136 THE FRESH-WATER LOCHS OF SCOTLAND 
Observations made on 14th August 1902 showed a surface temperature 
of 55°'0 Fahr. and a bottom temperature of 47°"9. Loch nah-Oidhche 
(Gairloch basin) has a maximum depth of 121 feet, a mean depth of 
o4 feet, and an area of 0*54 square mile. One would have expected 
to find a higher bottom temperature in this lake than in Loch na 
Sheallag, but observations made on 7th August 1902 showed a surface 
temperature of 51°'0 and a bottom temperature of 46'' "8. I am not 
acquainted with these lochs, and there may be something in their 
position and surroundings to account for these differences of tempera- 
ture, but the differences are not prima facie easy to explain. 
S. Loch Damh (Torridon basin) has an area of 1*32 square miles, 
a maximum depth of 206 feet, and a mean depth of 59 feet. Tempera- 
ture observations in this lake on 21st August 1902 showed a tempera- 
ture at the surface of 56° '5 Fahr. ; at 100 feet, 48° '5 ; and at 200 feet, 
42°-2 — a temperature usually found in August only at the bottom 
of the deepest lakes. The simplest explanation, and the one which 
I favour, is that an error of 5° was made in reading the thermometer, 
and that the temperature at 200 feet was 47° '2. A mistake of this 
sort is not uncommon with observers. 
4. The observations in I^och Dungeon (Dee basin) are also surpris- 
ing. In the deepest part of the lake (depth 94 feet) the following 
temperatures were recorded on 6th August 1903 : — surface, 5S°*2 Fahr. ; 
60 feet, 52°-2 ; 70 feet, 45°-2 ; 90 feet, 44°-6. If the observations are 
reliable they show a very marked discontinuity between 60 and 70 
feet, and for a lake with a mean depth of only 23 feet the bottom 
temperatures are very low. The lake is divided into three basins, 
and this may account for the low temperatures. 
5. Loch Muick [Dee (Aberdeen) basin] has an area of 0*85 square 
mile, a maximum depth of 256 feet, and a mean depth of 116 feet. 
Compared with its area and length (2J miles) Loch Muick is a deep 
lake, and accordingly there is a considerable range in temperature 
from top to bottom. Observations were made on 8th July 1905, and 
gave temperatures as follows : — surface, 56''*1 Fahr. ; 50 feet, 47° '2 ; 
100 feet, 44° -3 ; and 225 feet, 43° '0. 
6. Loch Frisa (Mull) may be contrasted with Loch Muick. It 
has a length of 4J miles, a maximum depth of 205 feet, and a mean 
depth of 76 feet. Observations made on 17th August 1904 showed 
a temperature at the surface of 59° '1 Fahr. ; at 50 feet, of 58° '7 ; and 
at 175 feet, of 55°-2 — 11 or 12 degrees higher than at a similar depth 
in Loch Muick, showing how important a factor is the relation 
between length and depth. The difference may be partly accounted 
for by the fact that the observations in these two lakes were made in 
different years. But a range fi-om top to bottom of only 4° Fahr. 
in a lake over 200 feet deep is exceptional. 
