478 Proceedings of the Royal Irish Academy, 
and between them is the horizontal index which points to the degrees 
on the graduated dial. The gut string is secured in its cone by a 
knob on its end, which cannot be drawn out of the hole. The silk 
string is secured in the other cone by a knot. The gut-string and 
silk then form one straight continuous line, held together by the little 
brass double cone connexion which keeps them firmly joined when 
the two pieces are screwed together with the index between. This 
is what I shall henceforward call the Compound- line : when in its 
place, the silk portion should be about one third of the length of the 
gut string, which, not including the part above the cross-bar, may be 
8l inches long, or more if found necessary. 
The compound-line is the important and acting portion of the 
whole instrument. The manufacturer of the gut portion, in fitting 
it for its uses, had twisted it while recent into several spiral convolu- 
tions ; after which it was dried. When fitted in the hygrometer for 
use, it absorbs moisture from the atmosphere, shortens a little, untwists 
more or less, and its position being vertical, and having the index 
attached horizontally to its lower end, where it joins the silk and has 
the greatest freedom of motion, it turns the index round over the 
horizontal graduated dial, and thus indicates the degree, of moisture 
which the atmosphere was at that time capable of imparting. This 
it does because the lower end is at liberty, while the upper end is 
confined: I say at liberty, for being rather loosely held by a few fibres 
of floss- silk, scarcely any obstruction is off'ered. 
As the efficiency of the hygrometer depends on the quality of the 
gut string and its management, a minute description will not be 
deemed unnecessary. The gut string employed measured three feet, 
and weighed three grains and a quarter : it is I believe the thinnest 
string in use. The necessary length being cut off, a knob is to be 
formed on one end by a momentary scorching in the flame of a candle. 
This knob is to be softened by immersion in water to the depth of ^ 
inch for half an hour. The gut being drawn through the hole in one 
of the brass cones, the knob will fill up its cavity ; and the silk line, 
knotted at one end, being similarly drawn through the other cone, 
* ' both cones are to be screwed together, and the compound line thus 
formed should be immediately transferred to the frame of the hygro- 
meter, the tightening pin being so regulated as to cause a tension. 
The knob of the gut line will in a few hours become hard, and will 
have assumed such a position in the cavity of the cone as will permit 
the index to traverse the circular groove without obstruction, as if 
turning on an axis. 
The chief points that demand attention regarding the compound 
line, the index and the dial, are as follows: the compound line must be 
central and vertical ; the index and dial must be horizontal and there- 
fore parallel. If these conditions be not accurately fulfilled, the 
instrument must necessarily prove a failure, as will hereafter fully 
appear. 
