BAROMETER. 
Whatever be the number of divisions in 
the scale of variation, and in the vernier, 
the height of the mercury in the barometer 
is easily discovered by a process similar to 
that already mentioned. 
There are several other kinds of barome- 
ters, of which it will be sufficient to give a 
short description. 
1. In the portable barometer, the lower 
part of the tube is bent upwards, and wider 
than the rest of the tube ; and in this recur- 
vated part the mercury is exposed to the 
pressure of the atmosphere ; or the mercury 
in the bason is contained in a flexible lea- 
thern bag, exposed to the same pressure. 
In this last, the mercury is forced into the 
tube so as to fill it, by a screw fixed in the 
bottom of a wooden box containing the bag, 
lest the motion of the mercury should break 
the tube. 
2. In the diagonal barometer, (fig. 11) the 
scale of variation is bent into the direction 
D R, making an obtuse angle with the verti- 
cal part B D. The scale of variation is by 
this barometer increased in the ratio of 
DR: DA; but this increase does not com- 
pensate for the friction and attraction of co- 
hesion upon the lower side of DR. And 
when the angle RDA is greater than 45°, 
the instrument is rendered useless by the 
separation of globules of mercury from the 
column. 
3. The wheel-barometer, (fig. 12) is a 
compound tube, SERB D, open at D and 
closed at E, the diameter of the highest part, 
S E R, being much greater than that of the 
rest, and filled with mercury from D to S R, 
and above that vacuous. Upon the surface 
of the mercury in the recurved leg there is 
an iron ball in equilibrio with another, H, 
by a string passing over a pully, P. As the 
ball at D rises and falls with the mercury, 
the string t urns the pulley, and an index, I N , 
fixed to it, which points to different parts of 
a graduated circle. It is clear, that by in- 
creasing the diameter of the circle, this con- 
trivance will shew the minutest variations of 
the air, provided the friction be inconsider- 
able, which is seldom true. 
4. The pendent barometer, (fig. 13) is com- 
posed of a tube of a very small bore, a little 
conical or tapering, closed at the smaller 
orifice, A, and filled with prepared mercury 
from A to B, whose distance is equal to the 
greatest altitude, or about 31 inches. Let 
the tube be suspended vertically, and the 
mercury will subside, and be quiescent in 
that part whose length is equal to the 
VOL. I. 
standard altitude at that time ; and sup- 
posing that to )>e the least, it will occupy a 
space F'E equal to 28 inches ; and conse- 
quently A F is the scale of variation. If 
AE = 60 inches, then AF = 32, when in 
the common barometer it is only 3 inches. 
The diameter of this barometer tube is very 
small, and consequently the " attraction of 
cohesion considerable, which prevents the 
freedom of motion necessary to ascertain 
minute variations of the air’s pressure. 
5. In the horizontal rectangular barometer, 
(fig. 14) the highest part of the tube, opposite 
to the scale of variation, is wider than the 
rest of the tube ; and the mercury, descend- 
ing 3 inches from A to 1>, will describe a 
much longer space in the horizontal leg 
F G, these spaces being to each other in- 
versely as the squares of the diameters of 
the tubes, and, that of F G being very small, 
its motion will be extremely sensible. But 
the free motion of the mercury in F G is 
impeded by friction, and the attraction of 
cohesion, which from the smallness of the 
tube is considerable ; and besides this, 
globules of mercury are apt to be separated 
from M, and flow out at G. 
By the above, and other expedients, as 
using water, or water and mercury, the 
scale of variation is enlarged ; but the com- 
mon barometer is the best, being subject 
to the fewest inconveniences. In the con- 
struction and use of it, the following parti- 
culars are to be observed. 1, The diameter 
of the tube should be fd or Jth of an inch, to 
prevent the effects of the attraction of co- 
hesion ; the length of the tube 33 or 34 
inches, with a bulb upon the top, into which 
the air may be diffused, should any remain 
in the mercury. 2. The diameter of the 
cistern containing the mercury should be 
large (at least ten times greater than that of 
the tube) that the addition or subtraction of 
the mercury, contained between the great- 
est and least altitudes, may not sensibly af- 
fect its depth ; for the numbers, marked 
upon the side of the tube, shew their dis- 
tance from a fixed point, and cannot shew 
the height of the column above the mercu- 
ry in the cistern, unless its surface coincide 
with this point, and be immovable. 3. The 
mercury should be free from any mixture 
of other metals, and purged of air by being 
boiled in a glazed earthen vessel, closely 
covered, and poured, when hot, through a 
glass funnel, with a long capillary tube, into 
the barometer tube washed with a rectified 
spirit, and cleaned with a piston of shammy 
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