484 



PROFESSOR CHRYSTAL 



It has not been possible to deduce any definite results from our observations under 

 the present head. 



7. Effect of Periodic Fluctuations of the Atmospheric Pressure. — Our observations 

 afford many examples of this cause of seiches. It must, however, be understood that 

 strictly periodic fluctuations of the barometric pressure of short period rarely if ever 

 occur. We often find, however, fluctuations extending over an hour or two in which 



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the undulations are approximately of equal length ; and still oftener we find two or 

 three consecutive undulations of approximately the same length. Such fluctuations we 

 shall describe in what follows as periodic ; and by the " period " is meant the average 

 of the intervals between the passage of corresponding phases (say maxima) of two 

 successive undulations at the same point. 



It follows from theory, and is confirmed by observation, that a periodic disturbing 

 cause is most effective when its period is not very different from that of the seiche in 

 question. In practice, however, the disturbing effect is considerable even if there is 

 considerable divergence between the two periods. It should also be noticed that, even 

 theoretically, if we consider only one or a limited number of oscillations, and neglect 



