\ 
Donovan — On a Comparable Hygrometer. 477 
Avhich, by absorbing moisture, increases in weight or bulk; or in some 
way alters its state of physical existence. To this end, animal, vege- 
table, earthy, or metallic substances have been employed. The variety 
of the materials and contrivances is an acknowledgment of the import- 
ance attributed to the subject. 
Of these different materials I have selected, as the hygroscopic 
medium, the string manufactured from the intestines of an animal, 
which being twisted and dried for the purposes of the arts, in that 
state arsorbs moisture from damp air, or gives it out in dry air, in both 
cases developing, in contrary directions, the force called Torsion, which 
is the agent in the instrument now to be described. 
It consists of a graduated circular brass plate, two inches in 
diameter, supported on a pillar and foot, and carrying a perpendicular 
stem four or five inches long, half of which slides up and down in a 
tube fixed perpendicularly to the edge of the circular plate, and may 
be held at any required height by a clamp-screw. (See PI. XXV., 
Science.) 
At the top of the stem, at a right angle, is a cross-bar holding a 
spring to which is attached one end of the gut string, the other end 
being connected with a silk string: this latter, passing downward 
through the centre of the graduated plate and pillar, is rolled round 
an adjustable tightening-pin acting underneath the foot. The junction 
of the gut to the silk string is efi'ected by a brass coupling joint 
holding a horizontal index, which, by the torsion of the gut stria g, 
points to the graduation of the plate, which it nearly touches, and 
indicates the degree of moisture existing in the atmosphere. Eut the 
torsion force which acts on the gut string, being not equipollent 
throughout, the degrees are not all of equal value. A shallow groove 
or channel traverses the edge of the circular plate, the use of which 
will appear hereafter. The circular plate shall henceforward be called 
the Dial : it is graduated into 100 degrees. 
Such is the general appearance of this instrument, in the principle 
of which there is so far little originality. I must now enter into par- 
ticulars relative to its parts ; and first, with regard to the gut string, 
the most important of all. I have tried a number of such strings, 
amongst others the gut strings of musical instruments — such as are 
not covered with wire, and selected from a number of violin, harp, and 
guitar strings a size that answers my purpose fully. As sold in music 
shops, these strings sometimes contain oil ; this must be removed by 
gentle rubbing with soft cotton wool ; forcible stretching is to be 
carefully avoided, as such would destroy or greatly impair the hygro- 
scopic power. The gut string being cut to the proper length, one end 
is to be connected with the end of a thin string consisting of a few 
fibres of floss- silk, the connexion being made by two minute conical 
pieces of brass, which screw together, base to base, leaving a cavity 
in their substance. A very small hole is drilled through the apex of 
each, one barely sufficient to admit the gut string, and the other the 
silk. The screw on one of the conical pieces is received into the other,. 
