154 Scientific Proceedings, Royal Dublin Society. 
two opposing tendencies will prevail: whether the water will 
evaporate or the vapor condense. A closer consideration shows, 
however, that the former is what should be expected.’ ” 
It will be observed that in each of these replies the question 
as to what will happen, supposing the temperature of the space 
to be raised, and not that of the water, is ignored, and the 
phenomenon treated as if the water and the space above are 
always at the same temperature. Dr. Buckingham does, indeed, 
notice it, but passes it over as an impossible state of things. Now, 
in nature it is seldom that an evaporating surface of water, and 
the space a little above it, are at the same temperature ; in fact 
(as appears from Tables 1 and 2), a small body of water in a 
screen is almost always cooler than air round about. In Kimberley 
it often happens that a warm north wind springs up early in the 
morning, and blows over the surface of the water that has 
cooled considerably during the preceding night; and, therefore, 
the question is a perfectly legitimate one, and not to he passed 
over as unworthy of consideration. 
Returning again to Tables 1 and 2 we see that shortly after 
5 p.m. the temperature of the air and that of the water in the 
metal tubes are about the same; before that time the air is the 
warmer, afterwards it is the cooler. Ii then there be also a 
humidity factor, we should expect, other things being the same, 
that the rate of evaporation would be greater during the day, 
before 5 p.m., and less afterwards. But, as we have seen iv 
Tables 10 and 11, the exact reverse is the case. And we might 
jump to the conclusion that by raising the temperature of the 
space to something higher than that of the water, we should 
indeed cause more of its aqueous molecules to strike the water, 
and become entangled therewith, than before ;1 while, since the 
water would retain for a time its original temperature, the number 
of molecules passing from the liquid to the air would be unaltered. 
We shall see presently that this is not a likely explanation of the 
fact. 
Assuming, for the sake of argument, that the stratum of air 
immediately in contact with a water-surface is completely saturated, 
we have the following quantities of evaporation, &c., corresponding 
to assigned relative humidities of the open air :— 
1 In the open air the aqueous molecules would also fly off in the opposite 
direction ; in a confined space they would not. 
