beam, which work under a simple and easy impulse, and to wickets in one piece which turn around a horizontal axle, without the complication of counterpoises, retaining-chains, props, or tripping-rods. In view of its unity and of the simplicity of its conception and its working, it is one of the most remarkable inventions that ever originated from the laborious investigations of engineers.

"But in consideration of the necessity of a carefully-constructed inclosure, this system, for the present at least, ought to be restricted to use on weirs.

"Besides the considerations just mentioned, it is of the greatest importance that a dam should be susceptible of being made water-tight, should necessity demand it. In this respect he thinks that there is a great deal yet to be done. The difficulty arises from the fact that the fall at a dam reaches its maximum when the river has its minimum discharge, and that at this time the water passes through the intervals with the greatest velocity, and more may thus be wasted than can be supplied from above, thus causing a decline in the level of the pool. At the same time, owing to the absence of back-water, the pressure on the needles is greatest, and they are often broken during the maneuvers necessitated by rises."

The following are M. Saint-Yves's conclusions as to the system that should be used in various cases:

"If the river in question can only be navigated by artificial waves, in in which case it is necessary to pour out in a short time the whole retained body of water in order to send forward a large volume, capable of increasing the navigable depth temporarily, but very appreciably, we think that it would be better to use Chanoine wickets, which seem preferable to the needle-dams, with escapements which are used on the Youne. The advantage to navigation appears to us to exceed the inconveniences which we have pointed out, and which result from a relative imperfection, which subsequent studies may cause to disappear.

"If the river in question has a regular regimen, with a discharge sufficient to assure, by means of pools suitably spaced, the draught necessary for a good navigation, we would not hesitate to choose Poirée dams, on account of their safety and facility of handling, and the means of communication which they give from one bank to the other for the superintendents, and for those who have the care and working of the

dams.

"In regard to weirs, a distinction must be made. Either self-regulation is indispensable, or it is only an advantage. If it is indispensable, we think that it is only so in order to secure an additional and supplementary opening to the waters which threaten to submerge the works, and not at all in order that this opening may be closed instantaneously. On this supposition the wickets of M. Chanoine are the best adapted; but we think it important to make a reservation, that is, that the swinging should not be limited to a slight lowering below the level of the pool, which makes it necessary to have an expensive and sometimes impracticable lengthening of the weir, but that the swing should be increased as much as possible by entirely taking away the power of spontaneous rising. In a word, the wickets of the weir should be considered as semiautomatic. A foot-bridge established above the wickets will permit the dam-tender to raise them mechanically. We think that in this way alone is it possible to give a sufficient discharge over a limited length of weir.

"If self-regulation is not necessary, M. Desfontaines' system ap pears to us preferable for crowning weirs. In fact, the simplicity of the

wickets, the ease with which they are worked, and the exact control of their fall, are precious qualities by which results as powerful as they are sure can be obtained, when the weir is not too long.

"In conclusion, we think that the comparison between different kinds of dams should be limited to the Poirée and the Chanoine, and that the Desfontaines wickets, thus far only applicable to weirs, should be considered as auxiliaries, but not as a system.”

In tome XVI, 1868, Annales des Ponts et Chaussées, M de Lagrenê replies to M. Saint-Yves, and the following is a brief summary of his remarks.

He begins by quoting, in reply to the latter's assertion that needledams are not dangerous, a statement of M. Cambuzat, who says, "With needle-dams there is really considerable danger to men who open them in rainy and frosty weather, and during floods, or at night, while in working wicket dams there is no danger at all to the attendants."

He adds, "that in 1859 a lock-tender was drowned at the Epineau Dam while opening it on the arrival of a small rise, and that very often the attendants fall into the water during the opening of needle-dams." "The danger seems to be much greater on the Yonue, where navigation is intermittent, than on the Lower Seine, where night-work is never necessary. On the Belgian part of the Meuse, as on the Yonne, the attendants often fall into the river, and in 1866 two of them were drowned at the Haignany Dam.

