Surron—Study of Evaporation from Water-Surfaces. 169 
humidity. Some supplementary process, in addition to those of 
difference of vapour-tensions, relative humidity, and wind-velocity 
is evidently required ; and if convection currents are not to be 
postulated, it is difficult to see what is. My observations made 
with the metal pipes do certainly seem to show that the rate of 
evaporation may go on increasing even though the temperature of 
the water-surface be falling, provided that the temperature of the 
air fall faster still; and that the rate may decrease under a rising 
temperature, provided that the air is warmed much more rapidly 
than the water; subject in both cases, however, to the governing 
elements of relative humidity. If future observation should 
confirm this conjecture, the possibility follows that the evaporation 
from the surface of the sea at night may actually be as great as 
or greater than it is by day. It has been stated before more than 
once that the evaporation from a large water area may be nearly 
as great at night as it is by day, because the temperature of the 
water does not vary much in the course of twenty-four hours under 
insolation and radiation. My contention is that the evaporation 
may actually be greater at night, not so much because the water is 
warm as because the air is colder. It would not be easy to prove 
this by direct observation with the evaporimeter and hygrometer ; 
but certainly indirect observation, so far as it goes, is not antago- 
nistic to the idea. To begin with, the sea is somewhat warmer at 
night and colder by day than the air. Then, according to the 
Challenger Report, the relative humidity over the open sea has its 
maximum value about 2 a.m., and not just before sunrise, when the 
air is coldest ; while the sky is most clouded at sunrise. ‘his is 
just what ought to happen supposing filaments of moist air from 
the surface to ascend more and more vigorously as the difference 
of temperature between air and sea became more marked, and to 
be replaced by descending filaments of drier air. Naturally, the 
hygrometer would not show that more water was being evaporated 
because the accumulation of vapour would occur in the higher 
levels. 
Davis’s Elementary Meteorology contains the following inte- 
resting statement :—‘“‘It is supposed that the energy needed in 
evaporation of water is expended in overcoming the attraction 
that exists between the molecules while the water is in the 
liquid state. The supply of energy to do this hidden work often 
SCIENT. PROC. R.D.S., VOL. XI., NO. XIII. xX 
