4 
Proceedings of the Koyal Society of Edinburgh. [Sess. 
change of temperature — followed by another part of the curve also nearly 
vertical. Fig. 2 illustrates the same phenomena by means of isothermal 
lines drawn in a diagram representing a longitudinal cross-section of a 
loch — for convenience the cross-section being taken as rectangular. In 
the first phase no isotherms can be drawn, as the water is all of uniform 
temperature. In the second phase the isotherms are closest together 
towards the surface, and the distance between the successive isotherms 
FIRST PHASE 
Fig. 2. 
gradually increases with their depth. During the course of this phase the 
loch is gaining in heat, and successive isotherms make their appearance at 
the surface. All the isotherms are gradually sinking, new isotherms 
appearing at the surface. In the third phase there is a bunch of isotherms 
forming the discontinuity layer. Above and below the discontinuity 
layer the isotherms are widely separated. The discontinuity layer 
gradually sinks deeper and deeper down, while isotherms separate them- 
selves from the bunch at the discontinuity layer and, rising into shallower 
