HYGROMETER. 
method. This philosopher employed, for 
the construction of his hygrometers, a very 
thin slip of whale-bone, which performs the 
same office as the hair in the hygrometer 
of Saussure. He kept this whale-bone bent 
by means of a spring, the action of which 
he preferred to that of a weight : he deter- 
mined the degree of extreme humidity, by 
immersing the slip of whale-bone entirely 
under water ; and to fix the opposite limit, 
which is that of extreme dryness, he made 
use of calcined lime, which he inclosed with 
the hygrometer under a glass bell., The 
choice of lime is founded on this, that the 
calcination having produced a higher de- 
gree of dryness, if it be afterward left to 
cool, so far that it may be placed without 
inconvenience under the glass bell destined 
for the experiment, it will be still found, as 
to sense, in the same state of dryness, since 
it is very slow in acquiring humidity ; and 
thus all its absorbent faculty will be em- 
ployed to dry up, by little and little, the 
air contained under the receiver, and to 
make the hygrometer itself pass to a state 
which approaches the nearest possible to 
extreme dryness. The hygrometer has 
been long neglected in meteorological ob- 
servations ; it is necessary to associate with 
it the thermometer and the barometer, to 
be in a state to unravel the complication of 
different causes which influence the varia- 
tions of the atmosphere ; and it is only by 
the aid of a long series of observations, 
made by these various instruments, toge- 
ther with all the indications which are de- 
duced from the state of the heavens, that 
we can obtain such data as will enable us 
to prognosticate, with great probability, 
the temporary changes, and to arrive at a 
plausible theory upon this object, so in- 
teresting, and so naturally calculated to ex- 
cite our curiosity. 
Mr. Marshall says, that a simple instru- 
ment of this sort may be formed by means 
of “ a flaxen line (large well-manufactured 
whipcord) five feet long ; and having a gra- 
duated scale fixed to an index, moving on a 
fulcrum. The length of the index, from 
the fulcrum to the point, should be ten in- 
ches ; that of the lever, from the fulcrum 
to the middle of the eye, to which the cord 
is fixed, two and a half.” He adds, that 
“ the principle on which this hygrometer 
acts is obvious. The air becoming moist, 
the cord imbibes its moisture ; the line, in 
consequence, is shortened, and the index 
rises. On the contrary, the air becoming 
dry, the cord discharges its moisture, — 
lengthens, — and the index falls. It may be 
true,” he says, “ that no two hygrometers will 
keep pace with each other sufficiently to 
satisfy the curious. He will venture to say, 
however, from seven month’s close atten- 
tion, that two hygrometers, on this simple 
construction, have coincided sufficiently for 
the uses of agriculture. It is true,” he adds, 
“ they diminished in the degree of action ; 
but as the scale may be readily diminished 
in extent, and as a fresh line may be so 
cheaply and so readily supplied, this is not 
a valid objection.” It is remarked, that 
“ this diminution, in the degree of action, 
depends considerably on the construction ; 
the propriety, or rather delicacy, of which, 
rests, almost solely, on this point: the 
weight of the index should be so propor- 
tioned to the weight of the lever and cord, 
that the cord may be kept perfectly straight, 
without being too much stretched. He 
made one with a long heavy index ; and, in 
order to gain a more extensive scale, with 
a short lever; but, even when it was first 
put up, it could barely act ; and, in a few 
weeks, it flagged, and was not able to raise 
the index, though the air was uncommonly 
moist. He therefore made another, with 
the same length, both of index and lever, 
but with a lighter index, and a heavier lever, 
so as to gain the proportion above-mention- 
ed; and it has acted exceedingly well.” 
He thinks that no farmer, “ who wishes to 
profit by the hygrometer, should have less 
than two. Three or four would be' more 
advisable. They would then assist, in cor- 
recting each other ; and, in case of renewal 
or alteration, there would be no danger of 
losing the state of the atmosphere ; which, 
if only one is kept, must necessarily be the 
case. The principle on which this hygro- 
meter is formed, is not, he says, confined to 
a small cord, and an index of ten inches 
long : it may be extended to a rope, of any 
length or thickness, and to an index and 
scale, of almost any dimensions and ex- 
tent.” But one, or more, on a portable 
construction, might, he thinks, be found 
useful. An axe is the form he has thought 
of ; the edge, graduated, will constitute the 
scale ; and the handle will receive the cord : 
this may be hung up, in the shade, exposed 
to the action of the air; or, by means of a 
spike in the end of the handle, it may be 
placed in the open field. By placing it on 
fallow ground, it may be actuated by the 
perspiration of the earth ; among vegeta- 
bles, by vegetable perspiration ; by the 
means of one, or, more probably, by the 
