1900—1901.] Prof. Knott on Solar Radiation. 
297 
sented by this quantity which gets stored up in the surface waters 
of the Mediterranean % Making a rough calculation, I found that 
this stored-up heat was more than could he reasonably accounted 
for if we accept Langley’s estimate of the solar constant. Ac- 
cording to Langley’s measurements, the solar energy which 
flows every minute normally across a square centimetre of the 
earth’s surface, after a portion has been absorbed by a clear 
atmosphere, is about 2 calories. In other words, if a cubic centi- 
metre of water were set with one face pointing to the sun, and if 
the solar energy crossing that face were all transformed into heat 
within the cubic centimetre of water, the temperature of the water 
would be raised 1° Cent, in one minute. Hence an accumulation 
of 1 460 calories under each square centimetre of the surface means 
that with a steadily vertical sun, and with no loss in other direc- 
tions, the sun would require to shine for 590 minutes, or nearly 
six hours. But six hours of a vertical sun is an impossibility, and 
it is certain that the solar radiation incident upon the face of the 
waters is not wholly transformed into heat within the water. 
A definite fraction is reflected, and a definite amount must always 
be passing out by convection, radiation, emission, and other pro- 
cesses. Taking all these conditions into account, we have great 
difficulty in believing that, between the morning and afternoon of 
each day, heat to the amount of 1460 units can be accumulated in 
the surface waters of the sea, unless we can discover some other 
source of heat than the direct radiation of the sun. 
To make the comparison more complete, I have made a detailed 
calculation of the amount of solar heat supplied to each square centi- 
metre of the earth’s surface in the latitude of the Mediterranean, 
the calculation being based on Langley’s broad results. To make 
an accurate calculation is at present an impossibility; for the 
necessary data are not yet to hand. Langley has shown indisputably 
that selective absorption in the atmosphere makes it impossible 
to treat the absorptive action of the air as a whole. That is to 
say, if the radiant energy of the sun is reduced from E to aE 
after transmission through a given mass of air, we cannot assume 
that it will be reduced to a n E after transmission through n times 
the given mass of air. The assumption may reasonably enough be 
made for each individual ray ; but, since the coefficient of trans- 
