[ 411 3 
required qualities. I had obferved that the key of 
an ivory cock was tighter or flacker, as there was 
more or lefs humor in the air. Ivory pallets, ufed 
for water-colours, fhewed no alteration, at lead none 
that was lafling. I knew alfo the elafticity of this 
fubflance, which feemed to fecure its coming hack 
to the fame ftate, on its return to the fame degree 
of moiftnefs. 
19. There ftill remained on this fecond head an- 
other object of inquiry, which was almoft neceflariiy 
connected with the third, viz. the fpecies of the de- 
grees to be given to the hygrometer. The beft 
form to be given to the ivory, in order to receive 
with eafe the impremons of the humor, and to have 
its effects meafured upon it, was to be determined. I 
firft thought of ivory rods, the lengthening of which 
fhould be meafured by a machine fimilar to the py- 
rometer. I likewife had fome notion of a large no- 
nius, formed of an ivory and a metallic rod. Either 
of thefe machines would admit of a fixed graduation, 
as both the dimenfions of their parts and proportions 
to one another could be determined. But then I ap- 
prehended that ivory might perhaps, like wood, have 
its longitudinal fibres but little liable to be extended 
by the humor, and that the imperfeftions of thefe 
two kinds of micrometers would occafion a confider- 
able irregularity in the hygrometrical degrees. I 
alfo feared that if ivory rods were made thick enough 
to prevent their bending, fuch a thicknefs might be- 
come an obftacle to their intire penetrability by the 
humor (14). I therefore concluded that the ivory 
fhould have fuch a form, that, though very thin, it 
might not warp} and that the meafurable varia- 
Vol. LXIII, Hhh tions 