"It seems to me that in every case where the discharge of the stream in question varies rapidly, or where there is navigation by artificial waves, the preference, if only on the score of humanity, ought not to be given to needle-dams, unless the methods of working them are changed, which is probably quite practicable."

M. Cambuzat recounts several instances of trouble with wicket-dams, such as tripping rods and horses broken, wickets remaining up, scour at the foot of the passes.

"Injuries to some of the parks appears to me to be accidents which prove nothing against the system; in fact, I can cite the Mehun Dam, whose navigable pass is furnished with fifty wickets, which were maneuvered during all the artificial floods of 1865, that is about fifty times, without a single wicket ever refusing to fall and without any damage whatever being reported." He acknowledges however that the construction of a Chanoine wicket dam demands very great care in the exact location of all the parts, and that the attendant must also be very watchfal when raising the wickets to see that no foreign matter interposes between the heurter and the foot of the prop, otherwise the tripping rod might not act on it.

"The weir that regulates the level of a pool is the most important part of a movable dam; it ought, if possible, to give passage for an overflow of such length that its depth shall in some measure suffice for regulating the level of the pool without any intervention of the lock-tender. if, in consequence of an increase in the discharge, the overflow no longer suffices to pass or rise, the weir ought by spontaneous movement to give an increase of discharge corresponding to the supply; that is, it should increase or decrease with the latter; finally, it is desirable that the lock tender should be able whenever it is necessary to control these spontaneons movements, and at any instant, to have perfect command over the apparatus for closing."

If a weir is so arranged as to fulfill the above requisites, it seems a matter of indifference how the object is accomplished, as where there is a continuous navigation the pass is but seldom manoeuvered. The

system that uses a needle dam on the weir, and wickets in the pass, seems objectionable, as it does away with a regulating overflow where it is needed and places it where it may be injurious.

The article concludes with a description of the Suresnes Dam, with which the author is very familiar.

"The navigable pass of this dam, or rather the pass adjoining the Suresnes lock, is closed by needles; the trestles, sixty-five in number, are 11 feet in height and are spaced at intervals of 2 feet 7 inches. The dam was first closed on the 2d of June, 1857, and since then the fall at this dam, which varies according to circumstances. has never exceeded 7 feet. The number of needles in use or in store on the 2d of June was nine hundred. Between the 2d of June and the 2d of October, 1867, that is less than five months, five hundred of these needles were broken. Leaving out of consideration two hundred which were broken by being struck by a boat, there still remain three hundred cases of rupture, which occurred while placing or removing the needles, and sometimes while they were under pressure without any apparent reason. These needles were 13 feet long with a cross section of 3 inches by 3 inches.

"While waiting for another supply of needles of northern pine, with a section of 3 inches by 23 inches, pine scantling, 9 inches wide and 23 thick, were used in place of needles, and they are still in use although some of them (about ten) are already broken. They were placed in very much the same way as the ordinary needles; the height at which one of them ought to rest against the supporting bar when in position was known beforehand; the scantling was then placed nearly horizontal, resting on the bar at the proper mark, and then the up-stream end was slightly lowered; the current immediately carried this end under, and righted the scantling; the head was held back in order to lessen as much as possible the shock against the sill. To withdraw a scantling, use was made of a boat with a windlass; this boat was suitably fastened to a mooring cable stretched across the upper end of the pass; each scantling was drawn out head foremost by means of the windlass, and taken on the boat."

This method of removing the needles of the Suresnes Dam is a necessity whenever the fall is as much as 64 or 63 feet. Information obtained from the attendants at other dams is to the effect that needles are only put in place or removed from navigable passes when the fall is reduced to 5 or 54 feet.

