632 HYGROMETR Y. 
end E hang a fponge, or other body, that eafily imbibes 
moifture. To prepare the fponge, it may be proper firft to 
wadi it in water very clean ; and, when dry again, in wa¬ 
ter or vinegar in which there has been diffolved fal ammo¬ 
niac, or fa.lt of tartar; after which let it dry again. Now, if 
the air become moift, the fponge will imbibe it and grow 
heavier, and confequently will preponderate, and draw 
the index towards C ; on the contrary, when the air be¬ 
comes drier, the fponge becomes lighter, and the index 
turns towards A ; thus fliowing the ftate of the air. In 
this hygrometer, Mr. Gould, in the Phil. Tranf. inftead 
of a fponge, recommends oil of vitriol, which grows fen- 
fibly lighter or heavier from the degrees of moifture in 
the air ; fo that, being faturated in the moifteft weather, 
it afterwards retains or lofes its acquired weight, as the 
air proves more or lefs moift. The alteration in this li¬ 
quor is fo great, that in the lpace of fifty-feveii days it has 
been known to change its weight from three drams to 
nine ; and has Ihifted a tongue or index of a balance 
thirty degrees. So that in this way a pair of fcales may 
afford a very nice hygrometer. The fame author fuggefts, 
that oil of fulphur, or campanam, or oil of tartar per deli- 
quium, or the liquor of fixed nitre, might be ufed inltead 
of the oil of vitriol. 
This balance may be contrived in two ways ; by either 
having the pin in the middle of the beam, with a {lender 
tongue a foot and a half long, pointing to the divifions 
on an arched plate, as reprefented in the laft figure. Or 
the fcale with the liquor may be hung to the point of the 
beam near the pin, and the other extremity made fo long, 
as to defcribe a large arch on a board placed for the pur- 
pofe ; as in fig. 6. 
Mr. Arderon has propofed fome improvement in the 
fponge-hygrometer. He directs the fponge A, fig. 7, to 
be fo cut, as to contain as large a fuperficies as poflible, 
and to hang by a fine thread of filk upon the beam of a 
balance B, and exaftly balanced on the other fide by 
another thread of filk at D, ftrung with the linalleft lead 
foot, at equal diftances, fo acljufted as to caufe an index 
E, to point at G, the middle of a graduated arch F G H, 
when the air is in a middle ftate between the greateft 
moifture and the greateft drynefs. Under this filk, fo 
ftrung with fhot, is placed a little table or flielf I, for 
that part of the filk or fhot to reft upon which is not fuf- 
pended. When the moifture imbibed by the fponge in- 
creafes its weight, it will raife the index, with part of 
the {hot, from the table, and vice verfa when the air is dry. 
From a feries of hygrofcopical obfervations, made with 
an apparatus of deal-wood, defcribed in the Phil. Tranf. 
N° 4.80, Mr. Coniers concludes, ift, That the wood 
fhrinks moft in fummer, and fw'ells molt in winter, but is 
mod liable to change in the fpring and fall, 2d, That 
this motion happens chiefly in the day-time, there being 
fcarcely any variation in the night. 3d, That there is a 
motion even in dry weather, the wood fwelling in the 
morning, and fhrinking in the afternoon. 4th, That the 
wood, by night as well as by day, ufually fhrinks when 
the wind is in the north, north-eaft, and eaft, both in 
fummer and winter. 5th, That by conftant obfervation 
of the motion and reft of the wood, with the help of a 
thermometer, the direction of the wind may be told nearly 
without a weather-cock. He adds, that even the time 
of the year may be known by it ; for in fpring it moves 
more and quicker than in winter ; in fummer it is more 
ftirunk than in fpring; and has lefs motion in autumn 
than in fummer. 
Drs. Hales and Defaguliers contrived another form of 
fponge-hygrometer, on this principle. They made an ho¬ 
rizontal axis, fig. 8, having a fmall part of its length cy¬ 
lindrical, and the remainder tapering conically with a 
fpiral thread cut in it, after the manner of the fufee of a 
watch. The fponge is fnfpended by a fine filk thread to 
the cylindrical part of the axis, upon which it winds. 
