138 THE FRESH-WATER LOCHS OF SCOTLAND 
at 100 feet 44° '9 — a range of 13° '5. These are remarkable ranges to 
find so late in the year as the third week of August. Large tempera- 
ture gradients are common in the beginning of summer, when the 
bottom waters have not yet become heated up, and when strong sun- 
shine rapidly heats up the surface-water — e.g. Loch Monzievaird 
(Tay basin), in which a range of 20° 6 Fahr. in 36 feet was observed 
on 8th June 1903. The low temperature of the bottom waters of Loch 
Achilty is probably due (1) to its comparatively great depth, (2) 
to its comparatively small drainage area, and (3) to its sheltered 
position. The month of June (when a lake is most sensitive to winds) 
was in the year 1901 very free from easterly winds, while in the year 
1902 there was a great excess of easterly winds. As Loch Achilty 
is more exposed to east than to west winds, this may account for the 
higher bottom temperature observed in 1902. 
12. In Loch Grlass (Conon basin) a very low bottom temperature 
of 42°'3 Fahr. was observed on 27th August 1902. The loch is 
4 miles in length, with a maximum depth of 365 feet. The axis of the 
lake runs from north-west to south-east, which is at right angles to the 
direction of the prevailing winds. It is also slightly crescent-shaped, 
and it is probable that it does not get the full force of wind-storms. 
In order to make a rough classification of the Scottish lakes, I 
endeavoured to find out which of them had never been known to 
freeze, or which only froze in very exceptional winters. It will readily 
be understood that it was frequently difficult, and sometimes impos- 
sible, to obtain information about lakes in remote parts of the 
Highlands, but I owe a debt of gratitude to the many gentlemen who 
took trouble in giving me information. 
The following are the lakes which, so far as I have been able to 
ascertain, seldom freeze over completely : — 
Loch. 
Maximum 
Depth. 
Mean 
Depth. 
Remarks. 
Feet. 
Feet. 
]\Iorar . 
1017 
284 
The deepest of Scottish lakes. 
Ness 
754 
433 
Occasionally freezes round shores. 
Lomond 
623 
121 
Freezes among the islands in severe 
frosts. 
Lochy . 
531 
229 
Shallows at north-east end freeze 
in very severe winters. 
Ericht . 
512 
189 
Was nearly, if not wholly, frozen 
over in 1895. 
Tay . . . 
508 
199 
Was nearly frozen over in 1895. 
Katrine . 
495 
199 
Rannocli 
440 
167 
Was frozen over in 1895. 
Treig . 
436 
20] 
Three-quarters of the loch were 
frozen over in 1895, but the 
tradition is that it had never 
been frozen over at all previous 
to that. 
