100 Proceedings of the Royal Society of Edinburgh. [Sess. 
It is a great improvement on any of the messengers which I have seen, both 
in point of convenience and of cost. 
The drainage area of Loch Garry is large, being 137*33 square miles — 
about 78'5 times the area of the loch, whereas in Loch Ness the drainage 
area is only 31*6 times the area of the loch. Assuming that the average rain- 
fall over this area is 72 inches (a moderate estimate), the volume of water 
received by this drainage area in the course of a year is, roughly, 23,000 
million cubic feet, and, assuming a 20 per cent, loss * by evaporation, 
absorption, etc., the amount of water flowing into Loch Garry in the course 
of a year is 18,400 million cubic feet, or nearly five times the normal 
capacity of the loch. These figures are given as showing the importance of 
rainfall in considering the temperature of lakes having a large drainage area, 
and in this connection it may be mentioned that the level of the loch rises as 
much as 25 feet above its summer level during floods. A rise of 10 feet in 
twenty-four hours is not uncommon. As a rule, however, the water which 
enters a lake is lighter than the bulk of water in the lake,*]* and spreads itself 
along the surface, and does not very greatly affect the abysmal temperature. 
Periodical observations were made of the temperature of the upper 
River Garry, which is by far the largest stream entering the lake. For 
reference these observations are given in the following table : — 
Table I. 
Temperature of Upper River Garry in 1908. 
Date. 
Temperature. 
Date. 
Temperature. 
Date. 
Temperature. 
Jan. 
31 
35*8 
Mar. 
16 
38-0 
May 27 
49-5 
Feb. 
3 
36*4 
11 
19 
39*2 
J une 2 
637 
11 
7 
39*0 
11 
23 
40*2 
„ 4 
62-6 
11 
19 
38-8 
AjDr. 
2 
40*2 
„ 11 
53-4 
11 
29 
34-0 
11 
6 
41-0 
„ 15 
52*6 
Mar. 
3 
35-9 
11 
17 
47*0 
„ 20 
51*2 
11 
6 
35-7 
11 
28 
43*3 
„ 26 
63*2 
ii 
9 
35*0 
May 
6 
49-2 
July 3 
72*0 
11 
13 
38-0 
11 
13 
48-5 
„ 8 
65*1 
11 
14 
38-5 
ii 
15 
48-7 
„ 11 
57*8 
The temperature of water in a shallow river responds rapidly to changes 
in the temperature of the atmosphere and to sunshine, and is therefore a 
fair guide to the quantity of heat which the lake is receiving — much more 
so than observations of extremes of heat and cold made with a maximum 
and minimum thermometer. 
* See Lake Survey Report, Geogr. Journal , vol. xv. p. 342, April 1900. 
t But see Forel’s Le Leman , vol. ii. p. 358. 
