NEW HYGROMETER OR DEW POINT INSTRUMENT. 23 
In the use of the instrument, a little dexterity of manipulation will of course 
be necessary, but this will easily be attained by practice, and the necessary care. 
Success will entirely depend on minute attention being paid to the various parti- 
culars which have been mentioned ; on the perfection of construction of the syringe 
and of its valves; on the various connections being quite air-tight; and on the 
steadiness and security of the clamp employed.* A point apparently unnecessary 
to be noticed, although really very essential and very apt to be overlooked, is that 
the internal diameter of the passage M, connecting the syringe with the bottle, shall 
throughout not be less than inch. 
After I had had the apparatus constructed, I made a Search through various 
books, with the view of discovering whether any similar idea had occurred to any 
one else. The only instances I could find, which would admit any supposition of 
resemblance, were in the cases of two hygrometers, the one contrived by Dosr- 
REINER, and described in Gitpert’s Annalen der Physik+ for 1822; and the other 
by Dr Cummine of Chester, of which an account is given in the Second Part of 
the Article Thermometer and Pyrometer, of the Library of Useful Knowledge. 
Both these instruments differ, however, essentially from mine, not only in arrange- 
ments, but in principle. They agree with Professor Danrext’s and with mine, in 
producing the necessary cold, by the evaporation of ether; but they produce the 
evaporation in quite a different way and on a different principle from either, viz., 
by blowing a current of air through the ether, and so producing and maintaining 
its volatilization. They use a syringe for this purpose, but it is not an exhausting 
syringe, it is a condensing one. They create no vacuum of air; on the contrary, 
they increase the amount of it present. In Dosrreinur’s, also, there is a rather 
complicated series of valves between the condensing syringe and the metallic 
tube, which contains the ether. Both instruments may possibly answer their end, 
but they differ entirely both from Danieiw’s and from mine. It is only since 
the paper was given in to the Royal Society, that I noticed in Ganov’s Traité 
de Physique, an account of ReaNavuut’s hygrometer, which effects the reduction 
of temperature on a similar principle to the two just mentioned, by passing 
through ether a current of air, which is set in motion by means of what chemists 
call an Aspirator, 7.¢., a large gas holder containing air from which there is a flow 
of water. 
My experiments with the instrument have as yet all been made in winter, or 
early spring, but I do not anticipate any additional difficulty in summer. Indeed 
the higher temperature will be favourable to the exhaustion, and consequent cold, 
and in such experiments as those made in a warm room, I have found the results 
quite satisfactory. See also note, p. 9, 
* As window sills are often inclined and uneven, one or two little wedges of wood may be 
employed to produce steadiness of attachment. Pieces of wood may be used to prevent fine tables 
from receiving injury, in attaching the clamp to them. 
+ Zehnterband, s. 135. 
VOL. XXI. PART I. G 
