B A R O 
in the well, it was conjectured by Galileo, who flouridied 
about that time, that there might be fome other caufe of the 
afeent of water in pumps, or at lead: that this abhorrence 
was limited to the finite height of thirty-three feet. Being 
unable to fatisfy himfelf on this head, he recommended 
the confideration of the difficulty to Torricelli, who had 
been his difciple. After fome time Torricelli fell upon 
the fufpicion that the pre'ffure of the atmofphere was the 
caufe of the afccnt of water in pumps ; that a column of 
water thirty-three feet high was a juit counterpoife to a 
Column of air, of the fame bafe, and which extended up 
to the top of the atmofphere ; and that this was tire true 
reafon why the water did not follow the fucker any farther. 
And this fufpicion was foon after confirmed by various ex¬ 
periments. Torricelli confiaered, that, if a column of wa¬ 
ter thirty-three feet high were-a counterpoife to a wdiole 
column of the atmofphere, then a column of mercury of 
about two feet and a half high would alfo be a counter- 
poife to it, fince quickfilver is nearly fourteen times hea¬ 
vier than water, and lb the fourteenth part of that height, 
or near two feet and a half, would be as heavy as the co¬ 
lumn of water. This reafoning was foon verified ; for, 
having filled a glafs tube with quickfilver, and inverted it 
into a bafon of the fame, the mercury prefently defcended 
till its height, above that in the bafon, was about two feet 
and a half, juft as he expected. And this is what lias, 
from him, been called the Torricellian experiment. 
The new opinion, with tjiis confirmation of it, was rea¬ 
dily acquiefced in by mod of the philolophers, who repeat¬ 
ed the experiment in various \fays. Others however ftill 
adhered to the old dobtrine, and raifed feveral objections 
againlt the new one ; fuch as that there was a film or im¬ 
perceptible rope of inercury, extended through the upper 
part of the tube, which fufpended the column of mercu¬ 
ry, and kept it from falling into that in the bafon. This 
and other objections were however foon overcome by ad¬ 
ditional confirmations of the true doiftrine, particularly by 
varying the elevation of the place. It was hinted by Def- 
cartes and Pafcal, that if the mercury be fuftained in the 
tube by the preffure of the atmofphere, by carrying it to 
a higher fituation, it would defcend lower in the tube, ha¬ 
ving a fiiorter column of the atmofphere to fuftain it, and 
nice verfa. And Pafcal engaged his brerther-in-law, M. 
Perier, to try that experiment for him, being more conve¬ 
niently fituated for that purpofe than he was at Paris. 
This he accordingly executed, by obferving the height of 
the quickfilver in the tube, firft at the bottom of a moun¬ 
tain in Auvergne, and then at feveral (fations, or different 
altitudes, in afeending, by which it was found that the 
mercury fell lower and lower all the way to the top of the 
mountain ; and fo confirming the truth of the doctrine re¬ 
lating to the univerfal preffure of the atmofphere, and the 
confequent fufpenfion of the mercury in the tube of the 
barometer. Thus, bv the united endeavours of Torricelli, 
Defcart'es, Pafcal, Merfenne, Huygens, and others, the 
caufe of the fufpenfion of the quickfilver in the tube of 
the barometer became pretty generally eftablifhed. 
It was fome time however after this general confent, be¬ 
fore it was known that the preffure of the air was various 
at different times in the fame place. This could not 
however remain long unknown, as the frequent meafuring 
of the column of mercury muff foon (hew its variations in 
altitude ; and experience and obfervation would prefently 
fhew that thofe variations in the mercurial column, were 
always fucceeded by certain changes in the weather, as to 
rain, wind, frofts, &c. Hence this inftrument foon came 
into life as the means of foretelling the changes of the 
weather ; and on this account it obtained the name of the 
zoeather-g/aj's, as it did that of barometer from its being the 
rneafure of the weight or preffure of the air. We (hall 
now (fate its various forms and ufes. 
