TIIUI. \VATK! 



TIDE MILL. 



-il 



roadsteads. There are, in fact, very few }ioitions in which tidal har- 

 bours ore free from inconvenience, arising either from bars or from the 

 advance of alluvial matter*. The tidal harbour. 

 Benin, in the .!. putm. nt of the Calvados, presents in this respect 

 some peculiarities to which it may bo desirable to call attention, on 

 account of the geological interest, quite as much as on account of the 

 leosons in engineering, which they furnish. Port en Betsin is erected on 

 coast entirely open to the north, north-east, and northwest; the most 

 dangerous storms blow from the north-east ; and the flood-tide comes in 

 from west by north. The jetty on the west side has been built with 

 openings, for the avowed object of allowing the flood to sweep through 

 the head of the harbour ; and an opening is left in the eastern jetty to 

 allow a fresh-water river, which rues from between the clay beds of 

 the lower oolitic formations on the shore, to escape. This river would 

 appear to be the continuation of the little river Drome, which loses 

 itself about two miles in the interior, there passing In'twecn the forma- 

 tion known locally as the " argile du Port en Benin " and the lower 

 oolite itself, until it thus escapes on the sea-shore. 



In consequence of the frequent occurrence of bars and banks at the 

 mouths of tidal harbours, it is more than usually necessary that tidal 

 signals, lighthouses, and fog-bells should be established upon them. 

 and that the navigable channel should be carefully buoyed. Those 

 precautionary details must not, of course, be neglected in floating tidal 

 harbours, if the Litter should present any local peculiarities ; but they 

 are usually less necessary than in dry harbours ; nor does there exist 

 {n the former the same necessity for the existence of a soft bottom, of 

 mud or of sand, that exists in harbours wherein vessels are likely to 

 take the ground. In floating or in ordinary tidal harbours which have^ 

 docks for the reception of large vessels, the entrances to the docks* 

 must be placed at a position in the outer basin removed from the 

 agitation of the open sea ; and it would appear from the practical 

 working of the docks at HAvre, Liverpool, Ac., that it is preferable to 

 moke the entrance to the floating dock from an exterior half-tide dock, 

 rather than from the outer harbour itself. There is, in fact, a danger 

 of the gates communicating with the outer harbour licing occasionally 

 forced open by the agitation of the latter ; and at all times the half- 

 tid. basins facilitate greatly the manoeuvres of a port. In some tidal 

 harbours, as at Havre, Dieppe, Ostend, tie., pains ore taken to break 

 the waves which may be driven in from the open sea, by the erection 

 of timber stockades and inclined floors of masonry ; but in the majority 

 of eases it is found that the waves become sufficiently stilled by the 

 fact of their passing through a narrow passage at the entrance, and 

 then widening out into a large sheltered area. 



(Sir .r. Hennie. on //arlmuri; Sganzin, Court de Conitructifin ; Miuard, 

 Travail* Jfyilrauliyiui A la Her; the Parliamentary Jtr/mrli H the 

 llifllilatui RoaJt, &c. ; Sineaton'a Re/xirt* ; Life of Tel ford ; &c., &c. 

 The Report of the Commission on Tidal /larbourt, 1845, may also be 

 consulted.) 



TIIiAl. WATKIIS AND CITRRKXTS. The partial streams of 

 water set in motion by the great tidal waves, which frequently How in 

 . :lercnt from, or even directly opposed to, the advance of 

 the great tide waves themselves, are known technically by the name of 

 irrrnti ; and they are of the greatest importance in all nautical, 

 or hydraulic, engineering operations, on account of the interference 

 they are able to produce in the conditions of access to harbours on the 

 sea-coast, or to the embouchures of rivers. These currents :u 

 duced either by the projection of head-lands, or by the retarding 

 influence of the sea-shore upon the advance of a portion of t ! 

 wave, and they are therefore subject to so many modifying conditions, 

 as to render it more easy to explain their nature l.y reference to some 

 of the most remarkable currents known to exist, A few illustrations 

 of them will then fore be given ; but before so doing, it may be desir- 

 able to observe that the great oceanic currents, such as the (iulf 

 stream, the current through the straits of Gibraltar and at the head of 

 the Mediterranean, Ac., have no connection with the class of phenomena 

 in question ; for their creation and propagation depend upon cosmical 

 causes of a very different and of a much more complicated nature than 

 those which produce the real tidal currents of the sea shores. 



Now, the great flood tide, following the impulsion it receives from 

 the attraction of the sun and the moan, and from the rotation of the 

 earth, advances normally from the west towards the east ; and in mid- 

 ocean the only tidal current is precisely in this uniform direction. On 

 approaching the shores of a Urge island, or of a continent, however, the 

 tide striking the advanced headlands is diverted from its course in 

 some cases, whilst in others, the great advancing stream may be carried 

 in its former direction past the portion of the coast immediately behind 

 the headland, and thus only allow the flood tide to exhibit itself by 

 derivation from the main stream. On the shores of the Atl.it. 

 instance, the flood wave striking the extremities of Ireland, England, 

 and the department of the Kiuisterrc in France, changes its course in a 

 remarkable manner. A ].tti..n i tin- llood continues to advance in 

 its original direction through the British Channel; a second portion 

 runs northwards along the western coast of Ireland and of Scotland, 

 giving off in its advance a branch, or subsidiary, current running up 

 through the St. (Jeorge's Channel, and flowing in a rather north-easterly 

 direction until It meet* a sin nt, flowing through the 



Mull of Cantire, from the N.W. to the S.E. ; a third portion striking 

 Cape Kinisterrc turns towards the south, and runs along the coast of 



the Bay of Biscay. The flood tide running up the Bri' 



puses through the strait* of Dover, and spreads itself ov. 



