MR. W. N. SHAW ON HYGROMKTRIC METHODS. 117 



the moistened-bulh hygrometer. His value for m is not far from the truth, being -j^ 

 for Cent, degrees or -fa for FAIIKKMIKIT." 



Ai-.ioiiN ti -t i -i I i lii- v.-iln.- ..r m iii .-i rariei | of ttnuuAuaam ';> bu wi n MgpimMDti 

 and by those of others. Assuming DE LA ROCHE and BRARD'S vulue of the specific 

 heat of air, he finds m for English inches and for FAHRENHEIT'S degree to be 

 -r = '01149 ; a posteriori he has determined it 



(1.) From experiments on the dew-point . . , . . . ..*.!;. .i ;' j '01151 

 (2.) ,, refrigeration in dry air . . . . V > . . . '01150 



(3.) air once saturated, then wanned '01140 



BOHNENBERGER'S value of the constant in reduced to inches and FAHRENHEIT 

 degrees ia m = '0114, which is practically identical with APJOHN'S values. 



KAMTzt employs a formula which differs only slightly from that given above, and 

 from observations on the Fuulhorn obtains a value '0118 for m when reduced to the 

 same units. 



KUPFFER I gives the value in = '01135. 



PIUNSEP has furnished us with a large number of valuable test experiments in a 

 warm climate. They are already referred to in the table on p. 115. The report 

 continues ; " When we find that Mr. PRINSEP once more coincides with Dr. APJOHN'S 

 numbers, only hesitating whether to prefer ^ to gV for the value of m, we are 

 prepared to admit that this problem is, practically speaking, completely resolved, and 

 this being the case, it is scarcely worth while to disentangle the various imperfect 

 steps by which so happy a consummation has been attained, and the hygrometer 

 rendered as commodious and as accurate as the common thermometer." 



These citations and references will perhaps be sufficient to show what were accepted 

 in England as the well-established facts in hygrometry before REONAULT introduced a 

 degree of accuracy into thermal experimental measurement that necessitated a revision 

 of the experimental work done in all branches of science connected therewith. It 

 would, however, appear that there was some valuable work done on the Continent, 

 which, if known in England, had not found its way into the summaries of the subject 

 that I have been able to consult. 



There is a very good account of the state of hygrometritfd science in the second 

 volume of the ' Corso Elementare di Fisica Sperimentale,' by GIUSEPPE BELLI, 

 published in 1831. The various rough methods (including DE LA RIVE'S sulphuric 

 acid arrangement) are described, and their weak points are thoughtfully laid bare. 

 DANIELL'S hygrometer is examined, and the reasons which may make its indications 



* ' Brit. Assoc. Report,' 1840, p. 98 (note). 



t ' Poggendorff's Annalen,' vol. 30, 1836, p. 43. 



J ' Bulletin de 1' Academic des Sciences de St. Petersbourg,' vol. 6, No. 22. 



' Journal of the Asiatic Society of Bengal,' 1836. 



