HYGRO 
ployed in the original conftfuetion and fubfequent ufe of 
the inftrument; in confequence of rvhich, the variations 
in the temperature of the air, though they produce their 
full effects on the inftrument as a thermometer, do not 
interfere with or embarrafs its indications as an hygro¬ 
meter. 
In the Phil. Tranf. for 1791, M. de Luc has given a 
fecond paper on hygrometry. This was chiefly occafioned 
by a Memoir of M. de Sauflure on the fame fubjeft, en¬ 
titled Effais fur I'Hygrometrie, in 4to. 1783. In this work 
M. de S. describes a new hygrometer of his conftnnftion, 
on the following principle. ^ It is a known faft that a 
hair will ilretch when it is moiftened, and contrail when 
dried ; and M. de Sauflure found, by repeated experi¬ 
ments, that the difference between the greateft extenfion 
and contraction, when the hair is properly prepared, and 
has a weight of about three grains fufpended by it, is 
nearly one fortieth of its whole length, or one inch in 
forty. This circumftance fuggefted the idea of a new 
hygrometer. To render thefe variations of the length of 
the hair perceptible, an apparatus was contrived, in which 
one of the extremities of the hair is fixed, and the other, 
bearing the counterpoife above-mentioned, furrounds the 
circumference of a cylinder, which turns upon an axis to 
which a hand is adapted, marking upon a dial in large 
divifions the almolt infenflble motion of this axis. About 
twelve inches high is recommended as the moft conveni¬ 
ent and ufeful; and, to render them portable, a contriv¬ 
ance is added, by which the hand and the counterpoife 
can be occafionally fixed. 
But M. de Luc, in his ldees fur la Meteorologie, vol. i. 
anno 1786, fhows that hairs, and all the other animal or 
vegetable hygrofcopic fubftances, taken lengthwife, or in 
the direction of their fibres, undergo contrary changes 
from different variations of humidity ; that, when immerled 
in water, they lengthen at firft, and afterwards fhorten ; 
that, when they are near the greateft degree of humidity, 
if the moifture be increafed, they fhorten themfelves ; if 
it be diminifhed, they, lengthen themfelves firft before 
they contraCt again. Thefe irregularities, which render 
them incapable of being true meafures of humidity, he 
fhows to be the necefl'ary confequence of their organic 
reticular ftruCture. De Sauflure takes his point of extreme 
moifture from the vapours of water under a glafs bell, 
keeping the fides of the bell continually moiftened; and 
affirms, that the humidity is there conftantly the fame 
in all temperatures; the vapours even of boiling Vrater 
having no other effeCt than thofe of cold. De Luc, on 
the contrary, fhows that the differences in humidity un¬ 
der the bell are very great, though de Sauffure’s hygrome¬ 
ter was not capable of difeovering them ; and that the 
real undecompofed vapour of boiling water has the di- 
reCtly oppofite effeCt to that of cold, the effeCt of extreme 
drynefs 5 and on this point he mentions an interefting 
faCl, communicated to him by Mr. Watt, viz. that wood 
cannot be employed in the fteam-engine for any of thofe 
parts where the vapour of the boiling water is confined, 
becaufe it dries fo as to crack as if expofed to the fire. 
To thefe charges of M. de Luc, a reply is made by M. 
de Sauflure, in his Defence of the Hair-Hygrometer, in 
1788; where he attributes the general difagreement be¬ 
tween the two inftruments to irregularities of M. de Luc’s; 
and affigns fome aberrations of his own hygrometer, which 
could not have proceeded from the above caul'e, but to its 
having been out of order. Sec. 
