344 



Mr. J. Y. Buchanan. 



[Feb. 2,. 



that the intervals at which successive marks passed out of the boat 

 must not be compared too rigidly. When the whole length of line, 

 50 or 55 fathoms, as the case might be, was paid out, it was allowed 

 to tauten itself, and the time observed when it became taut. The 

 bearing of the float was then taken, and that of the sun immediately 

 afterwards. The results of the observations are summarised in the 

 following table. 



Table II. — Summary of Current Observations made on the Afternoon 

 of the 21st of October, 1883, on the edge of the Dacia Bank, 

 lat. 31° 10' K, long. 13° 34' W., depth 175 fathoms. 



Time (p.m.) 



Direction (true) .... 

 Kate in knots per hr. 



1 hr. 55 m. 



2 hr. 15 m. 



2 hr. 40 m. 



3 hr. 30 m. 



4 hr. 6 m. 



N. 11° E. 



N. 11° E. 



N. 41° E. 



N. 56° E. 



1ST. 101° E. 





0-47 



0-30 



026 



030 



" It will be seen from these observations that, in two hours, the 

 current had shifted its direction through 90°, and had passed through 

 a minimum velocity of 0''26 per hour without there having been any 

 period of £ slack water.' The observations are too few in number to 

 make it worth while submitting them to analysis, but a little study 

 of them will show that they indicate a current which is the resultant 

 of a constant current and a periodic one. A constant current running 

 S.E. by E., combined with a tidal current running N.NYW. and 

 S.S.E., the maximum velocity of which, in either direction, is twice 

 that of the permanent current, would give a resultant current 

 agreeing fairly with that observed."* 



In these circumstances, during a complete tidal interval the water 

 flows along an S"hke path, and in the twenty-four hours it describes 

 two such figures, and moves on in a zig-zag course. It is apparent 

 that the integral drift in the twenty-four hours is that due to the 

 constant current alone independently of the tidal current. The- 

 same holds for twelve hours. Hence, if observations are carried on 

 at frequent and regular intervals for twelve hours, we are able to 

 determine both the constant current and the tidal current superposed 

 on it, and it is to be hoped that when they can be made they will not 

 be neglected by surveying ships and telegraph ships. 



Many banks are already known in the North Atlantic on which such 

 observations could conveniently be made, and there are probably many 

 more scattered about the ocean, for instance, on the ridge called after 

 the U.S.S. "Dolphin," which extends along a somewhat crooked line 

 from the Azores to Tristan da Cunha. It is of course important to 

 have fine weather ; then the boat in which the observations are made 



* ' Edinburgh, Boy. Soc. Proc.,' vol. 13, 1886, p. 437. 



