He then gives a description of a dam on the principle of the one suggested by General Moorhead. He says:

Other dams are so arranged as to fall when the pressure of the water increases. The wicket-gate with horizontal axis is an example of this class. The gate or wicket, when closed, rests at its lower end against a shoulder in the top of the dam. As long as the water above is below the horizontal axis of the wicket, its pressure increases its stability. But when the water rises above the axis, a counter-pressure is created. Both of these pressures only become equal (provided the down-stream water does not touch the wicket) when the up-stream water stands twice as high above the axis as this is above the top of the dam. If the water rises higher than this, the pressure on the upper part of the wicket becomes greatest, and it falls.

Figures 4, 5, 6, and 7 show a dam with this arrangement in the Riom, in the department of Puy de Dome. (Annales des Ponts et Chaussées, 1842, I, page 231.)

There are three openings in this dam, each 12 feet 9 inches in length; the wickets, consisting of a double layer of planks, are 2 feet 3 inches high. They extend into the abutments, where they turn in recesses whose edges support the wickets and diminish the leakage, besides preventing the wicket from turning down too far. The other peculiarities of this arrangement are shown by the drawings; but it must be borne in mind that when the water falls the wickets do not raise themselves, but must be lifted up from the abutments by means of hooks.

Similar turning gates or wickets may be held in an upright position by means of hanging weights. At high stages these counterpoises are submerged, and their loss of weight causes the wickets to fall down; when the water falls the weight of the counterpoises is restored and the gates are raised.

This arrangement has been practically applied ever since the year 1834, in the improvement of the Ouse above York. (Civil Engineering and Architects' Journal, 1840, III, page 284.)

The Ouse was formerly navigable only as far up as Selby. Farther up to York the depth on several gravel-bars at low water was only 5 feet.

To navigate this portion of the river it was, therefore, necessary to await the spring freshets, and the larger class of ships could not get up at all. After several of the gravel-bars had been removed by dredging a depth of from 11 to 12 feet was obtained at high water.

After this it was decided to extend the navigation of the river 20 miles farther up, to Borough Bridge.

A number of locks and dams already existed on this part of the river; but the depth of water in the intervening pools was not sufficient, and it was impossible to raise the dams on account of the low banks of the river. Nevertheless, at low stages in summer it was possible to raise the surface of the water 18 inches This purpose was subserved by the wicket arrangement just described.

Two wickets of this kind, each 74 feet long, 18 inches wide and 4 inches thick, constructed of pine wood, constitute the movable portion for increasing the height of the dam, whose whole length is 148 feet. They stand upon the top of the massive dam, and are held at every ten feet by strong iron hinges, let into and fastened with lead to hammered stone, which permit them to lie flat on the top of the dam. The movable arm of each hinge ends in a pin 1 inch in diameter, which projects above the wicket. To these pius flat chains are fastened, which wind up on an equal number of eccentrics. The latter are attached to two iron axles, each of which has the same length as the corresponding wicket. These axles rest on iron chairs, which are placed on the up-stream slope of the dam. Next to each of the abutments a cog-wheel is attached to the axle. This cog-wheel is driven by another wheel, which latter is conneeted with a pulley from which the counterpoise bangs.

When the counterpoise rests upon the abutment the wickets stand in a perpendicular position. This is the case as long as the water does not rise above 6 inches over the wickets, or 2 feet over the top of the dam. When the water rises above this point its pressure upon the wicket gains the ascendency; they lie flat upon the dam, and their influence upon raising the water is completely destroyed. As soon, however, as the water subsides, the counterpoise sinks, and the wickets resume their upright position. This arrangement was proposed and carried out by Engineer Rhodes, and it is said to have given results in every respect favorable.

Hagen then goes on to say:

Another ingenious arrangement of such wickets on dams was executed by Engineer Thénard, on the River Isle, in the Department de la Dordogne. It not only possesses the advantage of the one just described, in that the shutter falls flat upon the top of the dam, but it possesses besides the superiority in this, that no chairs or other parts of the apparatus project, and that, therefore, the greatest ice-field can pass over it without causing any damage. The raising and lowering, it is true, is not caused by the rise and fall

of the water itself, but the attendant must, by means of a mechanism which he controls from the shore, sever certain connections. There is, however, no disadvantage in this, as the employment of an attendant is already necessary at that point on account of the lock. The movement of the shutter is caused, however, by the pressure of the water, and results at once, without the application of ontside forces. These movable dams were partly constructed in 1825, and finished in 1832. In the report of the commission which examined them in 1841, no mention is made either of injuries or repairs, from which it must be inferred that such did not occur to any noteworthy degree.

