Jx&i 
BAROMETER. 
phere was the cause of the ascent of water 
in pumps; that a column of water 33 feet 
high was a just counterpoise to a column of 
air of the same base, and which extended up 
to the top of the atmosphere ; and that this 
was the true reason why the water did not 
follow the sucker any farther. And this 
suspicion was soon after confirmed by vari- 
ous experiments. See Atmosphere. 
It was some time, however, before it was 
known that the pressure of the air was vari- 
ous at different times in the same place. 
This could not, however, .remain long un- 
known, as the frequent measuring of the 
column of mercury must soon shew its varia- 
tions in altitude ; and experience and obser- 
vation would presently shew that those vari- 
ations in the mercurial column were always 
succeeded by certain changes in the weather, 
as to rain, wind, frosts, &c. : hence this in- 
strument soon came into use as the means of 
foretelling the changes of the weather, and 
on this account it obtained the name of the 
weather-glass, as it did that of barometer 
from its being the measurer of the weight or 
pressure of the air. We may now proceed 
to take a view of its various forms and 
uses. 
The common mercurial barometer, (plate 
Miscel. fig. 9.) or weather-glass, is a cylin- 
drical glass tube, whose diameter is gene- 
rally about |d or J-th of an inch in diameter, 
and length 34 inches, filled with prepared 
mercury; one end of the tube, A, is herme- 
tically sealed, and the open end, B, insert- 
ed into a bason of mercury. The tube and 
bason are fixed to a frame of wood, and sus- 
pended in a vertical situation. The height 
of the mercury in the tube above the sur- 
face of the mercury in the bason is called 
the standard altitude, and the difference be- 
tween the greatest and least altitudes is 
called the limit or scale of variation. 
The mercury in the barometer tube Will 
subside, till the column be equivalent to the 
weight of the external air upon the surface 
of the mercury in the bason, and it is there- 
fore a criterion to measure that weight, and 
chiefly directed to that purpose. In this 
kingdom the standard altitude fluctuates be- 
tween 28 and 31 inches; and from hence it 
is justly inferred, that the greatest, least, 
and intermediate weights of the atmosphere 
upon a given base are respectively equal to 
the weights of columns of mercury upon the 
same base, whose vertical altitudes are 28, 
31 inches, and some altitude contained be- 
tween them. 
The standard altitude 
be tlie 
same, whatever be the diameter of the ba- 
rometer tube; but when this diameter is 
very small, the attraction of cohesion be- 
tween the mercury and glass prevents a va- 
riation of altitude, which ought to be, and 
in larger tubes is, sensible from small dif- 
ferences in the weight of the atmosphere. 
Writers on this subject have given the fol- 
lowing lemma : — If a given line, L, be di- 
re-)- in 
vided into n equal parts, and L x 
be also divided into n equal parts, each di- 
vision of L will be less than that of Lx 
m -f- m , m 
— ~ — by L x 
n n 2 
When L is divided into n equal parts, each 
• 1 ji 
part is equal to L x or L x -? ; and each 
n nr 
, r T n -i- m . 
part of L x — - — , thus divided, is equal to 
T u -4- m 
^ X — r~ , which is greater than the for- 
, -r /« 
mer by L x 
7T 
If each inch of the scale of variation, A D, 
(fig.10.) of a barometer tube be divided into 
ten equal parts, marked with 1 , 2, 3, &e. in- 
creasing upwards, and a vernier or nonius, 
L M, whose length is j^ths of an inch, be di- 
vided into ten equal parts, marked with 1 , 
2, 3, &c. increasing downwards, and so 
placed as to slide along the graduated scale 
of the barometer, the altitude of the mer- 
cury in the tube above the surface of that 
in the bason may be found in inches and 
hundredth parts of an inch by this process. 
If the surface of the mercury in the tube do 
not coincide with a division in the scale of 
variation, place the index of the vernief, M, 
even with the surface, and observing where 
a division of the vernier coincides with one 
in the scale, the figure in the vernier will 
shew what hundredth parts of an inch are to 
be added to the tenths immediately below 
the index. Let, for instance, the surface of 
the mercury be between 7 and 8 tenths 
above 30 inches, and the index of the vernier 
being placed even with it, and the figure 5 
upon the vernier being observed to coincide 
with a division upon the scale, the altitude 
of the barometer will be 30 inches and 
T6oths of an inch : for each division of the 
vernier being greater than that of the scale 
hy tfeth of an inch (lemma,) and there being 
five divisions, the whole must be ^ths of an 
inch above the number 7 in the scale, and 
the height of the mercury is therefore 30.75 
inches. 
mMKmmmm 
■HR 
HP 
