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III. — On a New Hygrometer or Dew Point Instrument. By A. Connell, 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 Roi, Saussure, and Dalton, 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 Daniell'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 is a little round bottle of thin brass, well polished on the outside, and capable 

 of holding, when filled to the bottom of its neck, half an ounce of liquid. Its diame- 

 ter is about l^o inch. Its neck is £ inch high, and about j§ inch wide, and flashed 

 out a little at top. The passage M, which conducts into the neck, has throughout 

 an internal diameter of \ 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 



