C 437 ] 
be neceffary to make frefh experiments, in order to 
determine the length of time that the ivory pipes 
ought to remain in water, and how long they muft 
afterwards be expofed to the viciffitudes of the air (28), 
or in general to what preparation they mu ft be fub- 
mjtted, in order to acquire a lafting degree of con- 
fidence before they are made ufe of. For this pur- 
pofe it will aifo be expedient to compare hygrome- 
ters recently made, with older ones, both to afcer- 
tain whether they have undergone any alterations, 
and in what degree. I likewife am of opinion, that 
when we with to fix the point of extreme humi- 
dity, we muft be very careful not to make ufe of 
any ice but what is very clean, as well internally as 
externally; left any duft fhould ftick to the ivory 
pipes, which might hinder the water from pene- 
trating into the pores : this is what I thought of my- 
felf too late. I do not know whether for the fame 
reafon it would not be right to wafti thefe tubes 
with fpirits of wine before we put them into the 
water, to remove any greafy feurf they may have 
.gathered by handling; and afterwards to repeat this 
at times, in order to carry off any little depofit of 
various kinds, which may in courfe of time have 
been left upon them by the air. Moreover it will 
be right to inquire whether there is not a difference 
between the effects of the heat upon the ivory of 
the hygrometers, and upon the glafs of the ther- 
mometer, fenfible enough to be attended to, in cor- 
reding the effeds of this caufe upon the hygrome- 
ter. 
74. Having already difeovered fo many caufes, 
more or lefs probable, of the differences I obferved 
L 1 1 2 in 
