TEMPERATURE OF SCOTTISH LAKES 
97 
(3) its latitude, (4) its orientation, (5) its surroundings, and (6) its 
general shape. 
(1) The Effect of Depth and Superficial Area. — The part 
which the size of a lake plays in the determination of the temperature 
of its waters may easily be understood by imagining two lakes situated 
side by side, and thus subjected to similar external influences. In 
the first place consider the case of two lakes of the same surface 
area, but of unequal depths. For present purposes it may be assumed 
that the only channel by which heat enters or leaves a lake is by the 
surface, so that the two lakes which are to be considered have equal 
opportunities of gaining or losing heat. But if one lake is deeper 
than the other it contains a greater quantity of water, the tempera- 
ture of which will be raised or lowered by the heat entering or leaving 
the lake by the surface. A concrete example will illustrate my 
meaning. Assume that the lakes have rectangular basins and that 
the deeper of the two is 400 feet deep and the shallower 200 feet 
deep. Further, assume that at the time when they begin to gain 
heat the waters of the deeper lake have a uniform temperature of 
40° and the waters of the shallower lake a temperature of 37'. By 
the time the water of the deeper lake has an average temperature 
of, say, 50°, there must have entered through the surface 10 V units 
of heat, where V represents the volume of the lake. As the surface 
of the shallower lake has the same area, the same quantity of heat 
should have entered its waters ; but as the volume of the shallower 
lake is only V/2, its mean temperature will have been raised 20° by 
the lOV units of heat which have entered at the surface. That is 
to say, its mean temperature will be 57°. This example is very 
crude and has not much reference to actual facts, but it is suflicient 
to show the importance of depth in determining the temperature of 
a lake. In the same way a rough measure of the importance of surface 
area can be obtained. Consider two lakes similarly placed and of equal 
volume, but the surface area of the first double that of the second. As 
the heat is supposed to enter solely by the surface, the first lake will 
receive twice as much heat as the second ; and as the volume of the 
two lakes is the same, a rise of 20° in the mean temperature of the 
first lake would correspond to a rise of 10° in the mean temperature 
of the second. This is, of course, an extreme case, but it serves 
to illustrate the importance of surface area in determining the 
temperature of a lake. 
The size of a lake has another effect, which is well illustrated by 
some of the observations in Scottish lakes referred to at the end of 
this paper (see page 135). The longer and the straighter the axis of a 
lake is, the more effect has a wind blowing along its surface in mixing 
the water and producing currents. The ratio of depth to length in a 
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