he Teemed to require four 
times as long a fpace to perform fome experiments on the 
top of the mountain as he would have done at the foot of 
it. It muff be obferved, however, that thefe fymptoms 
did not begin to appear till he had afeended two miles and 
a half perpendicular above the level of the fea. The 
mountain is only about a quarter of a mile higher; and in 
this fliort (pace lie was reduced to the lituation juft men¬ 
tioned. But it is improbable that fo fmall a difference, 
even at the end. of his journey, fliould have produced fuch 
violent effects, had not fome other caufe concurred. A 
caufe of this kind he himfelf mentions, viz. that the at¬ 
mofphere at the top of the mountain was fo much im¬ 
pregnated with fixed air, that lime-water, expofed to it, 
quickly became covered with a pellicle, occafioned by the 
abforption of that fluid. Now it is known, that fixed air 
is extremely pernicious to animals, and would bring on 
fymptoms fimilar to tliofe above-mentioned. Thereisno 
reafon, therefore, to have recourfe to the rarity, of the at¬ 
mofphere for folving a phenomenon which may more na¬ 
turally be accounted for otherwife. 
It is not eafy to affign the true reafon for the variations 
that happen in the gravity of the atmofphere in the fame 
place. One caufe of it, however, either immediate or 
otherwife, it feems, is the heat of the fun ; for where 
this is uniform, the changes are fmall and regular; thus 
between the tropics it feems the change depends on the 
heat of the fun, as the barometer conffantly finks about 
half an inch every day, and rifes again to its former ftation 
in the night-time. But in the temperate zones the baro¬ 
meter ranges from twenty-eight to near thirty-one inches, 
filewing, by its various altitudes, the changes that are 
about to take place in the weather. If we could know, 
therefore, the caufes by which the weather is influenced, 
we fliould alfo know thole by which the gravity of the at¬ 
mofphere is affedted. Thefe may perhaps be reduced to 
immediate ones, viz. an emillion of latent heat from the 
vapour contained in the atmofphere, or of elefifric fluid 
from the fame, or from the earth ; as it is oblerved that 
they both produce the fame effect witli the folar heat in the 
tropical climates, viz. to rarefy the air, by mixing with it, 
or letting loofe a lighter fluid, which did not before acf in 
fuch large proportion in any particular place. 
With regard to the alteration of heat and cold in the at¬ 
mofphere, many reafons and hypothefes have been given, 
and many experiments made. In general they feem to 
depend entirely upon the light of the lun refledfed into 
the atmofphere from theearth ; and, where this reflection is 
deficient, even though the light fliould be prefent ever fo 
much, the molt violent degrees of cold are found to take 
place. Hence, on the tops of mountains, the cold is ge¬ 
nerally excellive, though by reafon of the clearnefs of the 
atmofphere the light of the fun falls upon them in greater 
quantity than it can do on an equaWpace on the plain. In 
long winding paffages alfo, fuch as the caverns of Etna 
and Veluvius, where the air lias room to circulate freely, 
without any accefs of the fun, the cold is fcarcely tolera¬ 
ble ; whence the tile of thefe for cooling liquors, prelerv- 
ing meat, &c. 
The coldnefs of the atmofphere on the tops of moun¬ 
tains has. been aferibed by M. Lambert and De Luc, to 
the igneous fluid, or elementary fire, being more rare in 
thole elevated fituations than on the plains. M. Lambert 
is of opinion that it is rarefied above by the adlion of the 
air, and that below it is condenfed by its own proper 
weight. He confiders fire as a fluid in motion, the parts 
of which are feparable, and which is rarefied when itsve- 
locity is accelerated. He does not decide with regard to 
the 
