REGNAULT’S HYGROMETRICAL RESEARCHES. 631 
of the same kind, divested of grease in the same operation, do 
not exactly keep pace, but that nevertheless they do not differ so 
much in most of the observations as to be regarded as not com- 
parable. 
But if we compare in Tables I. and IJ. the numbers indi- 
cated by the hygrometer of Paui with those of Bunten, we find 
such great differences that it is impossible to regard these in- 
struments as comparable, even in observations in which we 
should be content with general approximations. 
It will be objected that the hygrometer of Paul was very old, 
and consequently that the hair was altered. It is impossible 
for me to decide whether this hygrometer presented, when it 
was first constructed, a course identical with that observed at 
present ; but I can state hat this instrument is as delicate and 
as regular in its indications as any of the hygromeiers of mo- 
dern construction which I have had occasion to examine. 
At any rate, this observation alune proves that hygrometers, 
constructed with hairs of different nature and prepared in dif- 
Serent manners, may present very great differences in their indi- 
cations, even when their fixed points agree. 
This inequality in their range depends probably in great 
part on the manner in which the hair is prepared,—perhaps also 
on the length of time it has been used in the apparatus. I have 
ascertained, in fact, that hairs of very different natures, but pre- 
pared in the same operation, do not present any great irregu- 
larities in their range. 
Five kinds of hairs, as different as possible in their colour and 
degree of fineness, were freed from grease in the same operation, 
following exactly the directions of Saussure. They were mounted 
on similar frames, and their fixed points were simultaneously 
determined in the same vessel. Afterwards these instruments 
were placed in a vessel containing successively more or less 
concentrated solutions of sulphuric acid. 
Hygrometers of Bunten. 
The hygrometers 13 and 14 kept pace perfectly ; the hygro- 