Figures 8, 9, 10, and 11 show one of these dams, the elevation looking down stream, the plan, and two sections. The whole length of the dam is 151 feet; at one end there is a lock and two sluices. The dam is created by the down-stream shutters, which fall down stream. When they are raised, as figure 10 shows, they are held in position by iron props which abut against iron plates ou the top of the dam. It is only necessary to trip these props, and the shutters fall down under the pressure of the water. For this purpose an iron rod (figure 12) is used, which extends lengthwise over the dam, and is provided with teeth at one end, which are grasped by a cog-wheel. This cogwheel is put in motion by a lever which is attached to its axle. The props are not tripped at once, but in succession, a different one being tripped each time that the bar advances 1 inches. This is accomplished by means of projections on the iron rod, whose distances apart are a little greater than the distances between the props. The drawing back of the rod before the shutters are again set up is accomplished by the same machinery.

The raising of the shutters against the pressure of the water would, however, be very difficult if this pressure could not be removed while doing so. The counter-shutters, which fall in the up-stream direction, are used for this purpose. They generally lie flat, both in high and low water, and grasp, by means of a spring-latch, a catch which is fastened to the top of a post in front of each. These latches can be moved in the same manner as the props which sustain the lower shutters. As soon as one of them is sprung the water from above raises the counter-shutter as far as the chains will admit. When all the counter-shutters are raised the water is dammed, and several minutes intervene before it flows over. During this interval the lower part of the top of the dam is dry, and the attendant has the opportunity, without the least danger, to descend by means of a ladder, and to raise each section of the dam in succession, replacing its prop in its proper position. The operation is begun on the side from which the attendant descends, and is continued to the other side. He then waits until the water flows over the upper edge of the counter-shutters, and the real shutters become the dam. As soon as this occurs the pressure ceases against the counter-shutters, and they can be easily and rapidly pushed down in succession by a pole, so that the spring-latches again grasp the catches and hold them firmly.

The commission which examined this construction on the Isle expressed the opinion that these shutters could safely be made from 3 to 4 feet high, but in the latter case it would be well to limit the length of the sections to 34 feet.

Hagen then mentions that movable dams have been constructed which turn around vertical axes like lock-gates. The axis is placed either in the center of the gate or near it. They are maneuvered by means of valves in each leaf, assisted by machinery. The advantage is that as the gates are high the machinery for turning them can always be used. The disadvantages are that they can only be made of narrow spans : that, when open, they stand in the stream, and are exposed to destruction by ice and drift; and that, when closed, only the larger leaf can be supported, the pressure of the water having a tendency to move the other leaf away from any support that might be constructed for it.

He then mentions another form of movable dam; this consists in laying a sill in the bed of the river, aud swinging, just above this, a beam from abutment to abutment, and then placing, by hand, small scantling, which he calls "needles," resting against the sill below and the beam above. This plan has been practiced in France for many years.

A great improvement on this plan was made by the French engineer Poirée a short time before 1840.

The improvement consists in this, that instead of having a single swinging beam for the whole span, this beam consists of different pieces resting upon movable trestles. The first dam of this kind is in the Yonne, just below the mouth of Bourgogne Canal, near the village of Epineau. Soon after (in 1840) a similar dam was built in the

Seine, near Paris. The result of both, so far as it is known, seems to have been satisfactory in every respect; and since that time they have been erected in many other streams in France, with frequent alterations, however, in their construction.

The following is a description of the first one erected, (near Epineau :) The movable portion of this dam is 223 feet long. Its masonry-floor has (as Fig. 13 shows) a width of nearly 32 feet, and is 154 inches below the lowest stage of water in summer. The floor is about on a level with the natural bed of the river. The masonry of the floor is on an average about 4 feet thick. A recess, 7 feet and 3 inches in breadth, extends lengthwise along the floor. In this the wooden frame which supports the movable iron trestles is laid and firmly wedged.

