340 



Mr. J. Y. Buchanan. 



[Feb. 2, 



February 2, 1888. 



Professor G. G. STOKES, D.C.L., President, in the Chair. 



The Presents received were laid on the table, and thanks ordered 

 for them. 



The following Papers were read : — 



I. " On Tidal Currents in the Ocean." By J. Y. BUCHANAN, 

 M.A., F.R.S.E. Communicated by the late Sir Frederick 

 Evans, K.C.B., R.N., Hydrographer to the Admiralty. 

 Received March 24, 1884. Received after revision January 

 23, 1888. 



It is frequently asserted and commonly believed that tidal currents 

 do not exist in the open ocean or in waters remote from land. 

 Oceanic currents, that is, streams which set more or less constantly 

 in one general direction, are well-known and, from their importance 

 to navigation, have been the objects of much study. Chief among 

 these may be mentioned the Gulf Stream and the Equatorial Cur- 

 rents in both oceans. The data on which almost all our information 

 connected with these streams rests are furnished by the logs of ships 

 traversing them. When the position of the ship is determined from 

 day to day by good astronomical observations on the one hand, and 

 the courses and distances sailed are carefully observed and noted on 

 the other, it is usual, after allowing for known perturbing causes, to 

 ascribe the differences between the positions as ascertained by " ob- 

 servation " and by " dead reckoning " to the effect of a current. As 

 in the ordinary routine of a sea-going ship, the positions are made 

 up from noon to noon, the strength and direction of the current so 

 deduced is the integral resultant " set " of the previous twenty- four 

 hours. The direction and strength of the current may have changed 

 in any way during that time, and it would be nearly impossible to 

 detect such changes. The period of twenty-four hours corresponds 

 closely with that of the tidal wave, consequently in the time elapsing 

 between two successive noons, whatever effect may have been due to 

 a tidal cause will have completely reproduced itself twice over and a 

 very little more. The resultant current due to the tide during the 

 two complete periods will be nothing, and the only resultant current 

 affecting the day's reckoning will be that due to the difference be- 



