PLYMOUTH SOUND: ITS TIDAL CURRENTS. 279 
from a south-easterly direction; the opposite quarter from which 
we should naturally look for it. It is true that the tidal wave ap- 
proaches the coast between the Lizard and the Start, generally 
speaking, at right angles, as the map of co-tidal lines or the plan 
of the crests of a tidal wave at each successive hour of its advance 
up Channel clearly shows.* We must divest our minds of the 
idea that the direction of a tidal current in the open sea coincides 
with the direction of progression of a tidal wave. The time of 
high water at the Eddystone is eighteen minutes earlier than in 
the Sound, and the currents there begin to change about the time 
of high-water or low-water in the Sound. The main Channel 
stream is observed to run on flood tide for three to three and a-half 
hours after the change at the Eddystone in the direction of the ebb 
tide. Similarly a flood current, or that which flows in the same 
direction as the advancing tidal wave, will continue running for 
the same time after high-water. 
A general and simple explanation of this apparently paradoxical 
fact may be found in the pendulum. During an oscillation the 
maximum velocity occurs when the pendulum is at its lowest level, 
and the minimum or no velocity at the half-swing, or half-way 
between two successive passages over the lowest point. So with 
the tidal currents in the Channel, the slack-water or no velocity 
occurs half-way between the crest of one wave and the hollow of 
another, or half-way between their point of highest and lowest 
level. We must not forget that when motion is imparted to water 
its surface of equilibrium is not a horizontal plane, as it is when 
at rest. We need not therefore be at all surprised from whatever 
direction the flood stream first makes off Plymouth, even leaving 
the Eddystone rocks out of the question, which have undoubtedly 
a great influence on the local channel currents. Numerous tide- 
linings are seen encircling and extending in a north-westerly and 
an easterly direction from the Lighthouse, clearly showing that a 
very considerable interruption takes place, and that it is not entirely 
caused by the cluster of rocks on which the Lighthouse stands, 
but mainly by its submerged ramifications extending in various 
directions. 
Whether the current, which makes into the bay from an easterly 
direction at the commencement of the flood, is a branch of the 
main channel ebb-stream deflected inshore, or an eddy caused by 
* Dr. Whewell’s Chart of Co-tidal Lines. Phil. Trans., 1833. 
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