Hydrhophic Substances of an Animal and Vegetable origin. 243 
small slips of this substance in the construction of hygrometers ; 
one of which, after about three years almost constant exposure 
In the open air, in situations where it was protected from rain, 
retained its sensibility undiminished. A hygrometer made of 
this substance is perhaps more liable to suffer injury from the 
contact of water in the state of a liquid, such as drops of rain, 
than those made of some other substances. But, on the other 
hand, it is exceedingly sensible ; requires no particular prepara- 
tion ; expands and contracts with great uniformity ; and, if ac- 
cidentally injured, may be repaired without any very great exer- 
tion of art. 
When rendered transparent, by the application of oil, it is 
equally sensible to the variations of atmospheric humidity as be- 
fore that operation ; and, in this state, it is less liable to suffer 
injury from drops of rain coming accidentally into contact with 
it : For, when the interstitial spaces are occupied by oil, the wa- 
ter is prevented from making a lodgment between the fibres. 
The construction of the instrument which I have found to be 
the most preferable, is that of a pulley, to which an index is at- 
tached, in the form of an arm or lever ; the scale, forming the 
segment of a circle, being attached at one side. To each end of 
the sentient slip is affixed, by means of thick varnish or sealing- 
wax, short doubled slips of very thin metal ; one of which is 
connected with a sliding-pin, which moves in the frame, and 
which serves to adjust the index ; the other has attached fo it a 
fine metallic thread, which passes round the pulley, and which it 
moves along with the index, when the sentient part contracts. 
When the latter expands, the index descends by its own gravi- 
ty, and thus the use of a spring or weight is rendered unneces- 
sary. This form of the instrument is much more simple and 
easily constructed than when a weight or spring is employed for 
the purpose of giving the index a circular motion ; and what is 
gained by the latter mode of construction, in regard to appear- 
ance, & c. is more than counterbalanced by its complexity and 
consequent liability to disarrangement. The expence attending 
the construction of all complicated instruments is a material ob- 
jection. 
When the substance from which the sentient part of this hy- 
grometer is formed has been imbued with varnish, its cohesive 
