16 
Proceedings of the Royal Society of Edinburgh. [Sess. 
ary return currents are felt to great depths. They are, I think, extremely 
slow. The current at the surface of discontinuity is slow, and the secondary 
return current induced by it must be very much slower if it is distributed 
over a considerable depth, as I think it is. To give a rough idea of the 
velocity of this current, assume that the rate of the ordinary return current 
is one mile per hour, which must be considerably over the mark. Assume 
also that this return current, which, it must be remembered, is considered as 
taking place above the discontinuity, induces a current 10 feet deep below 
the discontinuity, and of an average rate of half a mile per hour. Assume 
further that the secondary return current is distributed through a depth of 
300 feet, and it will be seen that its average velocity is only one-sixtieth of 
Fig. 10. --Loch Ness, 6 p.m., 13th September 1904. 
a mile per hour, or that it takes two and a half days for the return current 
to move one mile ; and as this is the average velocity of the current, its 
velocity at great depths may be much slower. 
These figures are only taken for the sake of example, and I have no 
idea as to their quantitative accuracy. They show, however, that an 
assertion that the secondary return current is felt to great depths does not 
mean that rapid changes take place in temperature at these depths. 
Changes, however, do take place, and to explain these changes it is necessary 
to assume some system of currents. 
For instance, it was found by the Lake Survey in Loch Morar * that 
* Geographical Journal , July 1904. p. 74. 
