of Edinburgh, Session 1871 - 72 . 
571 
It is plain, in the first place, that in a deep lake in this 
latitude, there is a very gradual and slight increase of cold in 
the warm season for the first hundred feet, viz., by 2 0, 5 only, 
then a sudden descent by 5 o "0 in the next 50 feet only; next 
another slow descent by 2°'5 in 150 feet ; and finally, below 
that a great substratum of 250 feet of water, and at a deeper 
spot of no less than 350 feet, at the uniform temperature of 42°, or 
a little less. Next, at Loch Lomond no change took place in the 
temperature of the bottom water during two months of unusual 
warmth for the months of September and October, and no change 
at 300 feet from the surface during five weeks prior to the middle 
of November. 
It seems certain that the temperature of the great substratum 
of cold water cannot be raised after the middle of November, when 
the cold season has fairly set in. Whether it is to be lowered 
during winter, or whether the substratum, without becoming colder, 
will merely have its upper level raised, is a question to be settled 
by observation at an early period of next spring. 
In the meanwhile, abstracting the highly improbable existence 
of strong springs at the great depths I have mentioned, it does not 
appear how this vast cold substratum could have been moved dur¬ 
ing last summer and autumn. Neither does it appear how it can 
be moved during the winter, unless the equally great stratum above 
it acquire a lower temperature than 42°, and so take its place; for 
the uniformity of the bottom temperature between 21st September 
and 18th November, when no additional cold could descend through 
the warmer stratum above, is sufficient proof that the influence of 
the heat of the earth beneath is too feeble in this latitude to make 
itself sensibly felt by motion of the water. 
Thus there is a probability, that when water once descends to so 
great a depth as the bottom of our deep lakes, it cannot ascend 
again except under rare and extraordinary circumstances. If this 
view be correct, the movement of the waters of a deep lake towards 
its outlet for escape, must be confined very much to the warm 
water at its surface, or to no great depth, and, therefore, mainly to 
the waters which are constantly supplied on all sides by its feeding 
streams. This must be the case in summer and in autumn ; it may 
be the case in winter also 