The Common Barometer. This is reprefented at 
fg. i, in the annexed Plate, and was the invention of Tor¬ 
ricelli. A B is a glafs tube, of a quarter, or one-third, or 
half an inch wide, the more the better, and about thirty- 
METER. m 
four inches long, being clofe at the top A, and the open 
end B immerfed in a bafon of quickfilver C D, which is 
the better the wider it is. To fill this, or any other baro¬ 
meter, take a clean new glafs tube, of the dimenfions as 
above, and pour fome purified quickfilver into it, with a 
fmall funnel either of glafs or paper, in a fine continued 
flream, till it wants about an inch of being full ; then, 
flopping it clofe with the finger, invert it flowly, and the 
air in the empty part will alcend gradually to the other 
end, collecting into itfelf fuch other fmall air bubbles as 
unavoidably get into the tube among the mercury, in fill¬ 
ing it with the funnel : and thus continue to invert it fe¬ 
veral times, turning each end alternately upwards, till all 
the air bubbles are collected, and brought up to the open 
end of the tube, and till the part filled fhall appear with, 
out fpeck, like a fine polifhed (teel rod. This done, pour 
in a little more quickfilver, to fill the empty part quite full, 
and fo exclude all air from the tube: then, flopping the 
orifice again with the finger, invert the tube, and immerfe 
the finger and end thus flopped into a bafon of the fame 
purified quickfilver; in this pofition withdraw the finger, 
then will the mercury defcend in the tube to fome pface 
as E, between twenty-eight and thirty.one inches above 
that in the bafon at F, as thefe are the limits between 
which it always (lands in this country on the common fur- 
face of the earth. Then rneafure, from the fur face of the 
quickfilver in the bafon at F, twenty-eight inches to G, 
and thirty-one inches to H, dividing the fpace between 
them into inches and tenths, which may be marked from 
a fcale placed againll the fide of the tube ; and the tenths 
are fubdivided into hundredth parts of an inch by a Aiding 
index carrying a vernier or nonius. Thefe three inches, 
between twenty-eight and thirty-one, fo divided, will an- 
fwer for all the ordinary purpofes of a ftationary or cham¬ 
ber barometer ; but forexperimentson altitudes and depths, 
it is proper to have the divifions carried on a little higher 
up, and a great deal lower down. In the proper filling and 
fitting up of the barometer, feveral circumftances are to 
be carefully noted ; as, that the bore of the tube be pretty 
wide, to allow the freer motion of the quickfilver, with¬ 
out being impeded by an adhefion to the fides; that the 
bafon below it be alfo pretty large, in order that the fur- 
face of the mercury at F may not fenfibly rife or fall with 
that in the tube ; that the bottom of the tube be cut off 
rather obliquely, that when it reds on the bottom of the 
bafon there may be a free paffage for the quickfilver; and 
that, to have the quickfilver very pure, it is bed to boil it 
in the tube, which will expel all the air from it. This 
barometer is commonly fitted up in a neat mahogany cafe, 
together with a thermometer and hygrometer, as repre. 
fented at Jig. 13. 
As the fcale of variation is but fmall, being included 
within three inches in the common barometer, feveral con¬ 
trivances have been devifed to enlarge the fcale, or to ren¬ 
der the motion of the quickfilver more feniible. Defcartes 
fird fuggeded a method of increaling the fenfibility, which 
was executed by Huygens. This was efteefed by" making 
the barometrical tube end in a large cylindrical vefiel at 
top, into which was inferted alfo the lower or open end of 
a much finer tube than the former, which was partly filled 
with water, to give little obdruidion by its weight to the- 
motion of the mercury, while it moved through a pretty 
long fpace of the very fine tube by a fmall variation of the 
mercury below it, and fo rendered the fmall changes in 
the date of the air very fenfible. But the inconvenience 
was this, that the air contained in the water gradually 
difengaged itfelf, and efcaped through into the vacuum 
in the top of the fmall tube, till it was collected in a body 
there, and by its eladicity preventing the free rife of the 
fluids in the tubes, fpoiled the indrument as a barometer. 
And this, it may be obferved, is the reafon why a water 
barometer cannot fucceed. This barometer is reprefented 
in fig. 2, where C D is the velfel, in which are united the 
upper dr fmall water tube A C, with the lower or mercu¬ 
rial tube C 13 . To remedy this inconvenience, Huygens 
thought 
