470 PROFESSOR CHRYSTAL 



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 to the impact of the 

 precipitated water. 



5. Sudden alteration of the atmospheric pressure, clue 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 fluctuations 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 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. 

 If we take an extreme supposition, viz. that the system of isobars travelled with a 

 velocity of 30 (mile/hour) in the direction of maximum gradient, which we further 

 assume 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 (if we suppose the gradient 

 uniform over Earn) can only generate the uninodal of Earn,* the period of which 

 we may take roughly to be 15 m . If now we suppose the time of action to be the 

 most favourable, viz. 7|- m , and the increase of the gradient to be uniform in time, 

 then, by the result given in the Appendix on p. 513, 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 our limnograms. It seems 

 hopeless, therefore, to look for an explanation of ordinary seiches in the variations of 

 the general system of isobars shown in the daily weather charts. 



2. Effect of Wind Denivellation. — It is well established by the researches of Sir 



* See Part V., p. 513. 



