Nichols.] 
66 
/ 
[October 6, 
It is very desirable that a more complete study should be made 
of the temperature of the great lakes at various depths, especially 
in the cooler parts of the year. 
An interesting question arises as to the manner in which the 
cooling of ponds takes place after the water reaches 4° C. Until 
that point is reached, the surface water as it cools sinks by virtue 
of its greater specific gravity, but between 4° and 0° the cooler 
the water the lighter it is. V arious theories have been held in the 
matter. 
Thompson (loc. cit.) supposed that “in consequence of the vio- 
lent agitation of the water by the cold winds which prevail in the 
early part of the winter, the whole mass of water is usually 
cooled down very nearly to the freezing point before any ice forms 
on the top, and that, after the waters are protected from the winds 
by a covering of ice, their temperature is gradually raised by the 
reception of heat from the earth beneath.” 
Forel, 1 moreover, describes a way in which the action of the 
wind may cause a very important circulation. He says, “ The 
wind sweeping over the water causes sometimes considerable 
differences of level ; it causes a lowering of the water-level at that 
portion of the lake which it first reaches, and raises the water at 
that end toward which the wind is blowing. 2 There results a 
deep return current, in the direction opposite to that of the wind, 
and thus a general circulation of the water. In summer this 
return current takes place at no great depth, because then the 
water is stratified according to its temperature, and there is con- 
siderable difference in density between the water at the surface 
and at the bottom, but in winter, when the density is uniform 
throughout the lake, this current can descend very low 3 ; conse- 
quently, when all the water is at 4°, a violent wind may bring 
!Bibl. Univ., Arch. Sci., IV (1880), pp. 89-106. 
2 Forel, F. A. Limnim^trie du Ldman, § XIII and § XIX. Bull. Soc. Vaud. Sc. nat. 
XV (1878), pp. 145 and 408. 
3 During the hurricane of the 20th of February, 1879, fishing nets at Ouchy were 
torn away and carried for a long distance by this return current, even when they 
rested on the bottom at a depth of two hundred and three hundred meters. Cf. Chate- 
lanat, Effets sous-lacustres du cyclone du 20fdvrier, 1879. Bull. Soc. Vaud. Sc. nat., 
XVI (1880), p. 533. 
