102 THE FRESH-WATER LOCHS OF SCOTLAIS^D 
considerable, but he does not seem to have carried his observations 
to any greater depth. In Loch Ness, by means of an electrical 
sunshine receiver used with the Callendar recorder, the sun's rays 
could hardly be detected at a depth of 15 feet, though they were 
quite appreciable at a depth of 10 feet. At a depth of 5 feet it was 
impossible to distinguish between the effect of sunshine and cloud. 
Attempts were also made to detect radiation from the loch during 
the night, but they were unsuccessful. 
The observations show that the direct effect of the sun's rays is 
not appreciable to any great depth, and that they do not directly 
cause the temperature changes which occur and are appreciable even 
at the bottom of the deepest lakes. ^ 
In the shallow water round the shores of a lake, especially where 
the shores are shallow and shelving, the effect of the sun's rays is 
more appreciable than in deep water, as the stones lying on the shore 
and in the shallow water are heated up, and consequently it is a 
common experience to find the shore waters of a much higher 
temperature than the water in the centre of the lake. For a similar 
reason water which contains a large amount of matter in suspension 
will gain more heat through radiation than water which is relatively 
purer. 
(3) Conduction. — There is a certain amount of heat gained by 
conduction from the atmosphere — in fact, this is considered by 
Richter ^ to be the main source of gain or loss of heat by fresh-water 
lakes. The coefficient of conduction of water is 0*074, while that of 
mercury is 0*91. It is thus seen that propagation of heat by 
conduction is extremely slow. But through the action of wind, etc., 
the water heated up at the surface by conduction or otherwise is 
quickly mixed with colder water, so that there is frequently a 
considerable difference between the temperature of the water at the 
surface and the temperature of the atmosphere, favouring a trans- 
ference of heat by conduction. 
There is also a gain of heat through conduction from the earth 
which forms the basin of the lake, but this is an almost negligible 
factor ; it has been calculated that the heat supplied from this source 
would only be sufficient to raise the temperature of a layer of water 
8 inches thick about 1° Fahr. in a year. 
^ See also page 110. See Fitzgerald, Boy. Duhlm Soc. Proc, vol. v. j)p. 169-170, 
1886. He made some observations in Lougli Derg (Ireland) in sunny weather, 
and from a calculation of tlie amount of heat entering the water he thought that 
only about one-fiftieth or less was used in heating it, the rest being probably 
spent in evaporation. He does not state how his calculations were made, but it 
is probable that he neglected the effect of wind and convection currents in 
reducing the temperature gradient at the surface. 
2 See " Seestudien," Geogr. Abhandl., Bd. vi. p. 121, Wien, 1897. 
