Donovan — On a Comparahle Hygrometer. 479 
As a summary : — the compound line passes from the spring at the 
top of the hygrometer through a very small guide-hole in the cross-bar 
down through the centre of the dial-plate : the silk portion passes 
through the pillar and through another guide-hole in the foot, where it 
is secured to the tightening-pin. The spring is necessary for preserving 
a certain tension which ought to be yery slight, and barely sufficient to 
preserve the rectilinear direction of the compound line. If stronger, 
it will interfere with the twist of the gut, and give a false result on the 
dial. 
It will always be a proof that the compound line and index are in 
proper condition, with regard to each other, when, on moving the 
index a little out of its place, it springs back beyond its previous 
situation, and vibrates a few times back and forward over the degree 
on which it is to settle; but not with much force, for that would be a 
proof that its tension is too great, and a false indication would be given. 
This vibration should be always sought as a test that the tension is 
sufficient to prevent the gut line from turning without carrying the 
index. The proper degree of tension can be obtained by the counter- 
acting influence on each other of the spring at the top and the tighten- 
ing-pin at the bottom ; and both of these agents can be so antagonized 
that the index will revolve without touching the groove, or passing 
over the markers when such are used. 
If the compound line and dial be not exactly at right angles, the 
index, in revolving, will occasionally rub the dial and obstruct the 
feeble torsion force acting on the distant end of the index. The suc- 
cess of the instrument depends mainly on the due freedom of the index, 
which has to fulfil auother important function hereafter to be described. 
But the index may be free in another sense which would altogether 
defeat the object of the instrument : if the two inverted cones, which 
contain the knobs of gut and silk, be not screwed so closely as to carry 
the index along with them, the latter will have no effect, and indicate 
nothing. 
It would be the parfection of the instrument if the small additional 
expense were incurred of gilding it, as it might then be occasionally left 
uncovered in the open air, even during a fall of rain or snow — occur- 
rences of no small interest to the hygroscopist. Whether gilt or not, 
it will be proper to append to the instrument a glass receiver ground 
air-tight to a thick glass plate, in the manner of an air-pump. With 
these, many interesting experiments and observations may be made ; and 
they will serve to protect the gut line, when not in use, from the action 
of the weather, as while under the receiver it does not obey the changes 
of the atmosphere. They will also protect the hygrometer from the 
corroding agency of damp and acid vapours, both of which would be 
specially injurious to the necessary polish of the groove. 
The glass plate and receiver are applicable to other and more impor- 
tant services. It may be occasionally found necessary to expose the 
hygrometer to an atmosphere of extreme dryness at a time when per- 
