1824.] Remarks upon Mr. Darnell's Work on Hygrometry. 215 



fixed in that position till the obversev changes it, if his eyes be 

 fatigued by the operation, he can leave it as long as he pleases, 

 and examine the accuracy of his work at a future occasion, or 

 have it verified by an assistant. The great condition necessary, 

 is that the faces of the crystals be perfectly plane, which unfor- 

 tunately is not always so easily fulfilled as might be wished. 



Article XI. 



Remark* upon Mr. Daniell's Work on Hygrometry . 

 (To the Editors of the Annals of Philosophy.) 



As none of the Scientific Journals in their notices of Mr. 

 Darnell's work on Hygrometry make any remarks on a part of 

 that work which, if I mistake not, is erroneous, I beg leave to 

 direct the attention of your readers to the subject. 



A principal object with Mr. Daniell, is the application of his 

 improvements to the correction of the barometer when that 

 instrument is used for ascertaining; the heights of mountains : 



O D 7 



but it is not a little surprising that he should have erred in his 

 manner of computing what is commonly the greatest and most 

 important correction required in using the instrument. 



That to which I allude, is the compensation for difference of 

 temperature at the two stations, which Mr. D. considers as a 

 case of apparent dilatation of the mercury, and gives, for the 

 purpose of correction, a table calculated by Mr. Rice, from the 

 results of Dulong and Petit's experiments: now, besides that 

 the last mentioned gentlemen are egregiously wrong in their 

 way of deducing the apparent expansion, it is not only inappli- 

 cable to the present case, but is as a standing number quite use- 

 less for every other purpose, varying as every one knows. 



It has always been understood that, other circumstances being 

 alike, mercury in the barometer will have its altitude affected by 

 the existing temperature in no other way than as that tempera- 

 ture alters its specific gravity ; so that whether the tube expand 

 or contract, or were it possible, do neither, whatever the material 

 • it' which it is made, whatever its sectional form, equality or 

 inequality of calibre, still the absolute dilatation and not the appa- 

 rent must regulate the correction for difference of temperature. 



After a detailed account of the many operations gone through 

 by Mr. Daniell and his fellow-labourer Mr. Newman, while 

 making a barometer for the Royal Society, during which every 

 thing is done to attain accuracy, we are told that "a scale is 

 engraved on the front, of the correction to be applied for the 

 dilatation of the mercury and the mean dilatation of glass, by 

 which the observation may be at once reduced to the standard 



