Oil the Hygrometer. 



980 



e^5 



i)4.25 



«8.25 



80.0 



70.25 



83.0 





74.5 



71.5 



69.75 



67.5 



09.5 



Q-SH5 



19.75 



16.75 



10.25 



8.75 



13.6 





&ao 



5a 



ii 



67.3 



64.8 



58.25 65.3 



58.75 62.7 



t 



S 



L 



9.3 



66.5 



6.6 



66.0 



7.05 66.5 



4.05 66.5 



59.0 65.3 



6.3 67.5 



•8 



5.0 



4.6 



4.9 



4.7 



5.2 



Ii 



14.7d 



12.15 



6.35 



4.05 



&3 



According to these experiments, if the temperature of the 

 dry and wet thermometers exposed to the sun be respectively 

 denoted by T and T'; then the ratio of T — T' to <— ^ be- 

 comes greater as T — t increases. This, it is presumed, is what 

 Dr I^ardner intended, and not, as Mr Meikle supposes him to 

 mean, that T' is absolutely less than f . It may also be con- 

 cluded from these observations, that T — T' is Bot proportional 

 to j^, — •^,, but that it is more nearly so as T — t decreases. 

 The temperatures T' and If differ remarkably in this respect, 

 that the latter is not affected by a current of air, while the 

 former is considerably reduced by fanning the wet bulb for a 

 few seconds. This might have been anticipated ; for, in the 

 former case, the increased circulation of the air, while it pro- 

 motes evaporation, provides exactly in the same proportion the 

 requisite heat ; whereas, in the latter case, the source of heat 

 being constant, the cold increases with the rate of evaporation. 

 Besides, when T' is greater than t, the air abstracts instead of 

 contributing heat, and that in proportion to the quickness of its 

 circulation. 



Before concluding this rather desultory paper, the writer will 

 make one remark regarding the process of evaporation. Ac- 

 cording to Dr Dalton, its intensity varies as y — /* under si- 

 milar circumstances. Now, if, as has been supposed, the air in 

 contact with the wet bulb of the thermometer is saturated at the 

 temperature ^, there seems no reason to doubt that it is also sa- 

 turated at ty when exposed to the surface of a body of water of 

 that temperature. But it is evident, that, supposing the dew- 



