TEMPERATURE OF SCOTTISH LAKES 
101 
large wedges of water are sometimes found at considerable depths m 
the neighbourhood of rivers of the same temperature as the river- 
water. These wedges gradually flatten out and spread themselves 
between the upper warm water and the colder water below — unless, 
indeed, the rivers bring down quantities of matter in suspension, 
which make the water heavy, and carry it, although perhaps warmer 
than the surface-water, down to considerable depths. 
(2) Radiation. — Radiation is a source of gain of heat which 
aflects the cycle of temperature changes in a lake more directly than 
rivers do, but it is a factor which is frequently overestimated. At 
first sight it may seem that direct insolation is the chief source of gain 
of heat in a lake, but it is not so. Although the sun's rays may be 
perceptible to depths of 100 feet or more (the depth being dependent 
on the nature of the water and the amount of matter in suspension), 
the heat rays do not penetrate to anything like so great a depth. 
There is also a large quantity of heat reflected from the surface, 
especially when the surface is calm and mirror-like. Dufour ^ found 
in the Lake of Geneva that more than one-half of the radiant energy 
received at the surface of the lake was directly reflected. 
Dr Knott has calculated for the latitude of Edinburgh the solar 
energy which crosses unit surface in a given time from insolation, and 
he finds that per square centimetre — 
Energy supplied during summer months = 114,840 gram calories. 
Energy supplied during winter months = 19,080 „ „ 
This gives, say, 134,000 gram calories per square centimetre per 
annum for the solar supply, or for the whole of Loch Ness a supply 
of 7 '2 X 10^^ gram calories. The actual amount of energy stored up 
in Loch Ness, as calculated from the temperature observations, was 
1*9x10^*^ gram calories,^ so that about three-quarters of the heat 
supplied was lost by reflection and radiation from the lake surface, 
and by the various other methods by which a lake loses heat. 
This seems a reasonable result, and it is confirmed by the observa- 
tions in Loch Garry, from which it appeared that five-sixths of the 
heat supplied was lost. 
Dr Knott's calculations were for the latitude of Edinburgh, but 
his values are probably correct within the limits of error for Loch 
Ness and Loch Garry. 
It is a natural question to ask to what depth the sun's rays are 
appreciable. Forel, by means of a black-bulb thermometer, found 
that at a depth of 1 metre the direct effect of sunshine was 
^ See Forel, Le Leman, vol. ii. p. 333, 1895. 
2 See Proc. Roy. Soc. Edin., vol. xxiii. p. 296, 190L 
^ See page 134. 
