100 THE FKESH- WATER LOCHS OF SCOTLAND 
trivial to consider in the present state of our knowledge. Among 
such methods may be mentioned the mechanical effect of the action 
of the wind on the surface of the lake, the beating of the waves on 
the shore, condensation and evaporation at the surface, the heat 
generated by organisms in the water and by decaying matter. These 
and others doubtless all have their effects, but these effects must be 
small.^ Moreover, they are more or less constant all the year round, 
and so cannot contribute appreciably to the changes from the cold 
of winter to the heat of summer and the subsequent cooling of the 
waters, which present the chief problems in dealing with the tempera- 
ture conditions of lakes. 
The more important factors which require to be dealt with are : 
(1) the influences of rivers and rain, (2) radiation, (3) conduction, 
(4) convection, and (5) wind. 
(1) Effect of Rivers and Rain. — It is difficult to estimate the effect 
of rivers and streams entering a lake, both because of the rapid 
changes which the temperature of streams undergoes, and because of 
the difficultv in determining the bulk of water entering the lake. A 
shallow stream very quickly heats up when the sun shines upon it and 
upon the stones which form its bed, while in winter melting snows 
cool the waters of streams, even in mild weather, nearly to freezing 
point. In large and deep lakes, such as Loch Ness, rivers have no 
marked effect on the cycle of changes which takes place. They do, 
of course, affect the quantity of heat entering or leaving a lake, but 
they do not leave any distinct trace on the temperature distribution. 
The effect of rainfall is also not directly traceable. Rain quiets 
the waves, and so retards to a small degree the mixing of the surface 
layers. As a rule, the rain-water is of a higher temperature than the 
surface-water, and so its direct effect is limited to the surface layers. 
Sometimes, however, after rain-storms, or when snows are melting, 
the rivers entering a loch come down in spate, and their effect is 
distinctly traceable. Some smaller lakes become transformed into 
mere enlargements of the streams entering, and there is a wholesale 
transference of the waters towards the outlets. When it is remem- 
bered that lakes of the dimensions of Loch Ness rise sometimes five 
or even ten feet in a very short period, it will be seen that even in 
large lakes the effect of rivers and rain is not negligible. The 
behaviour of lakes during spates gives some information as to what 
becomes of water entering : if the river-water is of higher tempera- 
ture than the surface-water, it remains on the surface and spreads 
over it ; but if the river-water is colder than the surface-water, as is 
sometimes the case, it sinks until it reaches water of like temperature 
with itself, and there intrudes itself. In this way, during big spates, 
1 See paragraph on freezing of lakes as to effect of rapid evaporation (page 114). 
