1907-8.] The Temperature Changes in Fresh-Water Lochs. 11 
Sir John Murray’s observations in Loch Ness on 26th April 1887, mentioned 
in my previous communication (Trans., vol. xlv., p. 417). 
During the second phase, my opinion is that the return current is still 
appreciable to great depths, but that it is not so evenly distributed through 
the loch, being stronger near the surface than at the bottom. This is, 
however, a mere opinion, in support of which I have no evidence to adduce. 
But this assumption makes it easier to understand the genesis of the dis- 
continuity layer. For if the return current falls off at the bottom and 
becomes stronger towards the surface, finally this and the other influences 
noted on page 7 will cause a gradual separation of the upper and the lower 
waters, and the formation of the discontinuity layer. 
During the third phase, when the discontinuity layer is distinct, the 
return current takes place chiefly along this layer. This is easily illustrated 
experimentally. Starting with a layer of fresh water on the top of brine 
and with a current of wind blowing, if a coloured liquid * is introduced at 
the end of the trough towards which the wind is blowing, it will be seen 
at once to be carried along the surface of separation between the water 
and the brine, leaving a long streak behind it. This return current does 
not extend to the brine at all, but, though at its commencement it is narrow 
and comparatively swift, it gradually widens out as it reaches the other 
* The colouring liquid I used in these experiments to make currents evident was black 
alcohol stain, formed by a mixture of alcohol and lamp-black. 
