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EVAPOEATION IN A CUEEENT OF AIE. 
(Part 1.) 
By J. E. Sutton, M.A., Sc.D., F.E.S.S.Af., Hon. Memb. E. Met. S. 
(Eead July 21, 1909.) 
The results of the observations upon the rate of evaporation made under 
natural conditions at Kenilworth (Kimberley) with gauges of various 
patterns have always shown a provoking disinclination to conform to 
Fitzgerald's formula — 
E = { A (T - ^) + B (T - ^)- } (I + C W)'^) , 
not only when monthly averages of evaporation, temperature, and wind- 
movement are considered, but also (and particularly) for short intervals 
of time. By natural conditions " is meant the ordinary meteorological 
conditions of observation — as distinguished from the artificial conditions 
of the laboratory — the water being heated by radiation from the sun or by 
conduction from the air. Only in the roughest way do the quantities of 
water evaporated in this way vary as the difference between the vapour 
pressures at the temperatures of the water and the dew-point, represented 
by the quantity T - ^ in the formula. Instead they suggest rather that 
the temperature of the air has first to be taken into account ; that is, that 
the relative humidity of the air is of more importance than the absolute 
humidity. Nor does the evaporation at Kenilworth correspond so simply 
to the movement of the wind as it seems to do at Boston, U.S.A., where 
Fitzgerald's observations were made. 
The main obstacle in the way of discovering the true law of evapora- 
tion is, of course, the difficulty of separating one determining factor from 
another. Consequently it has been the custom to deduce a law" from 
theoretical considerations, and to determine the numerical values of the 
terms in the mathematical expression of the " law " from observational 
averages. But the various evaporation factors are so indissolubly inter- 
