568 
Proceedings of the Royal Society 
m supposing that no prior observations of the kind have been 
made on our deep fresh ’waters. [See, however, p. 574.] 
In the course of the discussion of the St Mary’s Loch water- 
supply scheme, opposite opinions were expressed as to the relative 
advantage of drawing the water from the surface of the lake, or 
from a considerable depth; and weighty arguments, of a specula¬ 
tive nature, w r ere advanced on both sides of the question. It 
occurred to me, therefore, to consider what becomes of the deep 
water. Does it escape as that of the surface must do ? And if so, 
How? It appeared to me that during a winter of such protracted 
cold as that of 1870-71, the water at the bottom would probably 
acquire so low a temperature, that it must long remain there. For 
it can only rise again, either by its temperature falling below 39°’5, 
when its density decreases instead of continuing to increase, or by 
being heated by the heat of the earth beneath; and it is unlikely 
that the temperature of the entire w r ater of a deep lake will fall 
lower than 39°*5, or indeed so low, in this latitude, and the heat 
derived from the earth, in our latitude at the elevation of 800 feet 
above the sea, must be inconsiderable. It is well known that the 
bottom cannot be heated by conduction from the summer heat of 
the atmosphere above, as in the case of a solid substance; 
and the effect of the penetration of the sun’s rays, by which 
the water is heated to a certain depth, cannot descend very low r 
in a lake, the water of which is, like that of St Mary’s Loch, 
so coloured as to render a very white object invisible at the depth 
of 8 or 12 feet. The conclusion would be that the w T ater at the 
bottom of the deep parts of the lake, in the absence of strong 
springs—of the existence of which there is neither proof nor pro¬ 
bability—will remain at the bottom for w r ant of a current during 
the whole warm season, and perhaps longer. 
When I was first at St Mary’s Loch on 12th and 13th June, I 
had no suitable thermometer for taking observation of deep tem¬ 
peratures. But Mr Dewar kindly undertook to make the necessary 
trial a few days later in the same month. With a Six’s thermometer, 
whose graduation w^as subsequently tested and found correct, he ascer¬ 
tained that in 150 feet soundings, the temperature, being 56 at 
the surface, w T as 46° at the bottom. When I revisited St Mary’s 
Loch on 8tli September, nearly three months afterwards, the inter- 
