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 conical shaped 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. 175) 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.

A, mass of stone.

B, battery of masonry.

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, from 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 case, has been formed of stone in small blocks, and the facing of very large blocks. It is thought, however, that it would be more prudent to form the whole of large blocks, because, were the exterior to suffer damage, and experience shows that the heaviest blocks yet used have at times been displaced by the shock of the waves, the interior would still present a great obstacle.

From the foregoing details, respecting the cross sections of breakwaters, which from experiment have been found to answer, the proper form and dimensions of the cross section in similar cases may be arranged. As to the plan of such works, it must depend on the locality. The position of the breakwater should be chosen with regard to the direction of the heaviest swells from the sea, into the roadstead,-the action of the current, and that of the waves. The part of the roadstead which it covers should afford a proper depth of water, and secure anchorage for vessels of the largest class, during the most severe storms; and vessels should be able to double the breakwater under all circumstances

of wind and tide. Such a position should, moreover, be chosen that there will be no liability to obstructions being formed within the roadstead, or at any of its outlets, from the change in the current which may be made by the breakwater.

766. The difficulty of obtaining very heavy blocks of stone, as well as their great cost, has led to the suggestion of substituting for them blocks of artificial stone, formed of concrete, which can be made of any shape and size desirable. This plan has been tried with success in several instances, particularly in a jetty or mole, at Algiers, constructed by the French government. The beton for a portion of this work was placed in large boxes, the sides of which were of wood, shaped at bottom to correspond to the irregularities of the bottom on which the beton was to be spread. The bottom of the box was made of strong canvass tarred. These boxes were first sunk in the position for which they were constructed, and then filled with the beton.

767. Harbors. The term harbor is applied to a secure anchorage of a more limited capacity than a roadstead, and therefore offering a safer refuge during boisterous weather. Harbors are either natural, or artificial.

768. An artificial harbor is usually formed by enclosing a space on the coast between two arms, or dikes of stone, or of wood, termed jetties, which project into the sea from the shore, in such a way as to cover the harbor from the action of the wind and waves.

769. The plan of each jetty is curved, and the space enclosed by the two will depend on the number of vessels which it may be supposed will be in the harbor at the same time. The distance between the ends, or heads, of the jetties, which forms the mouth of the harbor, will also depend on local circumstances; it should seldom be less than one hundred yards, and generally need not be more than five hundred. There are certain winds at every point of a coast which are more unfavorable than others to vessels entering and quitting the harbor, and to the tranquillity of its water. One of the jetties should, on this account, be longer than the other, and be so placed that it will both break the force of the heaviest swells from the sea into the mouth of the harbor, and facilitate the ingress and egress of vessels, by preventing them from being driven by the winds on the other jetty, just as they are entering or quitting the mouth.

770. The cross section, and construction of a stone jetty differ in nothing from those of a breakwater, except that the jetty is usually wider on top, thirty feet being allowed, as it serves for a wharf in unloading vessels. The head of the jetty is usually made circular, and considerably broader than the other parts, as it, in some instances, receives a lighthouse, and a battery of can

non. It should be made with great care, of large blocks of stone well united by iron, or copper cramps, and the exterior courses should moreover be protected by fender beams of heavy timber, to receive the shock of floating bodies.

771. Wooden jetties are formed of an open frame work of heavy timber, the sides of which are covered on the interior by a strong sheeting of thick plank. Each rib of the frame (Fig. 176) consists of two inclined pieces, which form the sides,

-of an upright centre piece, and of horizontal clamping pieces, which are notched and bolted in pairs on the inclined and upright pieces; the inclined pieces are farther strengthened by struts, which abut against them and the upright. The ribs are connected by large string-pieces, laid horizontally, which are notched and bolted on the inclined pieces, the uprights, and the clamping pieces, at their points of junction. The foundation, on which this framework rests, consists usually of three rows of large piles driven under the foot of the inclined pieces and the uprights. The rows of piles are firmly connected by cross and longitudinal beams notched and bolted on them; and they are, moreover, firmly united to the framework in a similar manner. The interior sheeting does not, in all cases, extend the entire length of the sides, but open spaces, termed clear-ways, are often left, to

give a free passage and spread to the waves confined between the jetties, for the purpose of forming smooth water in the channel. If the jetties are covered at their back with earth, the clear ways receive the form of inclined planes.

The foundation of the jetties requires particular care, especially when the channel between them is very narrow. Loose stone thrown around the piles is the ordinary construction used for this purpose; and, if it be deemed necessary, the bottom of the entire channel may be protected by an apron of brush and loose stone.

The top of the jetties is covered with a flooring of thick plank, which serves as a wharf. A strong hand railing should be placed on each side of the flooring as a protection against accidents. The sides of jetties have been variously inclined; the more usual inclination varies between three and four perpendicular to one base.

772. Jetties are sometimes built out to form a passage to a natural harbor, which is either very much exposed, or subject to bars at its mouth. By narrowing the passage to the harbor between the jetties, great velocity is given to the current caused by the tide, and this alone will free the greater part of the channel from deposites. But at the head of the jetties a bar will, in almost every case, be found to accumulate, from the current along shore, which is broken by the jetties, and from the diminished velocity of the ebbing tides at this point. To remove these bars resort may be had, in localities where they are left nearly dry at low water, to reservoirs, and sluices, arranged with turning gates, like those adverted to for river improvements. The reservoirs are formed by excavating a large basin in-shore, at some suitable point from which the collected water can be directed, with its full force, on the bar. The basin will be filled at flood-tide, and when the ebb commences the sluice gates will be kept closed until dead low water, when they should all be opened at once to give a strong water chase.

773. In harbors where vessels cannot be safely and conveniently moored alongside of the quays, large basins, termed wetdocks, are formed, in which the water can be kept at a constant level. A wet-dock may be made either by an in-shore excavation, or by enclosing a part of the harbor with strong water-tight walls; the first is the more usual plan. The entrance to the basin may be by a simple sluice, closed by ordinary lock gates, or by means of an ordinary lock. With the first method vessels can enter the basin only at high tide; by the last they may be entered or passed out at any period of the tide. The outlet of the lock should be provided with a pair of guard gates, to be shut against very high tides, or in cases of danger from storms.