Kiis'AHAN — Tidal Currents and Wind-icaves. 



Uo 



:find that, towards tlie end of the flow of the tide, there is a counter in- 

 shore tide from the headland (h), in the direction of the arrow (d). The 

 figure represents a regular bay, with equal headlands ; but in nature 

 we often find the shore line of the bay more or less irregular, or a con- 

 siderable river flowing into it, which causes various complications in 

 the set of the secondary currents in the bay. Let us suppose one head- 

 land to be longer than the other ; if the first headland is the longer, 

 off-shore shoals may form across the mouth of the bay, and, after they 

 are formed, the force of the secondary currents seems to increase from 

 c^ to c^, fig. 1. The formation of shoals, however, seems to depend 

 also very much on the nature of the rocks forming the sea margin ; 

 for if the margin of the bay and the coast up-stream, or in the direc- 

 tion from whence the tidal current comes, are of hard rocks, there may 

 be no materials to form shoals out of ; while if the margin of the bay, 

 or even the coast line up-stream, is of frail materials, there will be 

 shoals : a river might, in some places, also bring down materials suffi- 

 cient to form shoals ; this, however, is an exceptional case in Ireland. 

 Or, as in fig. 2, a third headland (c) may be opposite the headland {b), 

 forming a narrow, as in the English Channel between Portland Bill and 

 Cape la Hogue,^' in which case the primary current (A) seems often to 

 strike against the second headland (h), while the secondary currents 

 increase in strength from d^ to d^. That the secondary currents vary 

 in power, as mentioned above, seems proved, for the following reasons. 

 In such a case as that represented in fig. 1, the beach margining the 

 bay is made up, in nearly all places, of materials very similar, both in 

 quantity and size, which travel round the beach from a to b; while in 

 such a case as represented in fig. 2, the materials forming the beach 



Fig. 2. 



increase considerably, both in quantity and size, in the vicinity of the 

 second headland {b), while the major portion of them travels across 



* Here, in addition to the narrow in the sea, the force of the cuiTcnt is increased 

 as it approaches Portland Bill, hy the " Nodal " or " Hinge Line" of the Tides in 

 the English Channel, being immediately east of Portland. — {Hmujhton^ on " The 

 Tides," ^-c, pp. 22, et seq.) To this, thus augmented current, is probably due the 

 assorting and piling of the gravel and shingle of Chesil beach on the east side of 

 Lyme Bay, immediately N.N.W. of Portland Bill. 



