28 
BATHYMETRICAL SURVEY OF 
from the shore 54°*6, while the water of the burn flowing into the loch 
had a temperature of 53°-6, and higher up the stream 53°-l. It thus 
appears that the waters of Loch Voil were warmer than those of Loch 
Earn, and in the case of Loch Voil the stream feeding the loch had a 
lower temperature than the loch itself, while in the case of Loch Earn 
the streams were warmer than the waters of the loch. 
Loch Luhnaig . — Observations were taken in Loch Lubnaig only on 
April 6 and 8, 1899, and showed that at that time the temperature of the 
water was nearly uniform from surface to bottom, the range being only 
from 41°-8 to 42°-7. 
From the point of view of temperature, the Scottish fresh-water lochs 
may be divided into those which freeze during hard winters, and those 
which never freeze. Those which freeze over in winter are shallow 
lochs, and when frozen the water-temperature beneath the ice is at the 
maximum density point of fresh water (39°*1) or lower. In spring the 
temperature of these shallow lochs rises much more quickly through 
the heat of the sun, and the whole mass of water attains a higher 
temperature than in the case of the deeper lochs; they also lose their 
heat much more quickly in the autumn than the deep lochs, and con- 
sequently have a much wider range of annual temperature. In the deep 
lochs — those with 400 or more feet of depth — the temperature of the 
water never rises so high in summer, nor sinks so low in winter, as 
in the shallow lochs, and the range is much less. The temperature of 
the bottom water in some cases does not change more than 1° Fahr. 
from year to year, and in the deepest lochs it appears to be practically 
constant at all times and seasons; 40° is the lowest temperature that 
has been recorded at the bottom in any of these deep Scottish lochs, so 
that the maximum density point is never reached. In summer, autumn, 
and even early winter, it is possible, by observing the temperature of 
the surface and sub-surface waters, to form a fairly accurate idea of the 
depth of a loch, the temperature being higher the shallower the loch. 
The waters from a deep loch — like Loch Katrine — are much the best for 
the water-supply to a city, for in summer the temperature is relatively 
low and in winter it is relatively high. 
The serial temperature observations taken in Lochs Chon, Ard, 
Menteith, and Leven are given in the following table, but many tem- 
perature observations were taken at the surface, which are not, of 
course, included in the table : — 
[serial temperature observations. 
