116 THE FRESH- WATER LOCHS OF SCOTLAND 
In lakes which do not freeze all over, but only round the shore, or 
in shallow parts, as in Loch Lomond, there is always a circulation 
of water going on. The water which is cooled at the ice-fringe is 
carried by convection currents towards the open parts of the lake, 
its place being taken by warm water rising from the bottom ; but 
when once the whole surface is covered with ice there is no circula- 
tion of this sort. Nor is there any circulation produced by winds, 
as the covering of ice prevents the winds forming any currents 
in the lake. The only disturbing influence is water brought down 
by rivers, which during severe frosts is of small quantity and 
of very low temperature, so that it freezes shortly after entering 
the lake, and produces thickening of the ice round the river- 
mouths. 
But it is not only in calm weather that ice is formed on a lake. 
Even when the surface of the lake is disturbed by wind and waves, 
freezing will take place if the temperature is sufficiently low. I have 
not myself observed freezing taking place in this manner, and I am 
therefore indebted to the accounts of other writers. The freezing 
commences by the formation of isolated flakes or crystals of ice, half 
an inch in diameter or less. By the action of the waves, all these 
flakes are kept separate from one another ; but by rubbing together 
the edges are worn ofl*, and the flakes assume a circular form. The 
small particles worn ofl' by friction between these small pieces gather 
round their edges, and surround them with a margin or wall of 
friable ice, which has the effect of making the ice-flakes, or pancakes^ 
as they are called, sink deeper in the water. These pancakes 
gradually increase in thickness by the freezing of the water round 
them, in the same way as an ordinary sheet of ice increases in 
thickness, and also increase in diameter by the accession of small 
floating ice-crystals, until they assume a diameter of as much as three 
feet, always surrounded by the wall composed of small fragments of 
ice. It is a result of the mode of formation that the section of these 
pancakes is more or less elliptical, being thickest at the centre and 
gradually becoming thinner towards the edges. Whenever the 
surface of the water becomes calm all the pancakes freeze together, 
and the surface becomes covered by a continuous, though rough, 
sheet of ice, and thereafter the lake behaves as one which has frozen 
over in calm weather, and the ice-sheet gradually grows thicker until 
a thaw sets in. 
If an ice-sheet, formed in calm weather, partially covers the lake, 
and stormy weather and continued low temperature follow, ice grows 
to the windward by the accretion of ice-crystals. These form even 
in disturbed water, and, coming in contact with the edge of the ice, 
freeze to it. Ice grows to the leeward by the extension of ice-crystals 
