Besults of Some Experiments uimn the Bate of Evaporation. 65 



If we remember that in consequence of the absorption of heat by 

 the green glass, the water-temperature will rise somewhat faster 

 under the less absorptive colours, this result is practically a repetition 

 of the former. Thus, in three different ways, in an excellent spot for 

 receiving the maximum of insolation, and the greatest consequent 

 heating of a gauge, we deduce an increased rate of evaporation due 

 to overheating that looks ridiculously small by the side of some of 

 the magnitudes advocated. It should be noted that observations of 

 the temperatures at night, and very soon after sunrise, proved the 

 temperatures of all the gauges to be equal, and almost uniformly that 

 of the air. We may then enunciate this rule : If the water-surfaces 

 of a numher of similar and similarhj -situated evaporation gauges 

 remain at a temperature in the shade the same for all, then if when 

 fully exposed to tJie sun's rays, under the same elements of climate, 

 they assume different maximum temperatures, the differences in. the 

 depth of water evaporated will be at tJie rate of 1^ inches per annum 

 for each excess of 1° of temperature {approximately). 



The care with which the tests were carried out, and the congruence 

 of the different sets all go to warrant some present confidence in the 

 .accuracy of this rule within the limits of meteorological requirements. 

 Some day I may be able to take the subject up again. 



