418 Transactions of the Boyal Society of South Africa. 
woven that this way of determining the value of the respective terros 
representing them in the mathematical expression of the so-called law is 
likely to lead to error. At Camden Square, London, for example, "the 
curve of wind-velocity appears to have very little relation to that of 
evaporation."''' At Kenilworth the curve of the monthly quantities 
of evaporation throughout the year follows closely the curve of the 
monthly wind movement. But this agreement is quite illusory, for it 
so happens that the air is dry on the whole in the more windy spring 
months, and relatively damp in the quiet autumn. On the other hand, 
if we compare one day with another, we find that there is no cor- 
respondence between the variations of evaporation and those of wind 
velocity. Here it is the disagreement that is illusory, because an excess 
of wind-movement generally means an increase of the moisture content 
of the air: one therefore neutralising the other.! 
It is with the object of getting a better idea of the value of the wind 
factor in evaporation that the experiments described in this paper were 
begun. They differ from my previous experiments in being made under 
artificial conditions — i.e., inside a room instead of in the open air, and in 
an artificial current of air generated by an electric fan instead of in the 
wind ; they agree with my former experiments only in so far as they 
show the extreme difficulty of handling the determining phenomena of 
evaporating water and water vapour. The method of the experiments 
was to place four silver-plated cups nearly full of water at different 
distances from an electric fan, the plane of rotation of the fan being at 
right angles to the line joining the cups. The cups were the same as 
those used in measuring the evaporation at the bottom of iron pipes of 
different lengths,]: their dimensions being approximately 2^ inches deep 
inside, about 2f inches diameter near the bottom, and about 2|- inches 
diameter near the top, their convenient working capacity being about 
2,250 grains of water. The temperature of the water in each cup was 
noted at the beginning and at the end of each experiment, as well as the 
temperature and moisture of the air of the room, and the velocity of the 
air current as it passed over each cup — this last being measured by means 
of a Davis air meter, reading in feet, placed for half a minute immediately 
in front of a given cup. The duration of each experiment was 50 minutes. 
The cups were numbered for purpose of identification, and placed in a 
different order each day with the idea of eliminating what may be called 
any personal equation on the part of a cup. 
* Mill, British Rainfall for 1907. 
f Sutton, "Results of some Experiments upon the Rate of Evaporation," Trans. 
S.A. Phil. Soc, vol. xiv., 1903. 
I Sutton, " A Contribution to the Study of Evaporation from Water-surfaces," Proc. 
R. Dublin Soc, February, 1907. 
