478 



PROFESSOR CHRYSTAL 



the lake were about 28 and 22 (mile/hour) respectively. To obtain some idea of the 

 effectiveness of such disturbances, we may take this velocity to be v, and such that 

 vT x = 2a, where T x is the uninodal period, and 2a the length of Earn. If we denote 

 the pressure disturbance by 



yfa Q = Ja{l -cosily- 0(1 + u>))}, 



where a = 8 mm., say, and f(w, t) = when vt - a( 1 + «>)> or <2tt, then formula (46) 

 of Part V. gives 



43/rj/3a= dw w dt n x sin n } t < 1 - cos — ±(vt -o(l + w)) > 



/«|l+w)/i> 



9/3 5 sin 2(9 2(1- cos 26) tt(1 + cos 26) tt sin 2(9 

 _ -COSJ0 + — - Q p + ^ ^— , 



where 3^ x is the increment of the amplitude of the uninodal seiche at the end of the 

 lake,* and = 27ra/yT 1 . 



In the case supposed 9 = tt, and we get 



9/Cj = 3a/4 = 6 mm. 



Owing to the strong embroidery on the binodal limnogram, it is very difficult to 

 estimate the actual increment of the seiche amplitude at either of the two dis- 



r * 



MMMav 



// HIS 



/x 



/J 



Fig. I 



continuities ; but it is clear that the results of calculation and observation are of 

 the same order of magnitude. 



It is interesting to note that the very strong short-period embroidery that blurs the 

 binodal limnogram was almost totally absent on the limnogram taken at Picnic Point. 

 During the day the wind had been variable in direction from south-east to south-west, 

 gusty but never very high. The surface waves on the lake were not high during any 

 part of the day ; they came from the east in the morning, and from the west in the 



evenino-. 



O 



On the 7th of September, about 8 h 30 m , occurred the greatest barometric fluctuation 

 of short duration which we observed, f The extreme range was 19*3 mm. (Aq.), the 



* I e. 1"7 times the amplitude at the binodal limnograph. 



t For further details, see my paper, Proc. E.S.E,, vol. xxviii., p. 457. 



