from ilie Cold induced by the Evaporatmi of' Water. 107 



the temperature of 59° 9» contained more moisture than at 46°.l ; 

 and that, had it contained as little, the depression would have 

 exceeded 24°.8. This renders it probable, that the proper 



T> 



multiplier should nearly equal 3q"» or follow the ratio of the den- 

 sity or pressure, agreeably to what we deduced above from theo- 

 retical considerations. In adopting this multiplier, I would of 

 course follow the author of the article in this Journal for Sep- 

 tember last, in applying the correction to the temperature of the 

 air, and not lo that of the moist bulb. But the result so ob- 

 tained would still differ widely from that of Dr Anderson, who, 

 though he uses the same multiplier, yet in effect applies the cor- 

 rection to the temperature of the moist bulb, and not to that of 

 the air, which generally renders the correction too great, and 

 often considerably so. 



Professor Daniel has also given a series of experiments on the 

 rate at which the depression of a wet thermometer varies in air 

 of different densities, [Essays, 2d edit. p. 499). But, in these, 

 the wet bulb was evidently not so fully exposed to the drying 

 influence of the sulphuric acid as in my experiments ; because 

 the Professor not only kept a vessel with water under the receiver, 

 but placed it directly between the wet bulb and the acid. His 

 depressions are therefore much smaller than mine; and they 

 likewise increase at a slower rate, though perhaps both might 

 have observed more nearly the same rate had the temperature 

 of the moist bulb been constant *. 



On another point still we differ. In this Journal for September, 



in both cases ; that is, the excess of the maximum force of vapour for 60°.7, 

 over that for 48^4, equals the excess of the maximum force for 52^.3, over 

 that for 35°.7. This, however, can scarcely be the law of nature; for, if the 

 temperature of the air were lower than 31°, the force of vapour at the dew- 

 point would need to be negative, which looks rather paradoxical. The air 

 was exhausted, through every different pressure, to '15 inch, but the dew- 

 point may be said to have remained the same, at least it did not follow the 

 exhaustion. 



• To increase the cold in a wet thermometer, Dr Lardner recommends 

 exposing it to the sun ; but whoever properly tries this, will find that the 

 sun, as well as the fire, has quite the contrary effect. Nay, it is obviously 

 just the same delusion, only on a greater scale, to expose, as the doctor di- 

 rects, bottles of wine, wrapped in wet cloth, to the sun, as a source of greater 

 cold. 



