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570 PHIPPS’S JOURNAL.
ternally two linesand a half in diameter. It is open only at one end; and the thicknefs
of its fides, for the length of two inches fix lines from the bottom, is but three fix-
teenths of a line. It is this thin part which does the office of an hygrometer; the
remaining part of the cylinder towards its orifice, mult be kept a little thicker, being
deitined tor joining it toa tube of glafs, thirteen or fourteen inches long. This junétion is
effected by means of a piece of brafs, and the whole is cemented together with gum lac.
M. De Lue’s reafon for choofing ivory as the hygrometer, is, that this matter ap-
peared to him more proper than any other for receiving the impreflions of the moifture
of the air, without fuffering thereby any effential change. ‘The cylinder made of it
becomes more capacious, in proportion as it grows moifter. This is the fundamental
principle of the inftrument. M. De Luc has fince found, that, upon letting this cylin-
der lie fome time in water of an uniform temperature, it {wells to a certain point, after
which it dilates no further. ‘Yhis circumftance furnifhed him with a maximum of hu-
midity ; and, confequently, with one point of comparifon in the feale of the hygro-
meter ; and this point he has fixed at the temperature of melting ice. For meafuring
the differences in the capaciiy of this ivory cylinder, and thereby difcovering its different
’ degrees of moifture, M. De Luc makes ufe of quickfilver, with which he fills the cy-
linder, and a part of the communicating glafs tube. ~The more capacious this cylinder
is, or, which is the fame, the moitter it is, the lower does the mercury ftand in the glafs
tube; and vice verfd. Now M. De Luc has found, that the loweft point to which it
can fink, is that where it {tands when the ivory cylinder is foaked in melting ice: he
therefore names this point zero, in the fcale of his hygrometer; and, confequently, the
degrees of this fcale are degrees of drynefs, counted from below upwards, as the quick-
filver rifes in the glafs tube.
‘Yo give thefe degrees a determinate length, and thus render the hygrometers capable
of being compared with each other, M. De Luc employs in conftru@ing them fuch
glafs tubes as have been-previoufly prepared, by being made into thermometers, and
filled with mercury, fo as to afcertain upon them the points of melting ice and boiling
water, and to take exattly the diftance between thofe points by any {cale at pleafure.
‘That done, the bulb of this preparatory thermometer muft be broken, and the quick-
filver it contains exactly weighed. It is by knowing the weight of this, togethere with
the diftance between the fixed points of the thermometer, that the fcale of the hygro-
meter is determined. For inftance, let the weight of the quickfilver be one ounce, and
the diftance between the two abovementioned points, one thoufand parts of a certain
feale: then fuppofe that the quickfilver in the hygrometer, to which this tube is to be
applied, weighs only half an ounce; this will give a fundamental line, confifting of
five hundered parts of the fame fcale. ‘The fundamental line thus found, is applied to
the feale of the hygrometer, beginning at zero, and meafuring it off about four times_
ever, that the whole variation of the inftrument may be comprehended. Each of thofe
{paces being afterwards divided into forty equal parts, gives fuch degrees as M. De Luc
has found moft convenient. In general terms, the length of the fundamental line of
the hygrometer, mutt be to the interval between the two fixed points of the prepara-
tory thermometer, as the weight of the quickfilver in the hygrometer is to the weight
of the quickfilver in that thermometer.
This proportion between the feale of the hygrometer and that of the preparatory
thermometer, furnifhes an eafy method of correcting in this inftrumenggdileseieds of
heat upon the mercury it contains. :
It will eafily be conceived, from the conftruétion of the fcale of this hygrometer,
that if its cylinder of ivory was fuddenly changed into glafs the inftrument would be-
come:
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