[ 41 ° ] 
have been fo long overlooked. But I afterwards-ac- 
counted for this, from the difficulties which ! met 
with in the difcovery. The notion of an hygro- 
meter being both complex and unfettled, all the ob- 
ftacles prefented themfelves at once, and this multi- 
tude of ideas exceeded the power of attention. The 
very firft fleps were apt to miflead. On the one 
hand, I looked for an hygrometer with a head full 
of the matters already uled for hygrofcopes, which 
always are more or lefs altered by water j and, on 
the other, the name of humidity was applied to that 
caufe, the effeds of which I wiffied to meafure ; and 
both points of view turned afide the mind from the 
idea of water, as being proper to afford the required 
fixed point in an hygrometer. 
1 7. The firft difficulty had notefcaped me ; but, 
confidered initfelf, it did not appear unfurmountable. 
I was in hopes that a fubftance might be found ca- 
pable of being affeded by the {baking power of 
water, without being altered by it. As the nature 
of this fubftance was to determine not only the form 
of the hygrometer, but alfo the fpecies of the de- 
grees, which were to indicate the different quantities 
of humor, I concluded that my fecond object ought 
to be the difcovery of this fubftance. 
18. In this refearch, I again divided theobjeds, 
by confidering feparately the three kingdoms, viz. the 
mineral, the vegetable, and the animal. The two 
firft offered no fubftance fit for my purpofe, viz. 
none that would obey the impreffions of humor, 
without being altered either by it or by other caules. 
But in the animal kingdom bones drew my atten- 
tion, and ivory, in particular, feemed to poffefs the 
■ f . ' required 