Sea, meeting a portion of the flood 



trcmity of Scotland, and down the eastern coast of England, n. 



embouchure of the Thames; so that the united til" How up th.it 



river, sometimes synchronously, sometimes at alight intervals. In thu 



former case, the tide in the Thames is a simple one ; in the l.v 



may present the peculiarity of a double rise in the level of the water ; 



and the same phenomenon of the existence of more than . - 



flood may be perceived iu other rivers of the east, and even of the 



south, coasts of England, as well as on the northern coast of Franco, 



in consequence of the creation of subsidiary tidal current* by advancing 



Thus, the Bill of Portland projects so much into the line of advance 

 of the flood tide that the latter sweeps along in the offing at .111 

 tion above the water in-shore, until it reaches the Needles point, of 

 course parting with some of its own body, by derivation, i- 

 sheltered bay. At the Needles the flood divides, one portion flowing 

 from the W. by S. into the Southampton Water, the other continuing 

 up the Channel, but as it passes the Spitbeod Passage it gives off a 

 branch current, which flows through that channel from S. E. to N.\V., 

 until it meets the ebbing tide from the Southampton Water 

 running out. The Spithead flood current drives back the South:, 

 ebb, and thus makes the peculiar double tide of that river ; and it also 

 superposes itself in a manner upon the ebb of the coast lying - 

 t'alshot Point so as to create a double tide, known locally i 

 of the Quitter, which only ceases to be felt, .it the Hill of Portland. 

 Very much the game kind of action may be observed on the I 

 coast between the rocks of the Calvados and the Cape Antifc r ; for the 

 first flood tidal current in the bays at the mouths of the ' 

 the Seine, derived from the edges of the great flood tide in the offing, 

 is maintained for a variable period, by the return of a suhsidi 

 current deflected by the projection of the cape. At > -n, the 



double tide keeps the water at nearly a constant height for the space 

 of two hours; at Havre it remains in this state for hour 



and a .purler ; whilst at the mouth of the Orne, the flood tide 

 up for about one hour. The advantage thus produced to the naviga- 

 tion of those localities by these peculiar tidal currents is < 



The tidal current at Havre is, moreover, accompanied by a further 

 peculiarity. As was before said, the main fl<x>d tide in the ( i 

 runs in the offing with a direction from W. to E. until it strik. 

 Antit'er; there it divides, one portion or tid.-d current continuing to 

 flow up the Channel, the other running into the bay of the Seii 

 E. to W., until it strikes the jetty at the entrance to the port, which 

 deflects it a little, and causes the bulk of the current to run on until 

 it meets the Cape du Hoc. A portion of the flood is the: 

 and turns back towards the mouth of the harbour, with a L'. uer.il S.K. 

 by S. direction. A ship making this port at the flood tide, has thus to 



less than three tidal currents, flowing in diflercn' 

 tions ; and as all the shores swept by these currents are composed of 

 rocks consisting of materials easily transportable by the water, there Is 

 a constant formation of shifting bonks at the mouths of the Seine 

 taking place at a short distance from the shore. 



The effect of tidal currents upon the outline of a coast is one of tin. 

 most important forms of their action. For instance, it will be found 

 that all the bays exposed to the action of strongly defined flood 

 towards tlieir line of advance; and it also generally is the case that the 

 olent storms blow in the same direction, that is to say, in the 

 direction of the flood tide. In fact, the water is driven with the most 

 violence upon the shores by the flood tides; and they ,-ue tie 

 agents in removing and transporting the materials del 

 the shores. In all works for coast defences it is import ant. tin 

 to calculate all the existing, and the probable future, i-ffecS f the 

 tidal currents, observing always that they may depend in many cases 

 on modifications of the outline of the coast lying faraway from the, 

 locality considered. The advance of the flood tide on the eastern shore 

 of England from the north to south is, for instance, an effect of the 

 same circumstances which cause the flood tide on the west coast of 

 I, and of Scotland, t" How from south to north; mid in the 

 British Channel from west to east. The direction of the bars at the 

 mouths of rivers quiring into seas exposed to be cro.-vd by tidal 

 currents, it may be added, follow as a general rule the di; 

 resultant between the lines of flow of the ebb from the river, and of 

 i icing flood. 



The greatest amount of theot, tic.d information on the subject of 

 the laws of hydrodynamics affecting tidal currents, is to be found in 

 Vcntiiroli, ' .Sur la communication lat.'rale du mouvi-m. 'in 



rand Emmy, ' Sur les dudes;' in Sgan/in and Minimi, 

 '('ours de Construction;' in al Magazine;' in Lubbock, 



II. Airy, &e., ' (in Tides.' in the ' Philosophical Transact! 

 in Kennel's ' Investigation of Currents; ' Young ' On Tides,' in ' Ku- 

 cyclopadia Hritaiinica,' Ac., &c. 



TIDE MI 1. 1,. A mill driven by the water which is affected by the 

 rise and fall of the tides is called a tide mill, wh. : ieel be 



affixed to a building on the land, or to a .vessel floating in the river. 

 In the former case the water sometimes acts upon an un.l< 

 in escaping from a reservoir, which has been filled during the flood 

 tide ; but the working of the mill in this cose con only take place when 



