SECT. III. 
Of the Hygrometer. 
T HAD, for fome time, made ufe of Dr. Hooke's 
^ Hygrometer , made of the Beard of a wild Oat, 
let in a fmall Box, with a Dial-plate and an Index } 
but I foon found an Inconvenience, without the re- 
medying of which no Dependence could be had on 
this Machine j viz. its making more than one Re- 
volution in a Nighr. I endeavoured to remedy this 
by the following Method, deferibed in Tab I. Fig. 6. 
At the vertical Point, from which Moifture and 
Drynefs are graduated, I caufed a fmall Circle to be 
ddcribed; the lower Arch of which fhould juft in- 
terred! with that Arch , round which the Index of the 
Oat deferibed its Circuit. In the Centre of this 
fmall Circle I placed a Fin, eafily turning in the cen- 
tral Cavity, and furnifh’d with a flat Piece of thin 
Ivory on. its Head. This Piece of Ivory , interfe&ing 
with the Index of the Oat, by it was turned either 
to the moift or dry Side of its Graduation, as the 
Index made a double Revolution . I flatter’d myfelf 
with Succefs; but foon found, in the great Fogs we 
had laft Winter, that the wild Oat is not a fafe Ma- 
terial to make an accurate Hygrometer of : For, 
1. In the great Fogs it grew limber ; fo as that the 
Weight of the Index brought it down upon the Plate, 
where its Fri&ion prevented its further Motion, 
2 . It foon lofes its Senfihility , grows harfh, and is 
abfolutcly unfit for Ufe. So I immediately turned 
my Thoughts upon fome other for my Diary , and 
3 referved 
