when opened the surface against which the water presses shall be less than that of the smaller part. The play of the gate is thus rendered very simple; when the valve is shut, the pressure of water on the larger surface closes it against the sides of the sluice; when the valve is opened, the gate swings round and takes a position in the direction of the current. Various other plans for flashing, on similar principles, are to be met with.

848. When the obstruction in a river cannot be overcome by any of the preceding means, as for example in those considerable descents in the bed known as rapids, where the water acquires a velocity so great that a boat can neither ascend nor descend with safety, resort must be had to a canal for the purpose of uniting its navigable parts above and below the obstruction.

The general direction of the canal will be parallel to the bed of the river. In some cases it may occupy a part of the bed by forming a dike in the bed parallel to the bank, and sufficiently far from it to give the requisite width to the canal. Whatever position the canal may occupy, every precaution should be taken to secure it from damage by freshets.

849. A lock will usually be necessary at each extremity of the canal where it joins the river. The positions for the extreme locks should be carefully chosen, so that the boats can at all times enter them with ease and safety. The locks should be secured by guard gates and other suitable means from freshets; and if they are liable to be obstructed by deposites, arrangements should be made for their removal either by a chase of water, or by machinery.

If the river should not present a sufficient depth of water at all seasons for entering the canal from it, a dam will be required at some point near the lock to obtain the depth requisite.

It may be advisable in some cases, instead of placing the extreme locks at the outlets of the canal to the river, to form a capacious basin at each extremity of the canal between the lock and river, where the boats can lie in safety. The outlets from the basins to the rivers may either be left open at all times, or else guard gates may be placed at them to shut off the water during freshets.

CHAPTER X.

SEACOAST IMPROVEMENTS.

850. THE following subdivisions may be made of the works belonging to this class of improvements: 1st. Artificial Roadsteads. 2d. The works required for natural and artificial Harbors. 3d. The works for the protection of the seacoast against the action of the sea.

851. Before adopting any definitive plan for the improvement of the seacoast at any point, the action of the tides, currents, and waves at that point must be ascertained.

852. The theory of tides is well understood; their rise and duration, caused by the attraction of the sun and moon, are also dependent on the strength and direction of the wind, and the conformation of the shore. Along our own seaboard, the highest tides vary greatly between the most southern and northern parts. At Eastport, Me., the highest tides, when not affected by the wind, vary between twentyfive and thirty feet above the ordinary low water. At Boston they rise from eleven to twelve feet above the same point, under similar circumstances; and from New York, following the line of the seaboard to Florida, they seldom

rise above five feet.

853. Currents are principally caused by the tides, assisted, in some cases, by the wind. The theory of their action is simple. From the main current, which sweeps along the coast, secondary currents proceed into the bays, or indentations, in a line more or less direct, until they strike some point of the shore, from which they are deflected, and frequently separate into several others, the main branch following the general direction which it had when it struck the shore, and the others not unfrequently taking an opposite direction, forming what are termed counter currents, and, at points where the opposite currents meet, that rotary motion of the water known as whirlpools. The action of currents on the coast is to wear it away at those points against which they directly impinge, and to transport the débris to other, points, thus forming, and sometimes removing, natural obstructions to navigation. These continual changes, caused by currents, make it extremely difficult to foresee their effects,

and to foretell the consequences which will arise from any change in the direction, or the intensity of a current, occasioned by artificial obstacles.

854. A good theory of waves, which shall satisfactorily explain all their phenomena, is still a desideratum in science. It is known that they are produced by winds acting on the surface of the sea; but how far this action extends below the surface and what are its effects at various depths, are questions that remain to be answered. The most commonly received theory is, that a wave is a simple oscillation of the water, in which each particle rises and falls, in a vertical line, a certain distance during each oscillation, without receiving any motion of translation in a horizontal direction. It has been objected to this theory that it fails to explain many phenomena observed in connection with waves.