The whole arrangement is shown in figures 14, 15, and 16. The edges of the recess are not vertical, but inclined, the width on the bottom being greater than on top. On the down-stream side a wooden sill of corresponding form rests against the edge of the recess. The rest of the recess is then filled by a horizontal wooden frame, which is bound together at intervals of 3 feet 2 inches from center to center by braces mortised and tenoned into it. In the space between the sill and this frame double wedges are driven at intervals of 6 feet 6 inches, by which the whole wooden frame is held in the

recess.

This manner of fastening is so simple that it is not necessary to draw off all the water in order to take out or replace the frame. According to Chanoine, this can be done without difficulty in a 3-foot stage of water.

In later constructions the sill is entirely abandoned, the down-stream side of the recess being made vertical and of cut stone, so that the frame may be firmly wedged against it.

On the inner side of the longitudinal beams of the frame, and immediately over the center line of each brace, are laid the iron collars wherein the axles of the trestles turn. These collars are so arranged that the trestle may be taken out and replaced under water. Figure 17 shows the elevation of the up-stream collar. It is simply supplied with a round hole into which the axle is inserted. The opposite collar, however, (Fig. 18,) has, besides the hole in which the axle lies, a slot at one side enlarging upward, down which the axle must be shoved, and which is then closed by an iron plug. The iron trestles do not, however, rest entirely on these collars, as their axles touch the braces just below them, and a part of the weight is borne by the latter.

But

The trestles, nearly 7 feet in height, are constructed of bar-iron, jointed and riveted, except the upper end of the strut, which is held by a screw. The bar-iron is 14 inches in thickness, and also in width. The weight of each trestle is such that two workmen standing on the foot-bridge can easily raise it by means of the chain. This chain is fastened to the ring which projects from the head of the screw above referred to. this chain is in no wise indispensable, for an experienced workman can easily grasp the upper beam of the trestle and raise it into position. It is, therefore, customary during winter to remove the chains, because they are frequently caught by drift, and then are not only subject to injury themselves, but are apt to retain such objects at the dam, and cause heavy deposits of gravel.

In constructing the dam, each trestle is raised by the attendant and his assistant; as soon as it is nearly vertical, and only moderate strength suffices to hold it, the attendant leaves this part of the business to his assistant and seizes the trestle at the upper bar with the forward notch of the hook, (Fig. 19,) and then places the rear notch upon the last trestle raised, which is now firmly established. The foot-bridge is then extended by laying three short planks, about 4 feet in length, alongside of each other, and so far forward that they have a firm hold upon the newly-raised trestle. Through a notch in the planking a strong pin fastened to the upper bar of each trestle extends above the floor. This is partly to serve as a guide for properly laying the plauks, but mainly as a firm point against which the attendant can brace his foot during the operation of raising the next trestle.

As soon as the planks are laid, two iron clamp-bars, one inch in thickness and nearly two inches in width, provided with two notches, (see Fig. 20,) are laid upon the last two trestles, after which the hook which temporarily held the last trestle in position may be removed. One of these iron clamps is laid on the up-stream side of the heads of the screws, and forms the upper bar against which the "needles" or scantling rests. The other, which need not be so strong, and is often entirely omitted, rests, if it is used at all, upon the down-stream corner of the trestle.

In this manner the whole of the frame-work may be erected by two men, although it is generally customary to employ a third laborer to carry the planks and the clamps. The removal of the dam is carried on in the inverse order, and needs no description. It must be remarked, however, that it is not necessary to raise up or lower all of the trestles, as each one, when it once has its fuil connection on one side, is perfectly safe. It is, therefore, quite easy to make an opening at one end of this dam for passing vessels. The last trestle lies as flat as the rest in an opening provided for that purpose in the massive abutment.

Iron bars, projecting above the floor of the bridge, are fastened on the outside to the top of the posts of each trestle; but, in order to preserve clearness, they have not been indicated in the drawings. The one on the up-stream side is only a few inches high, and serves partly to prevent the clamp-bar from clipping off, and partly to hold the last "needle" in case this trestle becomes the end of the dam. The iron bar at the other side rises about one foot above the foot-bridge, and is also partly for the purpose of holding the other clamp-bar, but mainly to serve as a rest against which the “needles” are piled during the course of constructing or removing the bridge.

