142 Scientifie Proceedings, Royal Dublin Society. 
Now, according to the most popular current theories of 
evaporation, the numbers in these four Tables would indicate 
that the loss by evaporation from each type of gauge must be 
greatest in the afternoon, say from 2to5p.m. Stefan’s formula, 
for example, makes the loss by evaporation proportional to 
(P-p”)/(P-p’) where P is the barometric pressure, p’ the vapour- 
tension at the surface of the water, and p” the vapour-tension at 
the temperature of the dew-point.1 Fitzgerald’s formula makes it 
proportional to{(p’ — p’”) (1 + W/2), where W is the velocity of the 
wind at the surface of the water.?, None of my results, however, 
agree with either formula. Let us consider first of all a set ot 
observations of the evaporation from the tub made upon every 
day during the six months, October, 1905, to March, 1906. The 
following are daily averages for each month :— 
Tasie 5.—(1). Evaporation from the Tub. 
11 p.-8 a. | 8a.-11 a. | 11 a.-2 p.| 2 p.-S p. | 5 p.-S p. | 8 p.-11 p. | 
Oey | O88) HORA 058 059 047 043 1) 
NO, | We | OND | sy || Re 036 -037 | 
Dec., 092 | -050 056 +055 -046 LO |). 
Jan., 075 «| 044 056 | -049 -038 027 Pe 
Feb., 058 | -039 “045 -038 “029 “021 - 
Mar., -050 033 045 «=| +041 031 -018 
Mean, Ms | Oss “058 -050 -038 031 | 
1 See T. Preston, Theory of Heat, p. 291. I have not been able to consult 
Stefan’s memoir at first hand. Hann’s Handbook of Climatology (Prof. R. de C. 
Ward’s Ed.), 1908, p. 71, quotes it differently, i.e. “‘ According to Stefan, the rate 
of evaporation is proportional to the log. of a fraction whose numerator is the air- 
pressure, and whose denominator is the air-pressure diminished by the maximum 
vapour-tension.”’ 
* See C. Abbe, Meteorological Apparatus and Methods, 1888, p. 376. Stelling’s 
formula seems to be similar to Fitzgerald’s, but the coefficients vary with the season 
—which is fatal to it. See H. H. Kimball, ‘‘ Evaporation Observations in the United 
States,’’ Monthly Weather Review, Dec., 1904. 
