272 Prof. Apjohn on a Formula for ascertaining 



ters, and as soon as the instrument with moistened bulb 

 ceased to fall, the temperatures of both were noted. 

 ■ The following Table comprehends the results of five expe- 

 riments thus performed : 



Now, as in all these instances the observed depression dif- 

 fers from the true, this difference, though small, being always 

 on the same side, must be ascribed either to the coefficient m 

 being assumed too great, or to the method of experiment em- 

 ployed not being calculated to afford the extreme depression. 

 That this latter was the real cause of the discrepancy I was 

 disposed to believe from having observed that when the hy- 

 grometer in the course of an experiment became stationary, it 

 could be made to sink a little further by pressing with great 

 force upon the bag of air. In fact, this observation rendered 

 it probable that the tube between the lower and middle bottle 

 of the North did not afford sufficient air-way, and that, there- 

 fore, there was not a sufficient current from behind to propel 

 forward, and immediately remove from contact with the mois- 

 tened bulb, the air which had become saturated with its hu- 

 midity. To bring this conjecture to the criterion of experi- 

 ment it was obviously necessary to operate so that, while the 

 air underwent perfect desiccation, it was at the same time 

 made to pass over the thermometers in a strong and con- 

 tinuous current; and after some trials I found that both ob- 

 jects were secured by substituting for the Nooth a series of 

 three Wolfe's bottles containing oil of vitriol, and connected, 

 as in the process for preparing the water of ammonia, by glass 

 tubes and caoutchouc collars, the bag of air being attached 

 to a tube passing to the bottom of the first bottle, and the 

 thermometers being placed in the axis of a tube perforating a 

 cork inserted into one of the tubulures of the last bottle. 



The experiments recorded in the following table were made 

 with this apparatus. 



