Disturbances due to Tides and Waves. 151 



cannot be anticipated. It is not proposed to discuss the 

 disturbing conditions exhaustively, but merely to indicate 

 the general effect of some of them. 



The velocity of* a stream in general diminishes from the 

 centre towards the banks. In this case 



E=Vv S, 



where r is the mean velocity, S the total breadth of the 

 stream, and E the E.M.F. acting across it. 



Similarly C max . = Vv /p. 



In making observations in a broad channel, it would in 

 general be necessary to moor the stationary electrodes near 

 one shore. If the observed velocity in the experimental 

 area is u ls the relation 



e 1 = —YviS + Cps 



still holds ; but C may have a value as great as Vr /jO, in 

 which case 



£] = — Vr,5-j-Yv 



The effect would be to reverse the sign of e\ and give the 

 impression that the observations disagreed even qualitatively 

 with theory. 



The corresponding effect with drifting electrodes would 

 be to give an indicated E.M.F. above the theoretical 

 maximum. 



The tidal velocity diminishes not only toward the banks 

 but also toward the bed. Hence v and consequently e has a 

 different value for different strata. Near the centre of the 

 channel the equipotential lines would probably still be 

 vertical, C diminishing with the depth and possibly even 

 reversing in sign in such a manner as to make (Yvs — Cps) 

 constant. Near the banks C would probably have a vertical 

 component. If the banks were non-conducting the result of 

 the variation of v with depth would be the production of 

 electrical " convection " currents in the water (PL- II. fig. 2). 



If the sea-bed is conducting it will operate in the same 

 way as a stratum of water of zero velocity. Its general 

 effect would be to increase C, thereby diminishing e i9 and 

 increasing e^. It would also probably tend to reduce the 

 effects at a distance of non-uniform velocity. 



Other disturbing factors are irregularities of the shores 

 producing local variations in the direction of the tidal 

 stream. 



