TEMPERATURE OF SCOTTISH LAKES 
115 
of sheets of water must have frequently noticed oily patches {taches 
(Vhuile), which are popularly supposed to herald rain. The origin 
of these patches is not definitely known — it may be that oily 
substances are brought down by streams. Many explanations have 
been put forward, but none are conclusive. Their effect, however, is 
to produce variations in surface tension which hinder the formation 
of surface ripples. Schneider,^ in the Obersee, found that water in one 
of these oily patches was 2° '3 Fahr. colder than water with ripples on 
it in the immediate neighbourhood. This was, however, probably 
accidental, for numerous observations were made in Loch Ness with 
a view to correlating the appearance of these oily patches with 
temperature differences, but without result. It is possible that 
freezing takes place more readily on these oily patches, although 
Forel ^ and Halbfass ^ are of opinion that their effect is negligible. In 
forming their opinion, however, they had in view alteration in the 
rate of evaporation and radiation produced by the oiliness of the 
surface, and not the fact of the absence of ripples and small waves 
which mix the surface-waters. If there is less mixing of the surface- 
water in these oily patches, freezing will take place over them more 
readily than over the rest of the lake. It was observed in Loch 
Garry that where ice was melting an oily patch formed on the 
surface of the water. 
Another phenomenon connected with the freezing of lakes may be 
due to the same cause as the patchy formation of ice. It is a matter 
of common experience that, when a lake freezes over, there are 
frequently treacherous places on the ice, portions which remain 
unfrozen, or only become covered with a relatively thin sheet of ice. 
Popular theory attributes these to wells sending up a supply of 
water above freezing point to the surface of the lake. This may be 
a real explanation in many cases, but it must be remembered that 
either the force of the spring or well must be very strong and prevent 
freezing by the currents it produces, or else the water of the spring 
must be of less density than water at 3^°, when it will rise to the 
surface. This means that the spring water must have a temperature 
of at least 47°. 
Forel has discussed a immber of possible explanations,^ and 
he favoured the view that the mauvaises places^ as he called them, 
were simply portions of water which had been kept open by 
ducks, swans, and water-fowl w^hen ice was forming on other parts 
of the lake. 
1 "Der Obersee bei Reval," Arch.filr Biotitologie, Bd. ii., Berlin, 1908. 
2 Bull. Soc. vaud. Sci. nat.^ t. xxxiv. p. 498, 1898. 
3 Petermwnn's Mitt., Erganz. 136, p. 82, 1901. 
4 Bull. Soc. vaud. Sci. nat., t. xxxiv. p. 272, 1898. 
