Encyclopaedia Britannica, or, a Dictionary of arts, sciences, and miscellaneous literature : enlarged and improved. Illustrated with nearly six hundred engravings > Volume 13, MAT-MIC

Chap. HI.
Evapora- for thermometers, but with a much wider bore. This,
tion and in one tube, mult be from ^ to the of an inch in
Rain- diameter, and an exad calibre, at leaft not differing
v by T'o between both its extremities. To the end of it
a frnall piece of black enamel is attached, and blown
into an opaque ball, from 4 to of an inch diameter.
The correfponding tube may have its bore of the fame,
or rather a greater width, but its uniformity is not at
all effential. Near the extremity it is fwclled out into
a thin cylinder, almoft To an and from
To to T6o l°ng > inner cavity only being enlarged,
without altering the exterior regularity of the tube.
The Ihort bit of glafs where this cylinder terminates,
is now blown into a thin pellucid ball, as nearly of
the fxze of the former as the eye can judge. The ex-
aft equality of the balls would be unattainable, and
fortunately the theory of the inftrument does not re¬
quire it. When a dark and a bright objeft are viewed
together, the latter, from an optical deception, appears
always larger than the reality ; and for this reafon, fays
Mr Lellie, I prefer making the clear ball a flight de¬
gree fmaller than the black one. In the mean time
a coloured liquor is prepared by diffolving carmine in
concentrated fulphuric acid, in a phial with a ground
Hopper, taking care to avoid heat, as by this the colour¬
ing-matter would be charred, and the beauty of the
liquor deftroyed.
The tubes are now cut to nearly equal lengths, and
the end of each fvvelled out a little, to facilitate their
junftion. Clofe to the black ball, the tube is bent by
the flame of a candle into a fhoulder, fuch, that the
root of the ball fhall come into a line with the inner
edge of the tube. This ball, being then warmed, the
end of the tube is dipt into the acid liquor, and as
much of it allowed to rife and flow into the cavity, as
may be gueffed fufficient to fill both tubes, excepting
the cylinder. The two tubes are then, by the help of a
blow-pipe, folidly joined together in one ftraight piece,
without having any knot or protuberance. About
half an inch from the joining, and nearer the cylinder,
it is gently bent round by the flame of a candle, till
the clear ball is brought to touch the tube ^ inch
direftly below the black one. The inflrument is now
to be graduated ; and the fcale chofen by Mr Leflie is
that which correfponds to the centigrade thermometer.
Mr Leflie thus deferibes the mode of graduating the
inffrument.—The inftrument is held in an oblique
pofition, that the coloured liquor may colleft at the
bottom of the black ball, into which a few minute
portions of air mull, from time to time, be forced
over, by heating the oppofite ball with the hand.
In this way, the interpofed liquid will gradually be
made to defeend into the tube, and affume its pro¬
per place ; and it fhould remain for a week or two
in an inclined pofition, to let every particle drain
out of the black ball. If any trace of fluid collefts in
rings within the bore, they are eafily difpelled with
a little dexterity and manipulation, which, though it
would be difficult to deferibe, is moil readily learnt
and praftifed. The fmall cavity at the joining facili¬
tates the reftification, by affording the means of fend¬
ing a globule of air in either direftion. In fixing the
zero of the fcale, Mr Lellie fet the inftrument in a re¬
mote corner of the room, or partly clofed the window-
719
fhutters. When completely adjufted, the top of the Evapora-
coloured liquor, if held upright, fhould Hand nearly
oppofite to the middle of the cylindrical refervoir.
In this ftate of preparation, the inilrument is ready
for being graduated, i he clear bail and the conti¬
guous part of the parallel tube are therefore covered
with two or three folds of thin bibulous paper, moiilen-
ed with pure water, to make it aft as a hygrometer j
and there is attached to the fame tube a temporary
fcale, by means of a foft cement compofed of bees-wax
and rofin. A flat round piece of wood being provided
with four or five pillars that ferew into it, the inflru¬
ment is fixed to one of them in an ereft pofition, and
on each fide is difpofed a fine correfponding thermome¬
ter, inverted, and at the fame height, the one having
its bulb covered with wet bibulous paper. Then half
a yard of flannel is dried as much as poffible without
fingeing, before a good fire, and rolling it up like a
fleeve, it is lapped loofely round the lower part of the
pillars, and the whole is inclofed under a large bell-
glafs. The flannel powerfully abforbs moiliure from
the confined air, and creates an artificial drynefs of 80
or 100 degrees. In the fpace of a quarter or half an
hour, the full effeft is produced, and the quantities
being noted at two or three feparate times, the mean
refuits are adopted. The defeent, meafured by the
temporary fcale, being then augmented in the propor¬
tion of ten to the difference of the two thermometers,
will give the length that correfponds to too0. After
the ftandard inftrument is conftrufted, others are thence
graduated with the utmoft eafe > the firft being planted
in the centre, and the reft, with their temporary feales,
ftuck to the encircling pillars. For greater accuracy,
the obfervation fhould be made in a room without a
fire, or a fereen ought to be interpofed between the
fire and the apparatus.
The flips of ivory intended for the feales are divided
into equal parts, and fhould contain from loo° to 150°.
The edges are filed down and chamfered, to fit eafily
between the parallel tubes-, and they are fecured in
their place by a ftrong folution of ifinglafs. The
lower ball and its annexed cylinder, are covered with
thin filk of the fame colour as the upper ball, and a
few threads are likewife lapped about that part of the
tube which it touches. The inftrument is laftly ce¬
mented into a piece of wood, either end of which ad¬
mits a cylindrical cafe that ferves equally to proteft or
to hold it. On other occafions, the hygrometer is in-
ferted into the focket of a round bottom-piece where it
Hands vertical.
The above defeription refers particularly to fig. 14.
Fig. 13. differs from this, only in having the balls of an
equal height, and bended in oppofite direftions, which
Mr Leflie confiders as more convenient for fome purpofes
to which the inftrument is applied, to be mentioned
hereafter, but which renders the inftrument lefs portable.
The aftion of this hygrometer depends on the follow- Theory of
ing principle; That the cold produced by evaporation the inltru-
wlll accurately denote the degree of drynefs of the air, ment.
or its di/lance from the point of futuration. To difeover
the drynefs or humidity of the air, therefore, we have
only to find the change of temperature induced in a
body of water infulated, or expofed on all tides to eva¬
poration. The fteps which led Mr Leflie from thefe
Ample
METEOR OLOG Y.
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