468 PROFESSOR CHRYSTAL 



as the indications of the converted Sarasin limnograph were controlled by occasional 

 observations with the much more delicate index limnograph. 



For our disappointment in Tay and Lubnaig we find consolation in the beautiful 

 seiche behaviour of Earn, which we regard as a small but elegant daughter of Geneva, 

 the great mother of seiches. 



Origin of Seiches. 



Forel and his followers, Du Boys, Von Cholnoky, and others, have discussed the 

 causes of seiches ; and recently Endros, in his important memoir on the Chiemsee, has 

 confirmed the conclusions of his predecessors, and added some fresh details of great 

 interest. In what follows we shall not advance anything of great novelty ; but there 

 are two points of interest that may be worthy of the reader's notice. In the first place, 

 the use of the Dines-Shaw microbarograph enabled us to follow continuously the minute 

 variations of the atmospheric pressure with an ease and certainty hitherto unattainable.* 

 Also, in an appendix to this memoir the mathematical theory of the effect of pressure 

 disturbances of various kinds on an ideal lake, of form not very remote from Earn, has 

 been worked out, so as to show that the usually assigned cause of seiches, viz. the 

 minor local fluctuations of the barometric pressure, is in reality sufficient to cause the 

 disturbances observed, and is not a negligible quantity on ordinary lakes, such as the 

 tidal action of the moon can be shown to be.t 



Eegarding those causes of seiches which have never yet been proved to be other 

 than accidental, it may be of interest to record the fact that during our observations, 

 viz. on 21st September 1905, at 23 h 33 m , we were favoured with what Dr C. Davison,} 

 in a paper on the Ochil Earthquakes, calls a " principal earthquake." The estimated 

 duration of the shock was 3 "4 sec. Some of the members of my family who heard it, 

 took it for the rumble of a train passing at an unusual hour on the opposite side of 

 Loch Earn. The centre of disturbance seems to have been about 19 miles S. 39° E. of 

 St Fillans ; and the normal to Dr Davison's isoseismal 4 makes an angle of about 63° 

 with the axis of Loch Earn. 



At the moment the Waggon recorder was not working ; but the converted Sarasin at 

 the binode was running at high speed (158 mm. per hour) and giving a smooth trace. 

 The circumstances were as favourable as could be conceived for showing any seiche 

 disturbance due to the earthquake ; but none can be identified. There is, of course, no 

 reason to expect that the rapid oscillations of ordinary earthquakes could cause seiches. 

 Still, negative evidence in special cases is not without value ; because in exceptional 

 cases, such as the Lisbon earthquake of 1755, seiches have been produced, and we do 

 not as yet know the reason why. 



* A separate account of the observations with the microbarographs has been published in the Proceedings of the 

 Society, vol. xxviii., p. 437 (1908). 



t Kxcept in very large lakes, such as Erie. See Endros, Petermanns Geogr. Mittheilungen, 1908, Heft ii., p. 16. 

 + Quart. Jour. Geol. Soc., Ixiii., p. 366 (1907). 



