14 
Proceedings of the Royal Society of Edinburgh. [Sess. 
to account for the fact that the isotherms are not so close at Fort Augustus 
(the south-west end) on the 13th as they are at the north-east end of the 
loch on the 17th; for at the south-west end of the loch there are large 
rivers entering, and at the north-east end there is the sole outflow of the 
loch. Also, the loch narrows very much at this end, which must accentuate 
the effect of the wind blowing the surface water to that end. 
The experiments which have been described illustrate well the fact that 
the isotherms diverge from the end of the loch towards which the wind 
blows. The discontinuity between the upper and lower layer was always 
very pronounced at this end, and least pronounced at the end from which 
the wind was blowing. Where the return current began, the discon- 
tinuity was sharp ; but in its course it gradually drew with it, and became 
mixed with, some of the denser water of the lower layer. The result was 
that the return current gradually widened out and became denser than the 
water of the upper layer, though less dense than the bottom layer. The 
consequence of this was that, at the end of the loch from which the wind 
was blowing, the change of density with depth was gradual, and there was 
no well-marked discontinuity between the upper and lower layers. 
Another point was illustrated by the experiments. It was frequently 
noticed that the lower isotherms sloped in a different direction from the 
upper isotherms. On first thought it was natural to suppose that the wind, 
in driving the surface water to one end of the loch, would have produced 
a tilt of the isotherms all in one direction. This is very rarely the case, 
but such a distribution has occurred after a very heavy wind. As a rule, 
however, the isotherms arrange themselves like a fan, the upper isotherms 
sloping upwards and the lower isotherms sloping downwards. A density 
distribution corresponding to this was observable in the experimental trough 
when the difference in density between the two layers was not too great. 
The explanation is very simple. The return current which takes place at 
the layer of discontinuity acts at the surface of the lower layer in the same 
manner as the wind-current acts at the surface of the upper layer. It 
produces a slope in the isotherms of the lower layer in the same way as the 
wind produces a slope in the isotherms of the upper layer ; and as the wind- 
current and the return current are in opposite directions, so are the slopes 
of the isotherms in the upper and lower layers in opposite directions. 
Note added 8th July. 
It is a question to what depth the secondary return current of the lower 
layer is appreciable. The secondary return current is of course very much 
