74 ® 
BAR Ck METER. 
Tain is foretold a much longer time before it comes, than 
the vapour can be fuppofed to take lip in falling : that 
many times, and in different places, there falls a great 
deal of rain, without any finking of the mercury at all; 
as alfo that there often happens a fall of the mercury with¬ 
out any rain enfuing : and that fometimes the mercury 
will fuddenly fink, in a fliort fpace of time, half an inch 
or more, which anfwers to feven inches of rain, or about 
one third of the whole quantity falling in the whole year. 
M. de Lite fuppofes that the changes obferved in the 
prelfure of the atmofphere, are chiefly produced by the 
greater or lefs quantity of vapours floating in it: as others 
liave attributed them to the fame caufe, but have given a 
different explanation of it. His opinion is, that vapours 
diminifh the fpecific gravity, and confequently the abfo- 
lu v te weight, of thole columns of the atmofphere into 
which they are received, and which, notwithflanding this 
admixture, ffill remain of the fame height with adjoining 
columns that confilt of pure or dry air. He afterwards 
vindicates and more fully explains this theory, and applies 
it to the folution of the principal phenomena of the baro¬ 
meter, as depending on the varying denlity and weight of 
the atmofphere. 
Dr. James Hutton, in his Theory of Rain, printed in the 
Tranfadtions of the Royal Society of Edinburgh, vol. i. 
gives ingenious and plaulible reafons for thinking that the 
lelfening the weight of the atmofphere by the fall of rain, 
is not the caufe of the fall of the barometer ; but that the 
principal, if not the only caufe, arifes from the commo¬ 
tions in the atmofphere, which are chiefly produced by 
fudden changes of heat and cold in the air. “ The baro¬ 
meter, fays he, is an infirument necelfarily connected with 
motions in the atmofphere ; but it is not equally affedted 
with every motion in that fluid body. The barometer is 
chiefly affedted by thofe motions by which there are pro¬ 
duced accumulationsand attractions of this fluid, in places 
or regions of fufficient extent to affedt the prelfure of the 
atmofphere upon the furface of the earth. But as every 
commotion in the atmofphere may, under proper condi¬ 
tions, be a caufe for rain, and as the want of commotion 
in the atmofphere is naturally a caufe of fair weather, 
this infirument may be made of great importance for the 
purpofe of meteorological obfervations, although not in 
the certain and more Ample manner in which it has been, 
with the increafe of fcience, fo fuccfsfully applied to tiie 
meafuring of heights.” See Rain. 
In the Encyclopaedia Britannica there is another theory 
of the changes in the barometer, as depending on the heat 
in the atmofphere, not as producing commotions there, 
but as altering the fpecific gravity of the air by the changes 
of heat and cold. The principles of this Theory are, ill, 
That vapour is formed by intimate union between the ele¬ 
ments of fire and water, by which the fire or heat is fo 
totally enveloped, and its adtion fo perfectly lufpended 
by the aqueous particles, that it not only lofes its proper¬ 
ties of burning and of giving light, but becomes incapable 
of affedting the moll fenfible thermometer, in which cafe 
it is faid to be in a latent ftate and 2d, That, if the at¬ 
mofphere be affedted by any unufual degree of heat, it 
thence becomes incapable of fupporting fo long a column 
of mercury as before ; for which reafon it is that the ba¬ 
rometer finks. From theft axioms it would follow, that 
as vapour is formed by an union of fire with water, whe¬ 
ther by attraction or a folution of the water in the fire, the 
vapour cannot be condenfed till this union, attraction, or 
folution, is af an end. Hence the beginning of the con- 
denfation of the vapour, or the firlt figns of approaching 
rain, mull be the reparation of the fire which is latent in 
the vapour. In the beginning, this may be either flow 
and partial, or it may be fudden and'violent : in the firft 
cafe, the rain will come on flowdy, and after a confider- 
able time ; but in the other, it will come very quickly, 
and in a great quantity. But Dr. Black has proved, that 
when fire quits its latent ftate, however long it may have 
Lain dormant and inlenlible, it always realfumes its proper 
qualities, and affedts the thermometer juft the fame as if 
it had never been abforbed. The confequence of this is, 
that in proportion as the latent heat is difeharged fromdhe 
vapour, thofe parts of the atmofphere into which it isdif- 
charged mult be fenfibly affedted by it; and in proportion 
to the heat communicated to thofe parts, they will become 
fpecifially lighter,, and the mercury will fink of courfe. 
