296 Proceedings of Royal Society of Edinburgh. [sess. 
Solar Radiation and Earth Temperatures. By Professor 
C. G-. Knott. (With a Plate.) 
(Read January 21 and February 4, 1901.) 
At a recent meeting of the Society, Dr Buchan read a paper 
based on certain observations of the temperature of the waters of 
the Mediterranean, which had been made by the staff of the 
Austrian ship Pola. These indicated that the direct effect of solar 
radiation was felt to a depth of over 150 feet. At any rate, the 
facts were that the temperature of the upper stratum of water of 
this thickness was perceptibly higher at about 4 p.m. than at 
8 a.m., and that the difference was about 1°*5 Pahr. or 0°*8 Cent, 
at the surface, diminishing fairly steadily to value zero at a 
depth of fully 150 feet or 50 metres. It may easily he calculated 
that this excess of temperature at the afternoon hour means the 
accumulation of an amount of heat equal to 1460 units in every 
column of water 1 square centimetre in section; and this is 
accomplished within the eight hours from 8 a.m. to 4 p.m. It 
must he noted that this accumulation of heat is a daily occurrence. 
The whole process of the heating and cooling of any portion of 
the earth’s surface is a very complicated one. Doubtless there is 
constant radiation into space going on steadily day and night. 
During the day the solar energy enters the atmosphere and part 
of it reaches the earth’s surface, heating the matter there. At 
night this direct heating effect is absent. There must, therefore, 
result a steady periodic state of temperature change, a daily see- 
saw, as much on the average being lost every night as is gained 
every day. This daily fluctuation is of course subject to a seasonal 
variation, depending primarily on the declination of the sun, but 
also, as Langley has shown, on atmospheric conditions, the true 
nature of which is at present a matter of speculation. But what- 
ever these conditions may be, and whatever may he the real 
physical process by which the see-saw of temperature is pro- 
duced in the Mediterranean waters, we must regard this resultant 
accumulation of heat during the day as due to solar radiation, direct 
and indirect. And the first question which demands an answer 
is, what fraction of the whole heat supplied by the sun is repre- 
