E 4 I 3 3 
kind of connexion between the fcale of that inflru- 
raent and that of my hygrometer. I foon perceived, 
that, by applying to my hygrometer a thermome- 
trical tube, already graduated by means of two fixed 
points of heat, it would only be neceffary to know 
the proportion of the weights of mercury in this ther- 
mometer, and the hygrometer, to which its tube was 
to be applied, to have in this laft inftrument degrees 
as well determined as in the firft. Scales of a com- 
mon degree of exadtnefs were fufEcient to eftablifh 
between the refpe&ive degrees of both inftruments 
a proportion equal to that of their mercurial weights 
(42 and 43). 
24. Befides the eafe in the execution, this contri- 
vance afforded me a very fimple method to correct 
the effects of heat upon the mercury contained in 
the hygrometer. It is indeed obvious, that, abftraCt- 
ediy of the effects of the humor, the new inftrument 
muft in itfelf be a regular thermometer; and that 
confequently the variations of an adjoint thermo- 
meter were immediately to point out this correction. 
25. Every principle being thus fettled, nothing 
remained but to contrive its conftrudtion. I began 
by making fome experiments, on the nature and 
quantity of the adlion of water upon ivory. I made 
for that purpofe a fmall cylindrical ivory veffel, of 
an inch in diameter, and eight lines in length, and 
reduced its thicknefs to lefs than | of a line. I like- 
wife prepared a wooden cylinder, equal in its dia- 
meter to the internal one of the veffel. I then put 
this veffel into water, in fuch a manner that it only 
wetted it outwardly to the rim. In a very fhort 
time the wooden cylinder, which at firft filled the 
H h h 2 cup 
