414 
Proceedings of the Royal Society 
colder and lighter overlying water. After some hours even this 
surface current became so weak as not to he noticeable. 
Convection currents being thus shown to be incapable of lowering 
the mean temperature of the water in lakes below 39 ‘2°, I shall 
now call attention to a cause which, though not existing and not 
recognised in our laboratories, is yet in constant action in nature, 
and is quite sufficient to account for the thermal disturbance under 
consideration. The great vertical distributor of temperature in 
lakes is in all probability the winds which are constantly blowing 
over them. Previous to the freezing of the surface the winds were 
in constant action, producing currents in the water, which gave rise 
to the peculiar distribution of temperature we find after the lake is 
covered with ice. I have shown in a previous paper* that the great 
wind-driven currents of the ocean have but little influence on the 
vertical distribution of temperature of the oceanic waters. The cir- 
cumstances are, however, quite different in lakes. In the ocean the 
wind only affects one part of the surface of the water, and gives rise 
to a circulation the greater part of which is horizontal , the return 
current coming bacb. at a part of the ocean unaffected by the wind. 
But in a lake the wind blows in the same direction all over the lake, 
and the return current cannot come back on the surface, it is there- 
fore compelled to sink and return underneath the surface wind-driven 
current. The cold surface water is thus compelled to sink and carry 
its low temperature into the depths of the lake. When we consider 
that water of a few degrees above and of a few degrees below 39' 2° 
will have almost the same density, we will see that difference of 
density will oppose but little resistance to these wind-driven currents. 
And under almost all circumstances the “head” produced by the 
wind will be greater than that due to difference of density. 
The temperatures as given by Mr Buchanan, so far as they go, are 
entirely in support of this wind theory. For instance, Linlithgow 
Loch is much smaller and better sheltered from the wind than Loch 
Lomond, and we find the mean temperature of Linlithgow is higher 
than Lomond, which we would quite expect, as it will have a less 
wind-driven circulation. Again, as Linlithgow Loch is smaller than 
Loch Lomond, the wind-driven surface currents will not be so deep, 
nor the return currents so near the bottom. We would therefore 
* Proceedings of the Royal Society, Edinburgh, 1876-77. 