The "needles" are made of straight-fibered wood, and are 7 feet 9 inches long, 24 inches broad, and 14 inches thick. They rest at top against the clamp-bar above mentioned, and at bottom against the up-stream longitudinal beam of the frame, into which for this purpose an iron bar is let. In front of the axle of each trestle this iron bar has a small projection, which holds the lower end of the "needles" in the same manner as the top is held. The attendant thus has a guide at each trestle, by which he can keep the "needles" in the proper position.

The cost per running foot of this dam, including foundations, abutments, and all appurtenances, was 175 thaler, ($128.) After a few attempts it required 32 seconds per running foot to take down the dam, and 57 seconds per running foot to erect it. But it was soon shown that the work was done more rapidly as soon as the attendant had more experience.

It is undoubtedly a question of great importance to ascertain how this dam will act during high-water, and whether it will not cause large deposits of sand, which will make it difficult to raise the trestles, and therefore necessitate the resort to extraordinary measures for this purpose. From all information which has become public, and from what could be learned from private inquiry, it seems that these fears are unfounded. Chanoine says that it is true that the recess is sometimes filled up with deposit, but that even then the trestles can easily be raised or lowered if they are first worked up and down a little, so as to loosen the deposit, and cause the current to carry it off. The deposit that remains in the recess is, according to the same authority, easily removed by forcing a rapid stream of water over it, and for this purpose the dam itself furnishes every facility. It is only necessary, after the dam is constructed, to remove a few of the "needles" at the point where the scouring is to be done. In this manner there is no difficulty after the whole structure is erected in completely cleaning the foundation and recess. Even if the trestles were found so completely covered that they could not be lifted by either the chain or hook, the proximity of the already constructed portion of the foot-bridge would allow the easy removal of the deposit by handscrapers, and thus permit the trestle to be raised.

It is besides probable that in such a case this difficulty would be removed by putting the "needles" in place as far as the foot-bridge is constructed, as by this means the rapidity of the current would be so much increased that it would remove the deposit from the trestles that were buried. The conditions are precisely the same as in building a wing-dam, or dike. The portion of the dam already erected is nothing but a perpendicular wing-dam with a very steep face, and precisely as such a work under other circumstances causes a great scour at its outer end, so in this case will it remove the gravel and sand from the foundation of the dam.

In some instances, as in the Saône, between the Rhine-Rhone Canal and Lyons, the arrangement just described does not form the entire dam, but is only an addition to the permanent structure, which in this case rises to ten inches above the lowest stage of water. (Annales des Ponts et Chaussées, 1845, I, page 10.) In this arrangement there is undoubtedly no trouble with deposit, or at furthest only with that which the recess contains. But even this cannot occur here, as the foot-path consists of iron plates fastened to the upper bar of the trestles in such a manner that it lies down with the trestles, and thus makes a cover which effectually closes the recess and conceals the whole apparatus. The details of this arrangement are thus far unknown.

A great advantage of this dam is that the stage of water can be kept at any point in case of rises by simply removing a proper number of needles. It is advantageous, however, not to make this opening at one point alone, but to make numerous openings uniformly distributed along the whole dam. The attendants are able to judge with great accuracy in this matter, so that it is not necessary for them to make continual experiments, but they are immediately able to tell how many needles it is necessary to remove for any rise.

When the water below the dam is high, with a strong wind up stream, there is danger that the waves may lift the foot-path from its position and carry it away. This evil is averted by drawing the chain diagonally across the foot-bridge between the successive trestles.

The erection of the trestles and the placing of the "needles" does not generally require haste, as the low-water stage of rivers and streams generally comes gradually. On the other hand, rises, especially in mountain streams, (where these dams are generally used,) often occur suddenly, in which case the arrangement just described for removing the "needles" is not as rapid a maneuver as the circumstances require.

REPORT OF THE CHIEF OF ENGINEERS.

