66 
THE FEESH-WATER LOCHS OF SCOTLAND 
2. Sudden release of a denivellation, caused by the transport of 
water from one end of the lake to the other by a wind which has blown 
in one direction for a time and then fallen calm or reversed its 
direction. 
3. A sudden denivellation in one part of the lake due to very rapid 
flooding. 
4. A sudden denivellation due to a heavy fall of rain, snow, or 
hail over a part of the lake. This might be partly static, i.e. due 
merely to the gravitation of the precipitated water ; or it might be 
partly dynamic, i.e. due the impact of the precipitated water. 
5. Sudden alteration of the atmospheric pressure, due to the 
passage over parts of the lake of a local atmospheric disturbance 
(squall), such as is indicated by a disturbance on the microbarogram. 
6. The impacts of wind-gusts on the lake-surface. 
Among causes that might be expected to generate seiches gradually 
may be mentioned : — 
7. The action over portions of the lake-surface of small fluctua- 
tions of the barometric pressure which happen to synchronise more or 
less nearly with some of the seiche periods of the lake. 
8. Action similar to last of fluctuation in the velocity and pressure 
of the wind, as shown in the anemogram. 
1. Effect of the Progression of the General System of the 
Isobars. — In order to form an idea of the potency of cause 1, let us 
take an extreme case. The greatest gradient noticed on the weather 
charts for August and September 1905 was 2*5 mm. of mercury, i.e. 
34 mm. of water, in about 30 sea miles. Taking the length of Loch 
Earn as 6 miles, this would give a difference of pressure between the 
two ends of 6*8 mm. of water. At a distance of about 50 miles on 
the chart the gradient had fallen by about one-fifth. Taking an 
extreme supposition, viz. that the system of isobars travelled with a 
velocity of 30 (mile/hour) in the direction of the maximum gradient, 
which is further assumed to be in the axis of Loch Earn, then the 
decrease of pressure difference in an hour would be 6*8 x 3/25. A 
variation of this kind (supposing the gradient uniform over Loch 
Earn) can only generate the uninodal of Loch Earn, the period of 
which may be taken roughly to be 15™. If the time of action be now 
supposed to be the most favourable, viz. 7^"^ and the increase of the 
gradient to be uniform in time, then, by the mathematical theory 
above referred to,^ the increment in the range of the uninodal seiche is 
6-8x3/25x16= '051 mm. 
An alteration of this amount would, of course, be invisible on the 
limnograms. It seems hopeless, therefore, to look for an explanation 
1 Trcms. Roy. Soc. Edin.^ vol. xlvi. p. 513, 1908. 
