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 Dobe- 

 reiner, and described in Gilbert's Annalen der Physikf for 1822 ; and the other 

 by Dr Cumming 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 Daniell'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 Dobereiner'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, 

 bat they differ entirely both from Daniell's and from mine. It is only since 

 the paper was given in to the Royal Society, that I noticed in Ganot's Traite 

 de Physique, an account of Regnault'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, i.e., 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 



