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XLII. On the Use of the common Thermometer as a Hygro- 

 meter, and Dsscription of a Self-registering Hygrometer, 

 In two Letters from R. G. K s*. 



LETTER I. 



On the Use of the common Thermometer as a Hygrometer. 



Mr. Editor, — It is not my intention to enter into any long or 

 minutedetailofthenumerousinstrumentsthat have been proposed 

 for ascertaining the state of" the atmosphere with regard to mois- 

 ture, or to attempt deciding on the comparative merits of Saus- 

 sure's hair, and De Luc's whalebone. I believe it may be safely 

 affirmed, that a correct, at least a permanently correct, hygrome- 

 ter, never can be constructed on the principle of any such con- 

 trivance, and for this obvious reason : however accurately the 

 instrument may be originally made, it no sooner begins to ope- 

 rate than it begins to change, the alternate expansions and 

 contractions of the substance producing necessarily, however 

 lowly, some derangement in its natural texture. The contri- 

 vance itself may be extremely ingenious ; but from the very na- 

 ture of the materials employed, such hygrometers must be im- 

 perfect, in as much as they are subject to changes, the extent of 

 which it is impossible exactly to appreciate. Now, is it not very 

 strange, that after all the complaints that we have heard among 

 meteorologists, and philosophers in general, about the want 

 of a hygrometer on accurate principles, they should hesitate a 

 single moment about adopting one as simple and accurate as it 

 is elegant and philosophical? I allude to the diflferential ther- 

 mometer of Professor Leslie, which the ingenious inventor has 

 applied, among many other useful purposes, to that of measuring 

 the relative dryness of the atmosphere, and which does so upon 

 principles as fixed and determinate as those of the common ther- 

 mometer. For the sake of such of your readers as may not be 

 conversant with the subject, I shall give a short description of it 

 nearly in the Professor's own words : " It consists of a thermo- 

 meter tube, curved like the letter U, with a hollow ball at each 

 extremity containing air, and holding an intermediate portion of 

 sulphuric acid, tinged with carmine. When these balls are of 

 the same temperature, the liquor will remain stationary ; but if 

 one of the balls be warmer than the other, the !i(iuor, urged by 

 the increased elasticity of the air, will descend proportionally on 

 that side. To measure the difference of heat between the two 

 balls, the whole interval between freezing and boiling water is 



* From Blackwood's Edinburf^h Magazine for July 1817- 



Q 2 divided 



