170 Scientific Proceedings, Royal Dublin Society. 
comes from the sensible heat of some adjacent substance. When 
water evaporates from the sea or from a lake or river, or from the 
wet surface of the land, the energy needed to change its state may 
be derived, in part, from the heat of the adjacent water or land; 
but in the usual case of evaporation proceeding under sunshine, it 
is supposed that the energy of insolation may pass directly to 
the work of overcoming the inter-molecular attractions of the 
water, and thus changing it to the gaseous state, without taking 
the intermediate form of heat. ‘This is illustrated in the 
ordinary experience of a drying day after a rainstorm. The 
surface of the land, everywhere wet from the rain that fell from 
the clouds the day before, is then shone upon by the sun’s direct 
and indirect rays from the clear sky. Instead, however, of there 
being a rapid rise of temperature, there is a rapid drying of the 
ground ; the energy of insolation received upon the surface of the 
ground is expended in changing the state of the water more than 
in increasing the molecular activity of the ground or of the 
water. In the same way the strong insolation absorbed at the 
surface of the torrid oceans is devoted more to causing evaporation 
than to raising the temperature of the water; hence in good part 
for this reason the oceans around the equator are relatively cool.” 
It must be granted that this statement which Davis puts so well 
embodies a most important idea, and is worth careful attention. 
Nevertheless, my own experiments up to the present time do 
not conform very well with the suppositions. I have not been 
able to find that the energy of insolation, as such, and acting 
alone, is capable of evaporating water from either a water- or a 
land-surface. On the contrary, let the sun shine ever so brightly, 
there will not be a great evaporation unless the air be dry. But 
if the air be dry, the evaporation will be comparatively rapid, and 
this will prevent any great rise of temperature. It is well known 
that a porous surface, if kept saturated, will evaporate as fast as 
(perhaps faster than) a water-surface of the same area. In the 
five years, 1900-1904, for example, my observations gave the 
following average comparative quantities of evaporation from the 
tub, and from the Piche atmometer, using the factor ‘00415 for 
reducing the scale readings of the latter to inches :— 
1 W. M. Davis, Elementary Meteorology, 1894, p. 140. 
