94 REVIEWS 
the form and orientation of the lakes, and (d) the sources of supply, 
influence their temperature. The form and orientation of the lakes 
is of importance in connection with the winds. Lakes which are ori- 
ented so as to allow winds to exert their most important influence in 
the generation of currents, have their temperature equalized in the 
vertical sense, through the return currents. Circulation is thus shown 
to be of more importance than conduction in equalizing the tempera- 
ture between top and bottom. The rdle of affluents in determining 
lake temperatures is very variable. It depends on the size of the 
lake, the average stay of the water in the lake, and the nature of the 
affluents themselves. Following Forel, the author emphasizes the par- 
adoxical fact that the waters of the glacial Rhone raise the temperature 
of Lake Geneva, the temperature of the river being notably above that 
of the lower part of the lake at all times, while the large amount of 
sediment in the river water so increases its specific gravity as to cause 
it to descend much below the zone of lacustrine temperature corres- 
ponding to itsown. Sub-surface affluents exert an important influence 
in their immediate surroundings, or, if the lake be very small, on the 
whole volume of water. The temperatures of these affluents being essen- 
tially constant, the temperature of the lake is differently affected by 
them in different seasons. Lakes are classified by the author, follow- 
ing Forel, on the thermal basis, as ¢ropical, temperate, and polar. ‘The 
tropical lakes are those whose surface waters do not reach 4° C.; the 
temperate lakes are those the surface waters of which are sometimes 
below and sometimes above 4°; while the polar lakes are those whose 
surface waters never rise above 4°. Of the first class the larger part 
of Lake Geneva, and certain salt lakes near the shore are the only rep- 
resentatives. To the second class most of the lakes of France belong, 
but there are a few representatives of the third class in the Pyrenees, 
and in certain other high altitudes. 
Chapter VIII deals with the transparency, color, etc., of the lakes. 
The lakes are partly blue (few), partly green (the larger number), and 
partly yellow (a large number.) ‘The color is found to be influenced 
by (a) the dissolved organic substances, such as humic and ulmic acids ; 
(6) the presence of vegetable and animal organisms in the water; and 
(c) the inorganic sediment. The transparency is found to vary within 
a given lake with the season, and with the position of the station. It 
is greater in winter than in summer, and increases with increasing dis- 
tance from the debouchures of streams. The water of the blue lakes is 