This is particularly the case when the rise occurs at night, because then the work is delayed by darkness. Provision for the convenient and safe use of lanterns must naturally be made, but the lifting out of the "needles" may be avoided, and the stream may be made to push down the sections between the trestles, if the iron clampbar is arranged in the same manner as the turning-beam above described. This arrangement is made at the dam in the Seine at Saint Morton. (Annales des Ponts et Chaussées, 1843, I, page 250.) Each needle is provided with a strong ring at its handle. A line is tied to the ring of the "needle" next to each trestle, and passed through the rings of every "needle" in that section and then fastened to a strong hawser which reaches the whole length of the dam. In this manner all the "needles" remain hanging in bundles when the trestle is lowered, and as only one end of the hawser is fastened they are driven against the bank; afterward they are easily loosened and carried to the store-house.

The clamp-bar must be arranged differently from the manner above described if it is to be loosened when the "needles" are still pressing against it. The arrangement is quite complicated, and is made more so by the necessity of guarding against all accidental or malicious movements which would result in the opening of the dam. I will describe here only the material part of the arrangement. Figs. 21, 22, 23, and 24 show it in plan, and in front and side elevation. The clamp-bar has, in every instance, a hook 24 inches broad and 1 inch high. At one end it is provided with a hole, through which passes a pin which projects above the trestle. At the other end the hook is bent into a cylindrical form, which grasps the pin on the trestle. The front side of the hook forms an even surface, against which the "needles" rest.

To prevent the hooks from being loosened by the pressure of the "needles" there is an eccentric disk, which turns around the shank of the ring previously mentioned, to which the chain for raising the trestle is fastened. Figures 21, 22, and 23 show the eccentric disk in the position in which it holds the hook. In Fig. 24 it is turned so far that the hook no longer takes hold and the trestle may be laid down. Before the eccentric is turned into this position the trestle must be held by the light hook (Fig. 19) until the foot-bridge can be removed. The position of the eccentric disk is finally fixed by a key, which must be raised whenever it is to be turned. If the latter is in such a position that the eccentric disk is clear, as is shown in Fig. 24, a blow on the point of the disk will cause it to turn, and the pressure of the "needles," which is transferred to the hook, will complete it. The hook, however, is removed before the trestles are allowed to fall.

Finally, it must be stated that the abutments at St. Morton are not provided with recesses, but that the last clamping-bar, whose length is equal to the height of a trestle, turns on a vertical axis and is held in its position by a brace. It rests flat against the wall when the dam is lowered.

Becker also speaks of the great importance of movable dams, and then gives a description of several kinds already constructed, all of which, except one, are given by Hagen. of Thénard and Poirée. This one is a combination of the systems He says:

The systems of Thénard and Poirée can be combined with advantage to form a system peculiarly adapted for sluices which must be rapidly opened, or which are regulated by the water itself and need no attendance. Figs. 25, 26, 27, 28, and 29 show the construction of such a dam as it is built across the Seine, at Courbeton, in the vicinity of Montereau. A needle-dam, 124 feet 8 inches in length, abuts against the tail-wall of a lock. To carry off the surplus water in high stages, a sluice 39 feet 4 inches wide suffices. This sluice was, therefore, built according to this mixed system, and is separated from the needle-dam by an abutment 3 feet 3 inches thick.

When the sluice is closed, the shutters, Figs. 26, 28, and 29, are raised, and the iron trestles of the dam are also raised, and serve as support for the planks of a foot-bridge. The needles are not used.

When the water rises it flows into a culvert in the shore abutment, and thence to a small water-wheel 5 feet 3 inches in diameter and 20 inches wide. (Fig. 26.) On the axle of this wheel there is a bevel-wheel, which works into another bevel-wheel, on whose axle there is a rectangular cog-wheel which works into another wheel, whose axle is vertical and reaches down to the foundation of the dam. At the lower end of this axle there is another cog-wheel, which works into the toothed end of a rod, (Fig. 27,) whose motion, in a longitudinal direction, causes the successive falling of the shutters.

As soon as enough shutters have fallen to reduce the water to its former height, the water-wheel stops, and no further opening of the sluice takes place.

At high-water the effect of the water upon the water-wheel only ceases when all the wickets are down.

From this it is easily seen how the stage of water regulates itself, and prevents a flow over the top of the needle-dam.