COUNCIL - MARCH 1920 - APPENDIX — 70 — 



quite valueless as they are subject to very great probable errors owing to the fact 

 that the resultant tide is not a steady drift in a definite direction, but is subject 

 to great alterations both in direction and also in speed, even after the elliptical 

 movement of the daily tide has been eliminated. It is possible that if a fortnight's 

 observations were worked up in a similar manner, the law governing this variation 

 of the resultant tides might be apparent, but I am inchned to the belief that before 

 we shall be able to do anything towards discovering the law a continuous series 

 of observations taken on the same spot throughout the year is essential. 



(Signed) T. Edser. 26.11.1919. 



b. Resultant currents. 



Details and diagrams to be published in a forthcoming volume of [Fishery in 



vestigations. Series III, Vol. 11, Part. II, 1920]. 



From work already done it has been established that as a result of working 

 up resultant currents or drifts by a process of smoothing the curves of the N/S 

 and E/W componerls of actual observations separately, and reading off from 

 the smoothed curves the components at ^/g hourly intervals, and compounding 

 them for periods of 12, 12^/2 and 13 hours, the resultant currents of each period 

 show very material differences, and apparently irregular variations occur according 

 to the time of commencement of the period. Even such a short difference as half 

 an hour shows frequently a considerable alteration in the resultant. 



In order to throw further light upon this subject, a set of observations 

 covering 14 days taken at the Varne lightship in August 1913 was examined, 

 and for the tide at 10 m depth resultants covering various periods were compounded. 

 The dots on the attached diagrams, which are all prepared on the same scale, show 

 the ends of the vectors corresponding to these resultants. Each series was prepared 

 by commencing at 12 midday on 31st July and compounding half hourly vectors 

 to the end of the period for the first resultant. The next would commence at 12.30 

 p. m. on 31st July and extend ^/g hour later than the end of the previous period 

 and so on. 



Figure 1. shows the field covered by the 673 resultants of 12^2 hour periods 

 and from it it will be seen that the variation possible during the fortnight was 

 from practically nil to 25 cm/sec for speed and over the whole 360° for direction. 



Figure 2 gives similar information for 623 periods of 25 hours, the range 

 being from nil to 17 cm/sec and 360°. 



Figure 3 shows 550 periods of 74 hours each the range being from nil to 

 121/2 cm/sec and practically 180°. 



Figure 4 shows 549 periods of 74^/2 hours the range being from nil to 12 cm/sec 

 and 180°. 