In a recent French work on this subject, its author, Colonel Emy, an engineer of high standing, combats the received theory; and contends that the particles of water receive also a motion of translation horizontally, which, with that of ascension, causes the particles to assume an orbicular motion, each particle describing an orbit, which he supposes to be elliptical. He farther contends, that in this manner the particles at the surface communicate their motion to those just below them, and these again to the next, and so on downward, the intensity decreasing from the surface, without, however, becoming insensible at even very considerable depths; and that, in this way, owing to the reaction from the bottom, an immense volume of water is propelled along the bottom itself, with a motion of translation so powerful as to overthrow obstacles of the greatest strength if directly opposed to it. From this he argues that walls built to resist the shock of the waves should receive a very great batir at the base, and that this batir should be gradually decreased upward, until, towards the top, the wall should project over, thus presenting a concave surface at top to throw the water back. By adopting this form, he contends that the mass of water, which is rolled forward, as it were, on the bottom, when it strikes the face of the wall, will ascend along it, and thus gradually lose its momentum. These views of Colonel Emy have been attacked by other engineers, who have had opportunities to observe the same phenomena, on the ground that they are not supported by facts; and the question still remains undecided. It is certain, from experiments made by the author quoted upon walls of the form here described, that they seem to answer fully their intended purpose.

855. Roadsteads. The term roadstead is applied to an indentation of the coast, where vessels may ride securely at anchor under all circumstances of weather. If the indentation is covered by natural projections of the land, or capes, from the action of the winds and waves, it is said to be landlocked; in the contrary case, it is termed an open roadstead.

The anchorage of open roadsteads is often insecure, owing to violent winds setting into them from the sea, and occasioning high waves, which are very straining to the moorings. The remedy applied in this case is to place an obstruction near the entrance of the roadstead, to break the force of the waves from the sea. These obstructions, termed breakwaters, are artificial islands of greater or less extent, and of variable form, according to the nature of the case, made by throwing heavy blocks of stone into the sea, and allowing them to take their own bed.

The first great work of this kind undertaken in modern times, was the one at Cherbourg in France, to cover the roadstead in front of that town. After some trials to break the effects of the waves on the roadstead by placing large conicalshaped structures of timber filled with stones across it, which resulted in failure, as these vessels were completely destroyed by subsequent storms, the plan was adopted of forming a breakwater by throwing in loose blocks of stone, and allowing the mass to assume the form produced by the action of the waves upon its surface. The subsequent experience of many years, during which this work has been exposed to the most violent tempests, has shown that the action of the sea on the exposed surface is not very sensible at this locality at a depth of about 20 feet below the water level of the lowest tides, as the blocks of stone forming this part of the breakwater, some of which do not average over 40 lbs. in weight, have not been displaced from the slope the mass first assumed, which was somewhat less than one perpendicular to one base. From this point upwards, and particularly between the levels of high and low water, the action of the waves has been very powerful at times, during violent gales, displacing blocks of several tons weight, throwing them over the top of the breakwater upon the slope towards the shore. Wherever this part of the surface has been exposed the blocks of stone have been gradually worn down by the action of the waves, and the slope has become less and less steep, from year to year, until finally the surface assumed a slightly concave slope, which, at some points, was as great as ten base to one perpendicular.

The experience acquired at this work has conclusively shown that breakwaters, formed of the heaviest blocks of loose stone, are always liable to damage in heavy gales when the sea breaks over them, and that the only means of securing them is by covering the exposed surface with a facing of heavy blocks of hammered stone carefully set in hydraulic

cement.

As the Cherbourg breakwater is intended also as a military construction, for the protection of the roadstead against an enemy's fleet, the cross section shown (in Fig. 248) has been adopted for it. Profiting by the experience of many years' observation, it was decided to construct the work that forms the cannon battery of solid masonry laid on a thick and broad bed of beton. The top surface of the breakwater is covered with heavy loose blocks of stone, and the foot of the wall on the face is protected by large blocks of artificial stone formed of beton. The top of the battery is about 12 feet above the highest water level.

The next work of the kind was built to cover the roadstead of Plymouth in England. Its cross section was, at first, made with an interior slope of one and a half base to one perpendicular, and an exterior slope of only three base to one perpendicular; but from the damage it sustained in the severe tempests in the winter of 1816-17, it is thought that its exterior slope was too abrupt.

A work of the same kind is still in process of construction on our coast, off the mouth of the Delaware. The same cross section has been adopted for it as in the one at Cherbourg.

All of these works were made in the same way, discharging the stone on the spot, front vessels, and allowing it to take its own bed, except for the facing, where, when practicable, the blocks were carefully laid, so as to present a uniform surface to the waves. The interior of the mass, in each