488 



PROFESSOR CHRYSTAL 



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7th December, ca. 14 h 30 m (fig. 17). — About this hour was observed the greatest 

 total range of seiche found on Tay, viz. 80 mm. At that moment the range on Earn, 

 which at 8 h had been as much as 55 mm., was only about 25 mm. The explanation of 

 this is doubtless to be found in the well-marked period of 26*5 m shown in the micro- 

 barogram between 12 h and 16 h . 



l&th August. — Fig. 18 shows an interesting case of the gradual development of 

 a UB-dicrote seiche. The anemogram shows a fall of wind during this development ; 



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but it seems to have been too gradual to be the effective cause of the seiche. There 

 can be little doubt that the true cause was a periodic microbaric disturbance, which is 

 very faintly indicated in the microbarograms taken at Ardtrostan and Lochearnhead. 



The present is one of many examples found in the course of our observations which 

 prove that a lake-surface is much more sensitive to minor fluctuations of the atmo- 

 spheric pressure than any barometric apparatus hitherto constructed. 



We might produce many more examples ; but probably the above are sufficient to 

 establish that the synchronism of quasi -periodic disturbances of the atmospheric 

 pressure with the seiche-periods of a lake are a frequent cause of seiches. 



It is true that the resonance experiments which Nature performs in her own rough 

 laboratory have not the nice exactitude of those we devise and carry out in a physical 



