766 



DR E. M. WEDDERBURN AND MR A. W. YOUNG ON 



being half of that of the uninodal seiche, however, irregularities may be expected to 

 be greater in the case of the binodal seiche. 



4. Variation of Binodal Phase. — The phase of the binodal seiche is seen to be 

 very constant during the whole series, and suggests that the period of 10 hours is a 

 very close approximation to the true period. 



5. 10° Isothermal at Station 2. — The periodogram curves for this isothermal are 

 shown in fig. 18, the time scales being as in fig. 17. As regards amplitude and 

 phase they are seen to follow the 11° isothermal very closely. 



6. Mean Depth of the 10° and 11° Isotherrnals. — A comparison of the curves 



Harmonic Analys/s 



of 10° Isothermal. 

 Station N?H 



/9/4 August 24-29. 

 Period 20 Hours. 



Amplitude 



of uninodal seiche. 



Mean Depth 



OF 10° ISOTHERMAL. 



Amplitude 



OF BINODAL SEICHE. 



Phase 



of uninodal seiche . 



Phase 



of binodal seiche. 



Fig. 18. 



showing the variations of the mean depth of the isotherrnals for 10° and 11° at 

 Station 2 shows that there is a wide divergence between them. At the beginning 

 of the series the distance between them was about 2 '5 metres, but this decreases to 

 less than a metre in about 30 hours, and thereafter each rises gradually about 

 2 metres in the next two days. With a variation such as this in calm weather, it is 

 seen how impossible it is to find an accurate estimate of the period of the temperature 

 seiche. It has been previously suggested that there may be extremely slow oscilla- 

 tions formed by rhythmical variations of the isothermal layers, but the observations 

 are too limited to give much weight to this supposition. 



It will be noted that the average depth of the 11° isothermal at Station 2 is about 

 one-third of a metre greater than at Station 1. This suggests that in their position 

 of rest the isotherrnals are not plane surfaces but are slightly cup-shaped. 



Periodogram Analysis for Short Periods. — On the periodogram curve shown in 



