71 
of Edinburgh, Session 1878 - 79 . 
when it has been frozen by a moderate and comparatively long-con- 
tinned frost than when the ice has been formed quickly by very 
severe frost. For the more severe the frost the sooner will it be 
able to overtake the water leaving the ice fringe ; in other words, 
the stronger will be the current which it will be able to arrest, the 
greater the head which it will be able to stem But the head is 
caused by the higher temperature of the open water as compared 
with that under the ice. Hence the more severe the frost, the 
higher will be the temperature which it will be able to fix. 
Eighth. Condition seventh is affected by heat supplied from the 
bottom. The amount due to the internal heat of the earth is cer- 
tainly under present circumstances inappreciable. That due to 
decay of organic material in the water or at the bottom is also in all 
probability insensible in lochs of such purity as Loch Lomond, 
though it is of very serious importance in polluted lakes like Lin- 
lithgow. 
Ninth. The change of the water of a lake affects the distribution 
of temperature after it is frozen. When the loch is entirely frozen 
over, the supply is delivered entirely at the surface, and therefore 
sensibly at a temperature of 32° F., which is also the temperature 
of the outflow. This water finds its way from its source to the 
outlet close under the ice, and lowers slightly the temperature of 
the water in the neighbourhood of the ice. The water which finds 
its way from the open part to the outlet does so along the bottom, 
and it is taken from the deep warm water of the open part in virtue 
of the convection currents at the edge of the ice. The temperature 
of the supply thus furnished to the frozen basin depends first on 
that of its source, namely, the deep water of the open part of the 
lake, and then on the configuration of the bottom, more especially 
on the maximum depth on the ridge separating the frozen basin 
from the open one. The shallower the water on the ridge the 
nearer will the deep water have to come to the surface in order to 
surmount it, and consequently the colder will it become. 
The rising of warm water to the surface was very manifest at the 
passage between Inchtavannach and the Dumbartonshire shore of 
the loch. 
