TEMPERATURE OF SCOTTISH LAKES 
111 
sunshine and cloud; but to a depth of about 15 feet it was possible 
to distinguish between night and day by means of the records 
obtained. With the bolometer in air the deflection shown on the 
scale of the recorder was 150 times as great as when the bolometer 
was immersed at a depth of 10 feet. 
At first sight these observations in the Lake of Geneva and in 
Loch Ness seem irreconcilable, but this is not necessarily so, for the 
waters of Loch Ness are not so transparent as the waters of the Lake 
of Geneva, partly due to the composition of the water, and partly, no 
doubt, due to particles of matter in suspension in the water, which 
absorb radiant heat. The effect of the sun's rays on the Lake of 
Geneva will also be greater than on Loch Ness, owing to the lower 
latitude of the former, but 20 feet may be taken as an upper limit to 
the depth to which direct radiation is appreciable. 
How then is heat propagated to great depths ? Convection 
currents begin whenever the isotherms are not horizontal, and this to 
a small extent aids the propagation of heat to deeper waters. Con- 
duction also carries the heat gained at the surface from radiation, and 
from contact with the atmosphere, to the lower layers. Weber 
estimated the depth to which heat would penetrate into a lake in the 
course of a year, by conduction solely, at 6 metres. It is thus apparent 
that if the heating were due to radiation, conduction, and convection 
alone, the changes in the course of a year would be limited to the first 
20 or 30 feet of water ; but instead of this, temperature changes can be 
detected even at the bottom of the deepest lakes. 
This is due to some extent to warm waters brought down by rivers, 
which, if they are rendered heavy by matter in suspension, sink down 
into, and mix with, the colder waters of the lake. But the chief agency 
in distributing heat throughout a lake is wind-produced currents, 
which thoroughly mix the surface water, heated by radiation and con- 
duction, with the rest of the water of the lake, and which bring up 
from the depths of the lake cold water to be in turn heated at the 
surface. 
Many local variations of the temperature of the surface are notice- 
able. Owing to the action of winds, the warm water on the surface 
of a lake is carried to the lee end of the lake, with the result that 
a greater quantity of warm water is met with at the lee end than at 
the windward end. 
Heating and cooling of water take place more rapidly near the 
shore than in the deep parts of the lake, because of the greater quan- 
tity of matter kept in suspension by the beating of the waves on the 
shore, and also because the shores themselves absorb radiant heat much 
more rapidly than the water does, and thus help to heat up the shore 
waters. 
