TEMPERATURE OF SCOTTISH LAKES 
125 
biological as well as from the physical point of view, for on the circu- 
lation chiefly depends the amount of oxygen available for animal life 
which is found at the bottom of lakes. The quantity of oxygen at 
the bottom of a lake should be greatest in early spring, unless the 
lake has for a considerable period been covered by a sheet of ice. 
For during winter the whole lake is in circulation, and the whole 
body of water in the lake may become aerated. At the end of 
autumn the quantity of oxygen at the bottom of the lake should be 
at a minimum, as throughout summer and autumn the bottom waters 
are practically stagnant, and the available oxygen is used up by 
animal life and decaying matter. In the Lake of Geneva, however, 
the amount of oxygen at the bottom of the lake at the end of autumn 
was found to be practically the same as at the surface of the lake, 
which indicates that absence of life at the bottom of the deeper 
Scottish lakes must be due not to absence of oxygen but to other 
causes.^ 
If, however, the lake is frozen over for a considerable period the 
aeration of the water may not be so perfect, and indeed all the 
available oxygen in the water may be used up. This was found to 
be the case in Linlithgow Loch during a severe winter, when eels, 
which were abundant in the lake, in the search for oxygen forced 
their way into the drains from the town of Linlithgow in such 
quantities that the drain-pipes were choked. 
The Temperature Seiche 
Where two liquids of difi'erent densities rest one on the top of the 
other and do not mix, very slow waves ma}^ be propagated in the 
lower liquid ; the most important example of waves of this nature is 
the "dead water'' which is found in salt-water fjords, and in the 
neighbourhood of meltnig ice, where fresh water from rivers or from 
ice runs over the surface of the salt water without mixing with it, 
and forms a layer of some thickness at the surface. The progress of 
vessels in such water is at times greatly retarded, owing to the 
resistance caused to the vessel by its producing very slow waves in the 
salt water beneath. 
Another example of waves produced in the lower of two liquids of 
different densities is to be found in lakes when the discontinuity is 
marked between the upper warm water and the lower cold water. 
At such times it is possible to have a seiche of very long period in the 
lower layer. In a lake of rectangular section, and where the dis- 
1 See paper by Dr W. A. Caspari, p. 145; Professor E. A. Birge, "The 
Respiration of an Inland Lake," Poimlar Science Monthly, vol. Ixxii. p. 337, 
1908. 
