1836.] of the Wet -lulb Hygrometer. 399 



ing its actual state. M. Gay Lussac merely dried his air by chloride 

 of lime, and his depressions will be seen to be all below the mark. 



Professor Apjohn states, that he pressed air from a caoutchouc bag 

 through three of Wolfe's bottles, passing it thrice through the acid 

 on its way to the thermometers. This must have been inconvenient 

 and difficult to regulate, and the knowledge of the real condition of the 

 air was withheld ; although there can be little doubt that it was 

 thoroughly dried. My own method was to dry the air previously for 

 days or even weeks in a large gasometer, whence it could be driven 

 in a very uniform current. The secret of the facility I enjoyed in this 

 respect lay in the substitution of cocoanut oil for water in the reser- 

 voirs of my gasometers, which not only prevented the accession of 

 moisture, but preserved the gas unaltered for any length of time ; — I 

 have fearlessly lighted a jet of hydrogen that had stood two years in 

 my gasometer ! 



There are other modes of obtaining intermediate stages of dryness : 

 the most obvious is by using the atmosphere itself of a dry or damp 

 day, first ascertaining by Dalton's dewpoint experiment its actual 

 hygrometric state, and noting the corresponding indication of the wet 

 bulb thermometer; the averages of a good meteorological register 

 are of this kind. Again, when damp air is artificially heated by pas- 

 sage through a warm tube, the capacity of the warm air for water 

 being increased while the dew point remains unchanged, an effect tan- 

 tamount to using drier air may be obtained and exactly estimated. 

 The rarefaction of air also, (in the absence of the means of fresh 

 supply of water) produces a measurable diminution of the ratio of 

 humidity per given volume. These simple methods have been used 

 by all experimenters, particularly by Leslie himself, the original pro- 

 jector of the evaporation-hygrometer. 



In describing, therefore, my experiments directed to the two main 

 inquiries, it will save some circumlocution to designate the methods 

 pursued as, 1st, dry air current; 2nd, current of air having given 

 aqueous tension ; 3rd, heated air of known tension ; 4th, rarified air 

 do ; and 5th, dew-point comparisons. 



But there are other important branches of inquiry necessary besides 

 the above two, ere we can hope for a formula to satisfy all conditions 

 of the wet-bulb problem. 



III. The experimental effect of diminished and augmented atmo- 

 spheric pressure ? 



IV. The amount of depression in other gaseous media ? and 



V. The effect of greater or less velocity of the air on the tempera- 

 ture of evaporation ? This effect has been sufficiently examined by 



