TEMPERATURE OF SCOTTISH LAKES 
135 
for Loch Ness shows that the quantity of heat set free is equal 
to the heat which would be set free by the combustion of 2*4 million 
tons of coal — also a very large quantity. Without going into 
meteorological statistics, it may be stated that the winters along the 
shores of Loch Ness are so mild that snow never lies on the ground 
for any length of time. It is also said that potatoes and other 
root-crops may safely be left in the ground all winter without danger 
from frost. 
There is another method in which lakes influence the temperature 
of the atmosphere. During the period of freezing there is a consider- 
able quantity of latent heat set free ; and again, when the ice-sheet 
melts in spring, a considerable quantity of heat is absorbed. There 
is a resulting tendency, in the neighbourhood of lakes which become 
frozen over, for a spell of frost to be less severe at its commencement, 
but to be protracted longer than it would otherwise be. 
Comparison of Scottish Lakes 
Temperature observations were made in nearly all the lakes visited 
by the Lake Survey, and the observations are given in the descriptive 
reports. From the fact that the observations were made at widely 
different periods of the year, and that usually only isolated observa- 
tions were possible, it is difficult — indeed, I think, impossible — to base 
a comparison of the lakes, from the point of view of temperature, on 
these observations. The general factors which enter into and determine 
the temperature of different lakes have been mentioned in the preceding 
pages, and in what follows I wish to notice briefly some of the observa- 
tions in the Scottish lakes which call for special remark. 
1. Observations were made in Loch Calder (Forss basin) on 
6th October 190S, showing the temperature at the surface to be 51°*4 
Fahr., and near the bottom 51°*2 — a difference of only 0°'2. The 
area of the loch is 1*32 square miles, the maximum depth 85 feet, 
the mean depth 21 feet, and it lies about 205 feet above sea-level. 
Loch Brora stands 93 feet above sea-level, has a maximum depth of 
66 feet, and a mean depth of 23 feet, its area being 0*88 square mile. 
From these data no great difference in temperature M^ould have been 
expected between these lakes, but observations made in Loch Brora 
on 22nd October 1902 showed a surface temperature of 45°'8 Fahr. 
and a bottom temperature of 45° '0. The temperature gradient is 
greater than in Loch Calder, and the difference in temperature between 
the two lakes is surprising, especially as the mean air temperature for 
October 1902 was about 46°. 
2. Loch na Sheallag (Gruinard basin) has a maximum depth of 
217 feet, a mean depth of 103 feet, and an area of 1*37 square miles. 