This has drawm from M. de Luc a third paper on hy¬ 
grometry, publifhed in the Phil. Tranf. This author 
here refumes the four fundamental principles which he 
had fketched out in the former paper, viz. ilt, That fire 
is a fure, and the only fure, means of obtaining extreme 
drynefs. ad, That water, in its liquid ftate, is a fure, 
and the only fure, means of determining the point of ex¬ 
treme moifture. 3d, There is no reafon, a priori, to ex¬ 
pert, from any hygrofcopic fubftance, that the meafurable 
Vol. X. No. 692. , 
M E T R Y. 6 .13 
effects, produced in it by moifture, are proportional to 
the intenfities of that caufe. But, 4th, perhaps the com¬ 
parative changes of the dimenfions of a fubftance, and of 
the weight of the fame or other fubftances, by the fame 
variations of moifture, may lead to fome difeovery in that 
refpedt. On thefe heads M. de Luc expatiates at large 
in this paper, fliowing the imperfections of M. de Sauf¬ 
fure’s principles of hygrometry, and particularly as to a 
hair, or any fuch fubftance when extended lengthwife, 
being properly ufed as an hygrometer. On the other 
hand, he fhows that the expanlion of fubftances acrofs the 
fibres, or grain, renders them, in that refpedt, by far the 
moft proper for this purpofe. He choofes fuch as can be 
made very thin, as ivory, or deal fhavings ; but over all 
he finds whalebone to be far the belt of any, being of 
greater expanfibility than any other fubftance. A flip of 
whalebone will lengthen about one-eighth of itfelf front 
extreme drynefs to extreme moifture. It is a fubftance 
eafy to be cut into flips; and they have been made fo 
fine, as in a length of fix or eight inches to weigh only 
one-tenth of a grain ; on this account it is the moft fuit- 
able fubftance for a common hygrometer. 
At fig. 10 is fliown M. de Luc’s whalebone-hygrometer 
as now made for common ufe. The frame-work is of 
brafs, lightly conftrufted. The flip of whalebone is re- 
prefented by ab, and at its end a is fliown a fort of 
pincers made of flattened bent wire, tapering in the part 
that holds the flip, and preft'ed by a Aiding ring. The 
end b is fixed to a moveable bar, c, wdiich is moved by 
a ferew for adjufting firft the index on the dial ring. The 
end a of this flip is hooked to a thin brafs wire, to the 
other end of which is alfo hooked a very thin lamina, 
that has at that end pincers fimilar to thofe of the flip, 
and which is fixed by the other end to the axis of a pin 
in a proper hole. The fpring, d, by which the flip is 
ftretched, is made of filver-gilt wire; it acts on the flip as 
a weight of about twelve grains. The central pulley or 
axis has very fmall pivots, the fhoulders of which are 
prevented from coming againft the frame by their ends 
being confined, though freely, between the flat bearings 
of two ferews, the fronts of which appear in the figure. 
The dial ring at top is divided into one hundred equal 
parts, fliowing the expanfion of the flip from extreme dry¬ 
nefs, marked D, to extreme moifture, marked M. A 
principal nicety in conftrufting the inftrument is in fo 
proportioning the diameter of the axis or pulley, that 
this whole expanfion may exactly commenfurate the cir¬ 
cumference of the ring, as pointed out by the index. 
The form of the inftrument as now deferibed adapts it 
chiefly for ufe within doors. For expofure at the outfide 
of a window, the flip of whalebone is generally inclofed 
within a half brafs-tube perforated with two vertical rows 
of large holes, and a whole tube is made to go over this 
half one, which has alfo two vertical rows of holes at 
fuch a diftance from one another, that, when one of them 
correfponds to one of the rows of the half tube, the other 
is in front. The rows of holes of the tube are to be 
turned towards the room to prevent the rain from falling 
on the flip ; and, the dial with the axis and index being 
included in a box with a glafs in front, no rain can get 
into the inftrument. It mult alfo be fixed in a place not 
much expofed to the fun, or be fereened from it, without 
preventing the circulation of the air. In the preparation 
of the flip of whalebone, the points of extreme drynefs 
and extreme moiiture are to be carefully afeertained. The 
former M. de Luc directs to be found by means of large 
pieces of quicklime, taken from the kiln and fuffered only 
to lofe the red heat, put into a lime-veflel; and the lat¬ 
ter, limply by immerlion in water. 
The belt lime-veflel M. de Lup conftrufted and con¬ 
trived, is as follows: it conlifts of two tin veflels, the firft 
of which and the moft ufed is 16-’- inches high, 15^ inches 
wide, and five inches deep. The front of this veil'd is a 
plate of glafs, and the back a tin-plate Aider, which, being 
7 V take# 
