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751. In some of our rivers, obstructions of a very dangerous character to boats are met with, in the trunks of large trees which are imbedded in the bottom at one end, while the other is near the surface; they are termed snags and sawyers by the boatmen. These obstructions have been very successfully removed, within late years, by means of machinery, and by propelling two heavy boats, moved by steam, which are connected by a strong beam across their bows, so that the beam will strike the snag, and either break it off near the bottom, or uproot it. Other obstructions, termed rafts, formed by the accumulation of drift wood at points of a river's course, are also found in some of our western rivers. These are also in process of removal, by cutting through them by various means which have been found successful.

752. Slack-Water Navigation. When the general depth of water in a river is insufficient for the draught of boats of the most suitable size for the trade on it, an improvement, termed slack-water, or lock and dam navigation, is resorted to. This consists in dividing the course into several suitable ponds, by forming dams to keep the water in the pond at a constant head and by passing from one pond to another by locks at the ends of the dams.

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753. The position of the dams, and the number requisite, will depend upon the locality. In streams subject to heavy freshets, it will generally be advisable to place the dams at the widest parts of the bed, to obtain the greatest outlet for the water over the dam. The dams may be built either in a straight line between the banks and perpendicular to the thread of the current, or they may be in a straight line oblique to the current, or their

plan may be convex, the convex surface being up stream, or it

The three

may be a broken line presenting an angle up stream. last forms offer a greater outlet than the first to the water that flows over the dam, but are more liable to cause injury to the bed below the stream, from the oblique direction which the current may receive, arising from the form of the dam at top.

754. The cross section of a dam is usually trapezoidal, the face up-stream being inclined, and the one down-stream either vertical or inclined. When the down stream face is vertical, the velocity of the water which flows over the dam is destroyed by the shock against the water of the pond below the dam, but whirls are formed which are more destructive to the bed than would be the action of the current upon it along the inclined face of a dam. In all cases the sides and bed of the stream, for some distance below the dam, should be protected from the action of the current by a facing of dry stone, timber, or any other construction of sufficient durability for the object in view.

755. The dams should receive a sufficient height only to maintain the requisite depth of water in the ponds for the purposes of navigation. Any material at hand, offering sufficient durability against the action of the water, may be resorted to in their construction. Dams of alternate layers of brush and gravel, with a facing of plank, fascines, or dry stone, answer very well in gentle currents. If the dam is exposed to heavy freshets, to shocks of ice, and other heavy floating bodies, as drift-wood, it would be more prudent to form it of dry stone entirely, or of crib-work filled with stone; or, if the last material cannot be obtained, of a solid crib-work alone. If the dam is to be made water-tight, sand and gravel in sufficient quantity may be thrown in against it in the upper pond. The points where the dam joins the banks, which are termed the roots of the dam, require particular attention to prevent the water from filtering around them. The ordinary precaution for this is to build the dam some distance back into the banks.

756. The safest means of communication between the ponds is by an ordinary lock. It should be placed at one extremity of the dam, an excavation in the bank being made for it, to secure it from damage by floating bodies brought down by the current. The sides of the lock and a portion of the dam near it should be aised sufficiently high to prevent them from being overflowed by the heaviest freshets. When the height to which the freshets rise is great, the leaves of the head gates should be formed of two parts, as a single leaf would, from its size, be too unwieldy; the lower portion being of a suitable height for the ordinary manœuvres of the lock; the upper, being used only during the freshets, are so arranged that their bottom cross pieces shall rest, when the gates are closed, against the top of the lower portions. An arrangement somewhat similar to this may be made for the tail gates, when the lifts of the locks are great, to avoid the difficulty of manoeuvring very high gates, by permanently closing

upper part of the entrance to the lock at the tail gates, either by a wall built between the side walls, or by a permanent framework, below which a sufficient height is left for the boats to pass.

757. A common, but unsafe method of passing from one pond to another, is that which is termed flashing; it consists of a sluice in the dam, which is opened and closed by means of a gate revolving on a vertical axis, which is so arranged that it can be manœuvred with ease. One plan for this purpose is to divide the gate into two unequal parts by an axis, and to place a valve of such dimensions in the greater, that 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 sur

face 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, op similar principles, are to be met with.

758. 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.

759. 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.

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SEACOAST IMPROVEMENTS.

760. 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.

761. 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.

762. 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 twenty-five 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 NewYork, following the line of the seaboard to Florida, they seldom rise above five feet.

763. 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.

764. 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.

765. 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 land-locked; 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

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