360 
The common barometer answers sufficiently 
well for most purposes, when the observations 
are made at the same place; but as many of 
these necessarily imply a change of situation, it 
soon became an object of importance to construct 
barometers in such a manner that they might be 
conveyed from one place to another without 
much inconvenience or risk. Barometers of this 
kind, which are called portable barometers, are 
chiefly employed for measuring heights. They 
have assumed, under the hands of different artists, 
a variety of forms. S%gures 4 and 5, Plate VII. 
represent the portable barometer as constructed 
by Mr. Troughton, and first made by him in 1785. 
The greatest peculiarity in this instrument, ac- 
cording to the opinion of this ingenious and 
philosophical artist, consists in the excellent 
manner in which the mercury in the cistern is 
set to the zero of the scale of inches. For this 
purpose a glass cylinder of about 2°5 inches dia- 
meter, and as much in length, contains the mer- 
cury. An external covering of hollow brass, 
terminating in a female screw a little above and 
below the glass, admits male screw pieces, whose 
ends, well leathered, being pressed hard against 
the ends of the glass, prevent the escape of the 
fluid. Near the upper end of the brass cover are 
two slits made horizontally, one before and the 
other behind, exactly similar and opposite to each 
other. At bottom is a screw which, acting upon 
the usual leather bag, forces the quicksilver up- 
wards at pleasure, and by filling every part, ren- 
ders the instrument portable. But the primary 
design of this screw is, to furnish the means of 
adjusting the surface of the mercury in the glass 
cistern, so as to shut out the light from passing 
between it and the upper edges of the slitsin the 
brass cover. This is the mode of adjusting to 
zero; and it follows, that the upper edges of the 
slits must represent the beginning of the scale of 
inches. The frame is entirely made of a brass 
tube, and above the cistern is of about 1:1 inch 
diameter. The first ten inches of the lower end 
is occupied bya thermometer, whose bulb, bent 
inwards, is concealed within the frame. At about 
three inches higher, it attaches to the stand by 
a ving, in which the frame turns round with a 
smooth and steady motion, for the purpose of 
placing the instrument in the best light for read- 
ing off, &c. The actually divided scale com- 
mences at about 15 inches above the zero, and is 
continued as high as 33 inches, and, by the usual 
help of a vernier, is subdivided down to ‘001 of 
an inch. A longitudinal slit, from end to end of 
the divided part, exposes to view the glass tube 
and mercury within it. The whole of this part 
consists of two tubes of brass. In the inside of 
the interior one, slides a cylindrical piece, on 
which is divided the vernier scale, the index to 
which is the lower end of the piece. In taking 
the height of the mercury, this piece is brought 
down so as just to exclude the light from passing 
| between itself and the spherical surface of the 
BAROMETER. 
mercury. The screw at top, although but a short 
one, performs this office in whatever part of the 
scale the vernier piece may be; for it acts upon 
the interior long tube, in the inside of which the 
piece is sustained by friction; and in which it is 
on every occasion to be set by hand nearly. The 
tripod is altogether similar to what Mr. Ramsden 
used for the same purpose as far back, perhaps, 
as the year 1775. It affords, when closed, a safe 
and convenient packing-case for the instrument. 
The structure of the staff-head is curious. The 
principal part is a circle (/%g. 6.) about 75 of an 
inch broad, jointed in three pieces: these, al- 
though they seem in principle to be incapable of 
motion, yet, in practice, produce what is fully 
adequate to the purpose. The three joint-pins 
extend inwards, so as to pass through a circular 
rim, which they hold fast: within this rim is 
hung a similar one by two pivots; and inside the 
latter, at right angles to the pivots, are fastened 
two Y’s or angles, in which the barometer hangs 
by its gudgeons. Thus are brought about, in a 
small compass, the means of extending the legs, | 
of turning the instrument about respecting the 
tripod, and an universal joint, whereon it readily 
places itself perpendicular to the horizon. 
After it was observed that the different heights — 
of the mercury served, in some degree, to indicate 
the state of the weather, many attempts were 
made to enlarge the extent of the barometrical 
scale, in order to measure the smallest variations 
in the weight of the atmosphere. These attempts 
soon gave rise to a considerable variety of baro- 
meters, differing in form from the common baro- 
meter, and whose scales, though less accurate, 
were so much increased in extent, as to point | 
out the most minute changes in the pressure of 
the air. 
We are indebted to Hooke for the wheel-baro- 
meter, which he invented in 1668. This form of 
the barometer, on account of its exhibiting the 
rise and fall of the mercury in a very conspicu- | 
ous manner, is become extremely common. The 
tube is generally concealed in the frame; but, for 
the sake of representing the whole in one figure, 
we have made it to appear in front; it is about 
40 inches long, but six inches of the lower end is 
bent upwards, so as to become parallel to the rest 
of the tube. As an inch of rise of the mercury 
in the longer leg will cause an inch of descent in 
the shorter, the bores being equal, the two sur- 
faces will thus be two inches apart ; and this al- 
teration cannot be effected by a less pressure of 
the air than that which causes 2 inches of rise in 
the Torricellian tube. Hence the range of the 
scale is only half that of the common barometer. 
But this defect is compensated by converting the 
perpendicular motion of the mercury into a rota- 
tory one, and exhibiting it on a circular dial 
plate. For this purpose, a piece of ivory of a bell 
form is made to float on the surface of the mer- 
cury in the shorter leg, having a silk thread fas- 
tened to its upper end, which, passing over a 
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