3 
This is balanced by a fmall weight W, fufpended alfo by 
a thread, which winds upon the fpiral fufee. Then 
w'hen the fponge grows heavier, in moift weather, it de- 
fcends and turns the axis, and fo draws up the weight, 
which coming to a thicker part of the axis it becomes a 
balance to the fponge, and its motion is fh,own by an at¬ 
tached fcale ; and vice verfa-when the air becomes drier. 
Salt of tartar, or any other fait, or pot-afhes, may be put 
into the fcale of a balance, and ufed inftead of the fponge. 
Defag. Exper. Philof. vol. ii. 
The hygrometers which are conftrufted with an oat- 
beard index moving upon a dial, are in general made to 
go one way for moifture, and another for drynefs, the dial 
being numbered or graduated accordingly; but in Forfter’s 
winding oat-beard hygrometer, the graduated circle, 
fig. 9, is numbered completely round, inftead of "half 
round each way. Underneath the figure the parts are 
fhown diftinft. On the top of the oat-beard is cemented 
a circular piece of paper A, on which is fixed a tubular 
piece of ftraw B, which is capped with another piece of 
paper C. D, is a fupport to keep the beard upright, 
made of card-paper. On the ftraw tube, or little cylin¬ 
der, is fattened a piece of fine lilken firing, on which is 
hung a pea, to ferve as a weight to keep the firing 
flretched. As the oat-beard imtwifcs with moifture, the 
index (made of ftraw) moves the fame way round as the 
hand of a watch, and, thus moving, coils the firing round 
the ftraw tube or axis ; by which means the number of 
revolutions from any time obferved may be known ; and 
thus the confufjon will be avoided which is occafioned 
by the index moving more than once round, which it 
does in pafling from extreme drynefs to wet. The firing 
may be fo placed as to wind up when the index moves 
the contrary way, that is, from moift to dry, if the maker 
fo choofes; and in this cafe, the circle mull be numbered 
the contrary way from the above. The oat-beard is 
fixed, and the index is not to be turned or fet to a certain 
point, as is the cafe with the common hygrometer, by a 
contrivance behind the cafe. -The beard of the avena 
Jlcrilis, on account of its fize, is preferable to the beard of 
the common oat. 
The method of keeping a regifter of this hygrometer 
will be thus : When on the upper part of the axis there is 
One coil, and the index points at 6, fet down 1—6 
Two coils 6, 2—6, Sec. 
If the circle be divided into 100 divifions, the reckoning 
will be : No coil, and the hand at 10, fet down 10 
1 coil - - 10 - - - no 
a coils - - 10 - 210, Sec. 
which, if the circle be large enough, will be a very con¬ 
venient mode of regiflering. 
M. de Luc’s contrivance for an hygrometer is very 
ingenious, and on this principle. Finding that even 
ivory fwells with moifture, and contracts with drynefs, 
he made a fmall and very thin hollow cylinder of ivory, 
open only at the upper end, into which is fitted the un¬ 
der or open end of a very fine long glafs-tube, like that 
of a thermometer. Into tnefe is introduced fome quick- 
filver, filling the ivory cylinder, and a fmall part of the 
length up the glafs-tube. The confequence is this: when 
moifture fwells the ivory cylinder, its bore or capacity 
grows larger, and confequently the mercury finks in the 
fine glafs-tube ; and, vice verfa, when the air is drier, 
the ivory contrafts, and forces the mercury higher up the 
tube of glafs. It is evident that an inftrument thus con¬ 
ftrufted is in faftalfo a thermometer, and nrjft neceftarily 
be affected by the viciflitudes of heat and cold, as well 
as by thole of drynefs and moifture; or that it mull aft 
as a thermometer as well as' an hygrometer. The ingeni¬ 
ous contrivances in the ftrufture and mounting of this 
inftrument may be feen in the Phil. Tranf. vol. lxiii. art. 
38 ; where it may be known how the above imperfection 
is corrected hy fome fimple and ingenious expedients, em¬ 
ployed 
