THE FRESH-WATER LOCHS OF SCOTLAND. 
421 
deeper waters. This return current, however, acts on the water below 
the discontinuity layer just as the current of wind acts on the natural 
surface of the loch, and a secondary current is produced at the surface of 
discontinuity. This secondary surface current is much slower than the 
surface current produced by the winds, but to take the place of the 
water carried along by it there is a secondary return current at the 
bottom of the lake. The secondary return current is very slow, and 
its existence was first suggested to the writer by experiments carried 
out in a glass trough, but observations support the view taken. The 
current systems thus described are shown in Fig. 70. 
Another effect of the separation of the loch into two compartments 
by the surface of discontinuity, is to render possible the temperature 
seiche. The surface current produced by the wind transfers a large 
quantity of warm water to the lee end of the loch, with the result that 
the surface of discontinuity is deeper at the lee than at the windward 
end of the loch. When the wind moderates or ceases a temperature 
seiche is started, just as a seiche is started in a basin of water which has 
been tilted. The temperature seiche was also studied experimentally, 
and was made possible by superimposing a layer of paraffin on a layer 
of water. By driving the paraffin to one end of the trough by a current 
of air, the water, corresponding to the water below the surface of 
discontinuity in a loch, received a tilt, and when the current of air 
was stopped, a seiche started in the lower layer of water independently 
of the upper layer of paraffin. 
The temperature seiche was first described by Mr. E. H. Watson in 
the autumn of 1903, and a good deal of doubt was expressed as to the 
accuracy of his views, but the theory of a temperature seiche was 
established by the observations taken in 1904. For a considerable 
period observations were taken at Fort Augustus every two hours, so 
as to obtain a continuous record of temperature. Fig. 71 is drawn from 
the observations taken in July and August, 1904, and shows the tem- 
perature variations at Fort Augustus at the surface and at depths of 
50, 100, 150, and 200 feet. It will be observed that in July changes 
at the surface, which are chiefly produced by winds, are accompanied 
by similar changes at all depths, but that in August, when the dis- 
continuity layer has been formed, the temperature variations at the 
surface are independent of the variations at a depth of 100 feet, where 
the variations are principally due to the temperature seiche. 
Observations made at the two ends of the loch further support the 
theory, as showing that the layer of discontinuity was in general rising 
at one end when it was falling at the other end. Continuous records 
obtained from the Callendar recorder are also easily explained by the 
temperature seiche. Hough calculations were made of what should be 
the period of this seiche, based on the assumption that the loch contained 
two layers of water of different but uniform density. The observed 
