200 
common roach of our pools and rivers. 
One of those bladders should be chosen, 
which will contain from two to three 
drachms of pure mercury, and leave 
sufficient space above it to admit the 
tube I, K. When the membrane has 
been sufficiently dried, by exposure to 
the atmosphere of an ordinary room for 
the space of three or four days, pour in 
the mercury, and insert the tube above 
it, so that it comes quite in contact with 
the surface of the metal, or even dips a 
little into it. Now, take a strong, fine- 
waxed twine, and contract the capacity 
of the bladder, by winding the twine 
strongly around the lower end of the 
tube, until the mercury rises midway 
therein. Fix the tube in its proper 
situation, and expose the instrument, 
in this unfinished state, to a dense fog, 
or to the dampest atmosphere that oc- 
curs; or, what will succeed equally well, 
and save time in waiting such an oppor- 
tunity,—fasten it by a cord, and sus- 
pend it in a damp well for the space of 
twelve hours, Either of these methods 
will have the effect of dilating the blad- 
der to its greatest expansible capacity, 
and the mercury will have sunk very 
considerably in the tube. In an exact 
line with the surface of the metal, mark 
_the verge of the scale N, O, which will 
represent extreme damp. Next, let the 
dnstrument be placed in a secure posi- 
-tion, within three or four feet of a 
-moderate fire, until the mercury, from 
attentive observation, is found to rise 
no higher; and, observing the same 
mode as before, draw the line L, M 
parallel to N,O, which will give the 
point of extreme dry. A third line, 
drawn midway between the two, is the 
zero of the hygrometer. 
The scxle, on the left-hand, may now 
be divided and subdivided at pleasure. 
Mine is graduated to 50, both above 
and below the zero; and, as it answers 
admirably, I will recommend the same 
rule to be adopted by others who may 
choose to construct similar instruments. 
The right-hand side of the scale may 
be made to indicate the modifications of 
drought and dampness, as accurate and 
long-continued observation may enable 
the meteorologist to decide. I have 
given mere examples, in the accompany- 
ing’ sketch. 
The material of the scale may be of 
ivory, brass, or slate; or the graduations 
may be engraved on the surface of the 
box-wood of the instrument, A, B,C, D. 
I should prefer the slate, having found 
it to answer this and ‘similar purposes 
extremely well: another advantage it 
Description ofan improved Hygrometer. 
[April I, 
possesses is, that it requires but little 
skill or practice to engrave. | 
If, from long use, or casual injury, 
the fish-membrane decreases in its capa- 
city to contract and dilate (though this, 
I believe, will rarely, if ever, occur, 
excepting after a series of years, when 
it might be replaced by a new one), 
exposure, for a short time, to nitrous 
oxide gas, will speedily restore it to its 
pristine susceptibility. 
The air-bladder of the roach possesses 
a susceptibility of atmospheric change, 
which, I am persuaded, is not evinced 
by any animal membrane; and I have, 
some years since, tried the urinary blad- 
ders, and other fine membrafous parts, 
of most of the smaller animals of this 
kingdom, in my experiments, with a 
view to construct a simple and elegant 
hygrometer :—to which object, united 
to great accuracy, I venture to conceive, 
the instrument I have described makes 
some approximation.—W. H. WeEkes. 
Sandwich, Feb. 12, 1825. 
yin 
yy 
S| 
NM 
