r 260 ] 
groove of the pulley Ii ten times as much, becaufe it is 
ten times the diameter of G in the groove: and this mo- 
tion will caufe rhe cord I K to turn the pulley L (and draw 
up the weight N) ten times as much as the pulley G is 
turned. So that, if the pannel expands a tenth part of 
an inch, by moilt air, the pulley L will be turned quite 
round: and half round if the pannel expands but a 20th 
part of an inch: As the air grows dry, the pannel con- 
tracts, and the weight M defcends and turns all the pul lies 
the contrary way. 
The back of the plate A A (Fig. 2.) is fcrewed to the 
other fide of the frame (Fig. i.)lo as the ftraight edge of 
the plate may be even with the uppermoft fide of the 
frame, and the center B (Fig. 2.) may be direCtly over the 
center of the pully L (Fig. 1.) on whofe axis the index 
BC (Fig. 2.) is fixed. And as the pulley L is turned by 
the cord IK, the index will be moved on the plate, and 
Ihew the degrees of moiflure or drynefs of the air. 
If the expanfion and contraction of the pannel be fo 
great as to move the index beyond the limits of the degrees 
on the plate, this may be remedied by putting on a larger 
pulley at L. — But if not great enough, in very wet and 
very dry weather, to move the index through all the de- 
grees 011 the plate, the pulley L mult be made lefs in dia- 
meter accordingly. 
A r . B. In three or four years at moll, a new pannel fhould 
be put into the frame: becaufe, when the old one has been fo 
long expofed to the air, it will almoft ceafe to be affected thereby. 
And therefore, a large thick piece of deal Ihould be kept in re- 
ferve for that purpose ; and about the thicknefs of a card always 
planed off that fide from which the new pannel is to be taken. 
At G and M, there muft be fmall knobs of fome hard wood 
glued on the back of the pannel below the graduated plate, to 
make a proper thicknefs for holding the wires upright and fait on 
which the pullies G and M do turn : for othcrvvifc, the wires 
would foon loofen in the pannel. 
2 c 
XLVII. Exfe- 
