1907-8.] The Temperature Changes in Fresh- Water Lochs. 13 
wind, and it will be seen that the black stain at the bottom of the trough 
has in each case tailed out in the same direction as the wind, showing the 
existence of a current at the bottom of the trough in the same direction 
as the wind. The photographs were taken at night, a limelight lantern 
being used to illuminate the water. An exposure of about 7 minutes 
with rapid plates was given in each case. The first two photographs were 
taken after the wind-current had been stopped, but the third photograph was 
taken while the wind was blowing. In the third experiment the water of 
the upper layer was not coloured very darkly, but in the photograph there 
appears a broad dark line at the boundary between the two layers. This 
is probably an optical effect produced by the return current, which raises 
small waves at the surface of the lower liquid. The effect of these waves 
would be to reflect some of the light falling on them, and so to give a dark 
line in the photograph.* 
A3 - DISCONTINUITY LAYER 
C - SURFACE CURRENT 
D - PRIMARY RETURN CURRENT 
E - SECONDARY SURFACE CURRENT 
F - SECONDARY RETURN CURRENT 
Fig. 7. 
Figs. 8-13 show, by means of isotherms drawn at intervals of 2 
degrees, the temperature distribution in Loch Ness on 13th and 17th 
September 1904, at a time when the discontinuity between the upper and 
lower layers of water was fairly well marked. The gradual oscillation of 
the discontinuity layer in the three consecutive diagrams for these days due 
to the temperature seiche can be seen. On the 13th September, the tem- 
perature distribution was due to a prevalence of N.E. winds, and it will be 
observed that the isotherms are closer at the end of the loch towards which 
the wind is blowing, and that they diverge from that end. On the 17th 
September, however, the temperature distribution is due to a prevalence of 
S.W. winds, and the fan arrangement of the isotherms, diverging from the 
end of the loch towards which the wind is blowing, is much more evident. 
The local conditions at the two ends of the loch are, I think, quite sufficient 
* Fig. 7 shows diagrammatically the current-system which is indicated by these 
experiments. 
