1900-1901.] Prof. Knott on Solar Radiation. 
309 
solar constant, namely 6, we get the daily heating expressed in 
calories. The values are given in the third column. 
The particular values of the declination entered in the first 
column are the values at equal intervals of a month. With these 
as abscissae, and with the corresponding values of the energy 
supplied per day, we may construct a curve showing the manner 
in which the heating effect varies from day to day throughout the 
year. The curve is given in the Plate, fig. 3. From this 
curve by estimation of areas we can readily calculate the whole 
amount of radiant energy supplied by the sun during any assigned 
period of time. Thus we find 
Energy supplied during summer months, 114,840 
„ „ winter „ 19,080 
Roughly speaking, the sun supplies during the summer months 
in our latitudes nearly 100,000 units of energy per unit area in 
excess of what it supplies during the winter months. But of this 
amount only 1200 units accumulate in the crust in the form of 
heat. In other words only about 1 per cent, of the energy falling on 
the surface of the earth is allowed to accumulate in the crust of 
the earth as heat. The remaining 99 per cent, escapes by radiation 
and convection or is partly reflected back untransformed into 
heat. This seems to be quite a reasonable result, and contrasts 
markedly with the extraordinary result given in the first part of 
the paper. 
The above estimate is necessarily of a rough character. In this 
country the sunshine which reaches the earth’s surface so as to be 
propagated downwards as a wave of heat is on the average much 
less than what would be in a clear atmosphere similar to that in 
which Langley worked. Consequently the overplus of energy 
supplied in the warmer months of the year is probably over- 
estimated. Then again there is some doubt as to the surface values 
of temperature as deduced from the Calton Hill thermometers, for 
a complete account of which I refer to a paper shortly to be pub- 
lished in the Transactions of this Society by Mr Heath. Had I 
been aware sooner of the fact that Mr Heath was preparing an 
elaborate discussion of the Calton Hill rock thermometers, I should 
not have taken the trouble to make an harmonic analysis of the 
