Besults of Some Experiments upon the Bate of Evaporation. 59 



19 inches of water in a year at Paris ; or those of Mr. Townley, wlio 

 by a long-continued series of observations lias sufficiently proved that 

 in Lancashire, at the foot of the hills, there falls about 40 inches of 

 water in a year. Whence it is very obvious that the sun and the 

 wind are much more the causes of evaporation than any internal 

 heat or agitation of the water. The same observations also show an 

 odd quality in the vapours of water, which is that of adhering to the 

 surface that exhaled them, which they clothe, as it were, with a fleece 

 of vaporous air, which once investing it the vapour rises afterwards 

 in much less quantity : which was showed by the small quantity of 

 water that was lost in 24 hours' time, when the air was very still from 

 wind in proportion to what went off when there blew a strong gale, 

 although the experiment was made, as I said, in a place as close from 

 the wind as could well be contrived. For which reason I do not at 

 all doubt that had the experiment been made where the wind had 

 come freely it Avould have carried away at least three times as much 

 as we found, without the assistance of the sun, which might perhaps 

 have doubled it." " 



A few years ago I contemplated a series of experiments upon the 

 variations in the rate of evaporation from a given water surface, 

 under natural conditions of exposure, where any one factor was to 

 be caused to vary at will, w^hile the others remained unaffected. 

 But a satisfactory method of treating the humidity factor indepen- 

 dently has not hitherto suggested itself ; and the site of the station 

 would have made an inquiry into the effect of the wind to a large 

 extent futile. Wherefore the scheme eventually narrowed itself to 

 a consideration only of the effect of the varying temperature of the 

 w^ater. After a fair share of false starts, and a reasonable number 

 of unconvincing results, I managed in the winter of 1896 to devise 

 the following plan of varying the water temperature without altering 

 the other elements : — 



The gauges were ordinary tin mugs of approximately 4J inches in 

 diameter, enamelled a very light green (almost white) inside and 

 out. They were placed on and between bricks just above the 

 ground. They were covered by glass plates one-sixteenth of an 



* " Account of the Evaporation of Water, as Experimented in Gresham College, 

 &c." Phil. I'rans., xviii. Though only bearing indirectly upon the subject of 

 the text, some of Halley's concluding remarks are worth quoting: "This fleece of 

 vapour in still weather hanging on the surface of the water is the occasion of very 

 strange appearances by the refraction of the said vapours differing from that of the 

 common air. . . . And this may give a tolerable account of what I have heard of 

 seeing the cattle at high water- time in the Isle of Dogs from Greenwich, when none 

 are to be seen at low-water (which some have endeavoured to explain by supposing- 

 the Isle of Dogs to have been lifted by the tide coming under it)." 



