27 
of Edinburgh, Session 1872 - 37 . 
But the water of Loch Lomond is scarcely transparent enough to 
allow the sun’s heating rays to penetrate so deep as 500 or 600 feet, 
and the transparency of the lake of Geneva is not so much greater 
as to permit us to assume that the heating portion of the sun’s 
rays can penetrate to 620 and 950 feet. It may be, nevertheless, 
that a slight effect may be produced even at these great depths 
in this way. 
But there is still another heating power available for raising the 
cold substratum of water, and that is the heat of the earth at the 
bottom. At Loch Lomond, at 600 feet, this ought to be about 60°. 
At the bottom of the lake of Geneva it ought to be about 72°. It 
is true that the conducting power of the rocky exterior of the 
earth is too feeble to allow of much effect from this heating power, 
but it must have some influence, however small. In one way or 
another, — by heat from the sun’s rays, or heat from the bosom of 
the earth, or by the joint action of both, — it may be that the 
cooling influence of the atmosphere will be to some little extent 
counteracted. If so, the amount of this counteracting effect will 
vary according to the severity or mildness of the winter months. 
In short, the bottom temperature will rise a little in autumn after 
a very open winter ; and it will not stand so high after a very 
severe one. 
We have had an excellent opportunity of testing this view 
during the past summer and autumn, on account of the uncommon 
deficiency of cold weather last winter, — so great a deficiency that, 
as stated in my communication last spring, the mean atmospheric 
temperature of the six cold months was at Loch Lomond, by the 
calculations of Mr Buchan, 1 0, 4 higher than the average for thir- 
teen preceding years. 
Has this circumstance had any effect on the bottom temperature 
of Loch Lomond in deep soundings ? 
On 10th April, as stated in the Proceedings of the Society, the 
temperature at 594 feet, as near as possible to the place of obser- 
vation in September, October, and November last, was 42°, — exactly 
as in these months. On 0th May , much intervening sunshine 
having prevailed for nearly four weeks, but with a cold atmosphere, 
the surface-temperature at the same place had risen only from 43° 
to 44° *5 ; and the bottom temperature was 42°*1. I did not attach 