In the Memoirs of the Literary Society of Manchefter, 
vol. 4. is alfo a curious paper on this fubjedt, viz. Mete¬ 
orological Obfervations made on different Parts of the 
Weftern Coaft of Great Britain: by T. Garnett, M. D. 
This paper is compofed of materials furniflied by feveral 
obfervers; thofe of Mr. Copland, furgeon at Dumfries, 
are of fpecial importance. This gentleman is of opinion 
that the changes of the barometer indicate approaching hot 
and cold weather, with more certainty than dry and wet. 
“ Every remarkable elevation of the barometer, (fays he,) 
where it is of any duration, is followed by very warm or 
dry weather, and moderate as to wind, or by all of them; 
but heat feems to have mod influence and connexion ; and, 
when it is deficient, the continuance of the other two will 
be longer and more remarkable ; therefore the calculation 
muft be in a compound ratio of the excefs and deficiency 
of the heat, and of the drynefs of the weather in compa- 
rifon of the medium of the feafon; and with regard to 
the want of ftrong wind, it appears to be intimately con- 
nedted with the laft, as they (hew that no precipitation is 
going on in any of the neighbouring regions.” 
In his remarks, he obferves, ‘ That the barometer be-, 
ing lower, and continuing fo longer than what can be ac¬ 
counted for by immediate falls, or ftormy weather, indi¬ 
cates the approach of very cold weather for the feafon ; 
and alfo, cold weather,* though dry, is always accompa¬ 
nied by a low barometer, till near its termination. And, 
that warm weather is always preceded and moftly accom¬ 
panied by a high barometer; and the riling of the ba-' 
rometer in the time of broken or cold weather, is a fign 
of the approach of warmer weather : and alfo, if the wind 
is in any of the cold points, a fudden rift of the barometer 
indicates the approach of a foutherly wind, which in win¬ 
ter generally brings rain with it.’ 
In the two following remarks, Mi-. Copland had ex¬ 
plained certain phenomena from a principle fimilar to that 
on which Dr. Darwin has fo much infilled : viz. ‘ That 
the falling of the barometer may proceed from a decom- 
pofition of the atmofphore occurring around or near that 
part of the globe where we are placed, which will occa- 
fion the electricity of the atmofphere to be repelled up¬ 
wards in fine lambent portions; or driven downwards or 
upwards in more compacted balls of fire ; or laftlv, to be 
carried along with the rain, &c. in an imperceptible man- 
mer to the furface of the earth : the precipitation of the 
watery parts generally very foon takes place, which dimi¬ 
nifiles the real gravity of the atmofphere, and alfo, by the 
decompofition of fome of the more active parts, the air 
lofts part of that elaftic and repulfive power which it fo 
eminently polIeHed, and will therefore prefs with lefs force 
on the mercury of the barometer than before, by which 
means a fall enfues. 
‘ That the caufe of the currents of air, or winds, may 
alfo be this way accounted for : and in very fevere ftorms, 
where great decompolitions of the atmofphere take place, 
this is particularly evident, fuch as generally occur in one 
or more of the Weft-India iflands at one time, a great lofs 
of real gravity, together with a confiderable diminution 
of the fpring of the air, immediately enfues; hence a cur¬ 
rent commences, firft in that diredion whence the air lias 
moft gravity, or is molt difpofed to undergo fuch a change; 
but it being foon relieved of its fuperior weight or fpring 
on that fide, by the decompofition going on as fait as the 
wind arrives on the ifland, it immediately veers to another 
point, which then rufites in moftly with an increafe of force; 
thus it goes on till it has blown more than half way round 
the points of the conipafs during the continuation of the 
hurricane. For in this manner the Weft-India phenomena. 
