112 THE FRESH-WATER LOCHS OF SCOTLAND 
Other local variations are due to the state of the surface of the 
lake, which in some parts may reflect more heat than in others, and so 
produce local differences. The inflow of rivers also produces local 
differences. 
Even at one point of the surface there may be rapid changes of 
temperature, especially in early summer and in autumn, when heating 
and cooling are most rapid. Some interesting records showing this 
were obtained in Loch Ness by means of the platinum thermometers. 
During frosty weather the curves obtained by the recorder were of a 
much more ragged nature than the curves obtained in mild weather. 
The embroideries on the curves were caused by small changes of 
temperature which were probably due to convection currents mixing 
the water which had been cooled at the surface with the warmer 
water below. 
Towards the end of May 1904 the records of temperature at the 
surface began to show rapid changes of great amplitude. So erratic 
did the curves obtained by means of the Callendar recorder appear that 
they were at first attributed to instrumental errors. But the changes 
were checked by means of mercury thermometers. It was not possible, 
either, to attribute the changes to wind, for they occurred on the 
calmest days. Nor can they have been due to river influences, for they 
were also observed at Dores, where there is no river entering the loch 
to make the observations suspicious. On one occasion, in two minutes 
the surface temperature was found to change as much as 6° Fahr. On 
another occasion, when there was a quantity of pollen from flowers on 
the shore suspended in the loch, it was observed from the motion of 
the particles that different layers of water were moving in different 
directions, and the surface waters were evidently in a very agitated 
condition, although the surface of the water was quite calm. These 
and other observations indicate that, while the lake is gaining in heat, 
even in calm weather the surface-water to a depth of 5 or 10 feet is 
constantly being mixed up by convection. Were this not so, large 
temperature gradients would be observed at the surface, and this is 
the exception and not the rule. 
Formation of Ice 
It is only in lakes of the polar and temperate classes that freezing 
can take place. It must be borne in mind, however, that a lake may 
not always be in the same class. In a year with a mild winter it might 
fall to be classified as a tropical lake, while in another year with a 
severe winter it might come within the class of temperate lakes. More- 
over, it does not follow that because a lake is of the temperate or polar 
class it will become frozen over during the winter. The classification 
