Il.—On a New Hygrometer or Dew Point Instrument. By A. ConnetL, Esq., 
F.R.S.E., Professor of Chemistry in the University of St Andrews. 
(Read 3d April 1854.) 
How convenient soever the wet bulb thermometer and the various organic 
hygrometers may be for giving indications, by simple inspection, regarding the 
relative states of dryness and humidity of the atmosphere, it is scarcely possible, 
in conducting meteorological observations, to dispense altogether with instruments 
calculated for affording more direct information respecting the amount of aqueous 
vapour present in the air at any particular time. 
The old methods of Le Ror, Saussure, and Dauron, depending on the cooling 
action of water or saline solutions on glass or metallic surfaces, are always avail- 
able for that purpose; and some years ago I suggested an arrangement on the 
same principle, consisting merely of a little bottle of polished brass, and a small 
thermometer, into the former of which given measures of mixed nitre and sal- 
ammoniac, and of water, were introduced, and the temperature slowly reduced 
by simple agitation, so as to admit of an easy mode of noting the dew point by a 
single operation.* 
The elegant hygrometer of the late Professor DANIELL is sufficiently well 
known. It is a happy application of the ingenious Cryophorus of Dr WoLLasToN. 
I have now to submit to the notice of the Society an arrangement which has 
occurred to me for determining the dew point; and I think it will be found that 
this object may be accomplished by means of it, without much trouble. The me- 
thod proposed has this in common with Mr Dants11’s, that it produces the cool- 
ing effect on the observed surface by the volatilization of ether; but it entirely 
differs from it as regards the manner of removing the obstacles to that volatiliza- 
tion, and of keeping up the process of evaporation. 
It is in no respect a cryophorus, but produces and maintains the necessary 
rarefaction or vacuum, simply by the action of a small exhausting syringe. The 
accompanying figure will explain the nature of the arrangement. 
A isa little round bottle of thin brass, well polished on the outside, and capable 
of holding, when filled to the bottom ofits neck, half an ounce of liquid. Its diame- 
ter is about 135 inch. Its neck is ¢ inch high, and about * inch wide, and flashed 
out a little at top. The passage M, which conducts into the neck, has throughout 
an internal diameter of 1 inch, and it is very essential that it should not be nar- 
rower than this. 
* See Edinburgh New Philosophical Journal for 1835. 
VOL. XXI. PART I. E 
