TEMPERATURE OF SCOTTISH LAKES 
129 
The observations were designed to supplement observations made 
in 1899, on the depth to which the sun's rays penetrate. Owing 
to a beautiful temperature seiche, which was in progress during the 
observations, not much light was thrown on this point ; but results 
of greater importance were obtained. 
Observations were made at the six depths every three hours, with 
one or two breaks for repairs to instruments, etc. The observations 
from 1st to 6th August are shown in fig. 51. Those obtained by 
means of the first three thermometers show irregular changes of 
temperature, doubtless due to changes of wind and other causes 
which directly influence the temperature of the water at the surface. 
The most remarkable point about these observations is the frequent 
inversion of temperature that occurs. Some of these irregularities 
are probably instrumental ; but the weather was changeable, and, owing 
to the high surface tempeiature, the surface cooling on cold, cloudy 
days must have been rapid, and may well have caused inversion of 
temperature. 
The observations with thermometer IV., however, at a depth of 
about 24 feet, show a large temperature seiche, with a period of 
almost twenty-four hours. The range of the temperature for one 
oscillation was nearly 14° Fahr. The Wolfgangsee has a maximum 
depth of 374 feet and a mean depth of 155 feet, and from the experi- 
ence in Scottish lakes one would not have expected to find a discon- 
tinuity at so shallow a depth as to give a temperature seiche at a 
depth of 24 feet. When Exner's observations were commenced, on 
20th July, the surface temperature was only 63°, as compared with 
about 70° in the beginning of August, and there was not much 
evidence of a temperature seiche at 24 feet. It is probable that a 
discontinuity was formed at a shallow depth during the changeable 
weather which followed the beginning of the observations. There 
was a discontinuity at about 40 feet when observations were com- 
menced with thermometers V. and VI. on 24th July. This discon- 
tinuity was still quite distinct on 1st August, but in addition there 
was a less marked discontinuity at about 25 feet. The effect is most 
instructive. The observations with thermometer V. show a tempera- 
ture seiche of opposite phase to the seiche shown by thermometer IV., 
and this seiche is also shown by thermometer VI. The reason, as 
Exner indicates, must be that there was an oscillation of the large 
body of cold water below, say, 40 feet, and a separate oscillation, of 
opposite phase, in the layer of water between the upper and lower 
discontinuity. 
It is remarkable that the period of the oscillations should happen to 
be exactly twenty-four hours, and Exner suggests that the period may 
be forced by a periodicity in the wind. Such a periodicity would have 
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