(103) 
A LUNAR PEEIOD IN THE EATES OF EVAPOEATION 
AND EAINFALL? 
By J. E. Sutton, ScD., Hon. Memb. E. Met. S. 
Three factors are commonly regarded as fundamental to evaporation, 
namely, the temperature of the water, the humidity of the air, and the 
velocity of the wind. 
Actually, only the first of these three is fundamental. The second is 
subsidiary, acting to check the rate at v^hich the evaporation goes on. 
The third is a perturbation pure and simple ; its only function is to replace 
the damp air over an evaporating surface by other air ; so that if the other 
air be damper, or less damp, than the replaced air, the evaporation will be 
checked, or accelerated, accordingly. 
The present paper is intended to call attention to the possibility of 
another fundamental factor, not, apparently, hitherto suspected ; and 
which, although feeble, may have an important bearing on the kinetic 
theory. 
Briefly put, the object of the investigation here to be described was to 
determine whether a variation of gravity was directly or indirectly accom- 
panied by any appreciable variation of the motion of the molecules con- 
cerned in the evaporation from a water surface. If there be such a relation, 
it cannot be very prominent, and only to be discovered in the results of 
long and careful observation. The existence and magnitude of a tiny 
lunar atmospheric tide has long been known ; and because of this tide the 
barometer rises and falls slightly twice a day ; the corresponding maxima 
of pressure occurring when the moon is near the meridian, a.bove or below 
the horizon. There is doubtless, also, a related very small variation in 
the velocity of the wind the magnitude of which has still to be measured. 
Consequently, we should expect, to begin with, that if the moon has any 
influence at all upon the rate of evaporation, it would be exerted by means 
of the agencies of pressure and wind variation — both too minute, it would 
seem, to be of much account. 
However that may be, the results of the observations with the evapor- 
ating tank at Kenilworth have been examined for the purpose of ascertain- 
ing whether there is any lunar effect at all of any sort. 
A description of the tank, and the way it is used, will be found in the 
