/ 
1908-9.] Observations with a Current Meter in Loch Ness. 633 
which the balls reached the compass, would have been helpful. The observa- 
tion on afternoon of the 20th shows a N.E. return current at 150 feet, with 
a velocity of 3'5 cm. sec. An all-night observation beginning immediately 
after shows a small current with directions N. 80 E., 4 S. 50 W. 100 . Had 
the balls been distinguishable, and had it been seen that the N. 80 E. ball 
was the first indication of direction, less doubt would have been felt in 
making the statement that there is a tendency for the return current to rise 
towards the surface, and for this secondary current to appear. 
It was mentioned in connection with the Invermoriston observations 
that as the season progressed currents penetrated to greater depths. The 
observations at Fort Augustus show the same thing, and all the observations 
go to show that the surface current and the return current are chiefly con- 
fined to the water above the temperature discontinuity. There are small 
currents deeper down, but they are negligible in comparison to the upper 
currents. During the season of the year within which the observations 
were made the temperature discontinuity is gradually sinking, and therefore 
it is found (1) currents are appreciable to greater depths, and (2) the velo- 
cities in individual cases are less, owing to the larger mass of water affected. 
This confinement of the appreciable currents to the upper layer may be 
seen at once from the Fort Augustus observations. From 23rd September 
till 21st October there are eighteen observations at depths below 250 feet, 
and in only one case did the velocity exceed 1 cm. sec., viz. during a N.E. 
wind on 24th September. In several of the other observations the meters 
gave no indication at all. 
In this connection it is interesting to refer to Forel’s description of “les 
eaux troubles du Rhone ” ( Le Leman, vol. ii. p. 280). During a strong 
north wind when the surface of Lake Geneva is viewed from a height the 
lake appears to be separated into two compartments. For a mile or two 
from the shore of the lake the water is greenish, while the rest of the lake 
appears a deep blue. The line of demarcation is quite distinct but does not 
run parallel to the shore, and varies from time to time. Examined closely 
it is seen that the blue water is quite clear, while the greenish water is 
opaline with a low transparency, which is not due to matter in suspension. 
This greenish water Forel thinks is the water from the depths of the lake 
brought to the surface by the return current. 
The extent of the lake which takes on this greenish appearance 
indicates that the rise of the return current may be felt at a considerable 
distance from the windward end of the lake, and it is not surprising that 
one mile from Fort Augustus, or even at Invermoriston, there should 
appear to be inconsistencies in the current systems during S.W. winds. 
