Mr. T. Tate on the Laws of Evaporation and Absorption. 331 



magnets being mounted, and the clock once adjusted to time by 

 the pendulum screw, any future change of rate may be counter- 

 acted by advancing or withdrawing the fixed magnet through a 

 very small space. The magnets should not be placed too near 

 together, for which, indeed, there is no necessity, as a quite 

 sufficient amount of action may be commanded even when they 

 are separated by a comparatively large distance. 



Roval Observatory, Greenwich, 

 March 27, 1863. 



XLV. Experimental Researches on the Laws of Evaporation and 

 Absorption. By T. Tate, Esq. 



[Concluded from vol. xxiii. p. 289. J 



On the Cooling Effects of Evaporation. 



1. T\ ALTON determined the amount of evaporation from 

 JLr liquids at different temperatures. Now the object of 

 the present inquiry is to determine the cooling effects produced 

 by the evaporation of liquids at different temperatures. The 

 following law of cooling seems to be fairly established by the expe- 

 riments. 



The cooling effect of the evaporation of liquids, at a constant atmo- 

 spheric pressure, varies as the tension of the vapour at the given 

 temperature multiplied by the latent heat of the vapour at that 

 temperature. 



Thus, let v = the rate of cooling per minute, estimated in 

 degrees of the thermometer at T temperature, p pressure of 

 vapour, and L latent heat of vapour at that temperature; v x = 

 the rate of cooling of the same or any other liquid at Tj tempe- 

 rature, p l pressure of vapour, and L l latent heat of vapour ; 

 then 



v pxh 



Experiment I. A tin canister, clothed with woollen cloth, and 

 filled with hot water, had its mouth closed by a perforated cork 

 through which a thermometer passed into the centre of the hot 

 water. The whole was suspended in the air, and the intervals 

 of time requisite to cool down the thermometer every successive 

 ten degrees were noted, first when the woollen cloth was dry, and 

 second when it was damped with water. In the first case the 

 rate of cooling due to radiation alone was found, and in the 

 second case the rate of cooling due to radiation and evaporation 

 was found, the difference of these giving the rate of cooling due 

 to evaporation alone. The experiments were made when the air 



