ON THE SEICHES OF LOCH EARN. 471 



John Murray that a wind which has prevailed for some time causes transport of the 

 water of a lake in the direction in which the wind is blowing ; and the observations of 

 von Cholnoky on Lake Balaton show that in shallow lakes this wind denivellation may 

 be considerable, and that its sudden release may give rise to seiches. 



After a long and careful examination of our limnograms, we have arrived at the 

 conclusion that this kind of denivellation is very small on Loch Earn under ordinary 

 circumstances, and is rarely an effective cause of seiches. It is, however, not easy to 

 judge of this matter. When the wind is light, the effect is very small, and cannot be 

 separated from the denivellations due to precipitation and evaporation, and to variations 

 in the barometric gradient. When the wind is high it is usually accompanied by con- 

 siderable fluctuations of the barometric pressure, or by rainfall, or by both ; and again 

 the difficulty of separating the causes arises. That wind denivellation should be small 

 on Earn is not surprising, for, looking at the ratio of its depth to its length, we must 

 classify it as a deep lake ; and in such lakes, as is now well known, the return under- 

 current readily forms, and prevents the accumulation of wind denivellation. 



It may be of interest to record one or two of the more striking cases which were 

 examined. 



On 18th August at 16 h 46 m the microbarograph showed a nearly uniform increase 

 of 8*8 mm. (water) in the atmospheric pressure in about half an hour. At 17 h 5 m the 

 wind had fallen dead calm, and so continued for about 25 m . Then in 12 m to 13 m it 

 rose to a mean velocity of 25 (mile/hour), with an extreme of 38. Thereafter a mean 

 velocity of 18 to 20 was maintained for over three hours. The squall at 17 h 30 m was 

 very violent. One of my boys was out in a boat on the lake, and saw a large solitary 

 wave travel up from west to east. The water was calm in front, but very rough behind ; 

 and after the wave came the strong wind. He estimated the height of this wave at 

 2 feet ; but, as he was badly scared by the difficulty experienced in navigating his boat, 

 he most likely exaggerated. 



The effect of this remarkable wind-squall on the limnogram is comparatively slight. 

 There is a rise of 1 1 mm. or so, which took over two hours to develop. There is some 

 increase, but not much, of the characteristic wind embroidery, and a considerable disturb- 

 ance in the phases of the somewhat irregular UB-dicrote which had prevailed before 

 the squall. But there is no very marked permanent increase in the general range of 

 the seiche. 



From 22 h on the 18th there is a great increase in the range of the seiche, till it 

 reaches 79 mm. about 4 h 30™ on the 19 th (see fig. 4)— the greatest range we observed 

 on Earn. This increase was clearly due to the barometric disturbances indicated 

 by the microbarograph, which at the moment of maximum range had a period of 

 about 12 m 



It is specially notable that the seiche was not further increased by the great 

 increase of wind between 5 h and 7 h . The maximum of wind follows the maximum of 

 seiche in this case, as in several others that were closely examined. As the velocity 



TRANS. ROY. SOC. EDIN., VOL. XLVI. PART III. (NO. 20), 72 



