Besidts of Some Experiments upon tJie Bate of Ecaporation. 57 



of 94%, it was able to "evaporate" more than three-quarters of an 

 inch in depth per hour ! 



Nearly two centuries before, however, the immortal Halley had 

 attempted in much the same way to acquire " one of the most 

 necessary ingredients of a real and philosophical meteorology," 

 namely a knowledge of the quantity of water passing into the 

 atmosphere in the form of vapour. " I thought it might not be 

 unacceptable," he said, " to attempt by experiment to determine the 

 quantity of the evaporations of water, so far as they arise from heat ; 

 which, upon trial, succeeded as follows : We took a pan of w^ater, 

 about 4 inches deep and 7 '9 inches diameter, in which was placed a 

 thermometer, and by means of a pan of coals we brought the water 

 to the same degree of heat which is observed to be that of the air in 

 our hottest summers, the thermometer nicely showing it. This 

 done we affixed the pan of water, with the thermometer in it, to one 

 end of a beam of the scales, and exactly counterpoised it with 

 weights in the other scale, and by the application or removal of the 

 pan of coals we found it very easy to maintain the water in the same 

 degree of heat precisely. Doing thus we found the weight of the 

 water sensibly to decrease; " the final result being that at summer 

 temperatures the loss by evaporation was " a skin of w^ater" of a 

 depth equal to 0*1 inch in twelve hours.''' 



But a fatal defect in all such laboratory methods is that they do 

 not apply to any meteorological problem : their application is 

 strictly limited to a determination of the number of units of heat it 

 is necessary to ctpplij to a given volume of water in order to evaporate 

 a given portion of it, and so forth. Almost invariably the heat is 

 communicated from beneath, and the whole body of water experi- 

 mented upon must be, in consequence, at nearly the same tempera- 

 ture throughout. Hence each successive surface stratum will be 

 driven off as vapour without appreciably cooling that next below. 

 Under natural conditions of exposure, however, the changes of 

 temperature are' communicated from above, and there may be great 

 differences of temperature at different depths of the body of water. 1 



* Phil. TriDis., xvi., 1G87. " An estimate of the quantity of vapour raised out of 

 the sea by the warmth of the sun ; derived from an experiment shown before the 

 Eoyal Society." I have ventured to translate the given fractions into decimals. 



f This fact has been known for more than a century. See inter alia, " Of the 

 Saltness and Temperature of the Sea," by Bishop Watson, in his charming 

 Chemical i-J.s.sa/y.s', 1781. "On Dr. Hales's Ventilators ; also the Temperature and 

 Saltness of the Sea." H. Ellis. Fliil. Trans., xlvii., 1751. Oar Earth and its 

 Story,!., 331. R. Brown, 1887. "Lakes." J. Y. Buchanan, Kncij. Jh'it., xiv. 

 Ellis was captain of the ship F.arl of Halifax, and is to be considered happy in 

 having made it particularly wholesome and comfortable. His paper is interesting 



