1907.] 



on Seiches in the Lakes of Scotland. 



671 



Next I show you a rarity (Fig. 11), a short seiche from Lubnaig, 

 having a period of about 24 minutes. This is the best of only four 

 distinct seiches observed from that lake during six weeks. The modi- 

 fied Sarasin limnograph ran the whole time, and occasional observa- 

 tions were made with the index limnograph as a control. Besides these 

 four seiches nothing else was found but persistent wind denivellation, 



Micro bar. 



Fig. 10. 



and occasionally wind embroidery. Yet during all this time there 

 were continual seiches on Earn and Tay — much more regular on the 

 former than on the latter. 



The wonderful persistence of seiches on deep lakes, such as Geneva 

 and Earn, and the almost total absence of seiches on shallow lakes, 

 such as Lubnaig, is a remarkable confirmation of the ordinary theory 



6k 



^l^t^t :^f,.Tt^SxT7f^''-^'^~^ 



Fig. 11. 



of VISCOUS liquids, according to which the dissipation of energy is 

 mainly due to friction at the boundary. A striking example of" this 

 has long been known in the case of the ocean swell. This is often 

 propagated with little change of period or wave length for distances 

 of over a thousand miles. Xo one who has watched from Cliff House, 

 near San Francisco, the great billows that often break on the beach 



