the Dew-point by the Wet-bulb Hygrometer, 271 



remarkably close. Indeed, the difference, which is less than 

 three in the fourth place of decimals, is so small that they may 

 be substituted indiscriminately for each other without the oc- 

 currence, at least in ordinary cases, of sensible error. Had 

 values of m been calculated from the comparison alone of the 

 first series of observations in each table, with the subsequent 

 ones, the mean, it is worthy of remark, would be 01156, or 

 almost exactly -£ T ; and as, for such observations, F — /', and 

 D — rf are necessarily greatest, they are best calculated to af- 

 ford correct results, since any error of experiment would ob- 

 viously in their case exercise the least influence. 



The next test experiments performed were suggested by 



the formula itself. If/" =/'- -^ x ^, and/" be sup- 

 posed equal to 0, a condition which can only be fulfilled in 



d d 

 perfectly dry air, /' = — x — 9 an equation from which 



30 

 we deduce d = 87/ x — • Hence by determining experimen- 

 tally the depression of the hygrometer in perfectly dry air, we 

 shall be able to pronounce upon the validity of the general me- 

 thod under discussion. 



The first attempts for determining values of d experi- 

 mentally, consisted in suspending a pair of thermometers, one 

 of which had its bulb moistened, in a close-corked bottle, the 

 bottom of which was covered with a stratum of oil of vitriol ; 

 but this method was soon abandoned, as the depressions it 

 afforded were, on an average, one fifth less than they should 

 be according to the formula. In fact, the extreme depression 

 could not be expected here, for it is obvious that the air in 

 contact with the bulb of the moist thermometer is never per- 

 fectly dry except at the very commencement of the experi- 

 ment. 



The next contrivance to which I resorted was as follows: 

 A bag of India-rubber cloth, furnished with a cap and 

 stopcock, was inflated by a bellows, and thus connected by 

 means of a caoutchouc collar to a glass tube traversing a cork 

 fitted to the tubulure of the lower bottle of a Nooth's appa- 

 ratus. The middle bottle of the apparatus was next filled 

 two thirds with oil of vitriol, and the pair of thermometers 

 last described being introduced into the axis of a small tube 

 perforating a cork fitted to the upper opening of this bottle, a 

 stream of air was forced by pressing on the caoutchouc bag 

 through the oil of vitriol, and, of course, over the thermome- 