In the same volume of the Annals, M. M. Chanoine and Lagrene give an elaborate description of the twelve dams constructed on the Seine between Montereau and Paris, with full details of the methods used in constructing the foundations, and of all items of cost. This will be given at sufficient length for the purposes of the present report from the later report of M. Cambuzat, but reference is made to it for the benefit of those who may desire to go more fully into this subject. The following items are, however, quoted here, as they are not referred to by M. Cambuzat.

Careful observations on the river, before and after the construction of the dams, showed that they produced no sensible effect on the heights of floods. When they are submerged a very slight ripple on the surface is all that can indicate their pressure. The works themselves were not injured, especially if the lock-gates were opened before the arrival of the flood. The only effect was a slight deposit of sediment in the locks.

The time allowed for passing a fleet of eight boats through a lock is

twenty minutes, half of which is used in filling or emptying the lock, and the other half is needed for getting the boats in and out, and for closing and opening the gates. The use of a submerged chain for towing has increased this time to twenty-eight minutes. A single tow-boat can be passed through in fifteen minutes. The larger the lock the better, when navigation is habitually carried on by fleets, as there is a very great gain in being able to pass a whole fleet through in one lockage. The average time necessary for raising such wickets as are used on the Upper Seine is two minutes and ten seconds for each wicket.

In dropping a number of wickets there is sometimes considerable difficulty in moving the last ones, on account of the pressure on the tripping-rod of those first dropped. This is remedied by passing an iron band under the caps of the axle of the wicket, so that when the wicket is down it is raised a little higher above the tripping-rod, and there is less danger of the weight of water pressing it against the rod. As it is important that the lock-tender should have some index to show him, at all times, the number of the wicket on which the trippingrod is acting at the moment, several have been invented, three of which are described. They are either geared, so as to move with the trippingrod, or with the axle of the pinion that works it.

Should a wicket remain standing after the others have fallen, the locktender, after the current through the pass has somewhat lessened, approaches it from below in a skiff, and drops it by a side-blow on the prop.

If it becomes necessary to swing the weir wickets at a time when the overflow is nearly sufficient to swing them of itself, it is unsafe to use the service-boat, unless it is fastened to a line of piles above the weir, as any pressure against the wickets, even though the boat has long fenders, might swing them.

A full discussion is given of a modification in the manner of connecting the weir-wickets with their horses, so that after these wickets swing, the level of the pool may be kept constant as the discharge increases. This change, however, seems not to have been adopted in practice, and therefore need not be considered here.

In the same volume of the Annals is an article by M. Malezieux, engineer of ponts et chaussées, on the Joinville Dam, from which the following is taken. Joinville is about 11 miles above the fortifications of Paris, measured by the river, and about 44 measured by the Saint Maur Canal.

"A dam had been in existence on the Marne at Joinville for forty years. It was constructed in order to raise the water to a minimum depth of 5 feet in the Saint Maur Canal, and at the same time to create a fall, which caused the construction of important manufactories along the banks of this canal. The necessities of navigation having been greatly changed, the first thought was to raise the old dam 2 feet, which would be enough to give 54 feet of draught in the river between the upper end of the Saint Maur Canal and the lower end of the Chilles Canal. But the city of Paris became, in 1864, the owner of the manufactories of Saint Maur; and from that time the motive power was used to pamp up the Marne water, in order to supply the Vincennes lakes and the elevated parts of the capital. The city requested that the Joinville Dam should be raised 34 feet, and offered to share in the cost. This was about the highest elevation that could be permitted without serious inconvenience, if not on account of the submersion of the banks above, at least on account of the heights of the cargoes of the charcoal boats,

which pass through the tunnel of the Saint Maur Canal. The proposals of the city were accepted by the State."

It was found inadvisable to rebuild the old dam, and an entirely new one was built about 1,400 feet below the old dam, so as to leave a basin of a little more than 12 acres below the mouth of the Saint Maur Canal. This canal is a cut off by which boats pass through two-thirds of a mile of canal instead of 8 miles of river. As navigation goes almost entirely through the canals, there is little need of a navigable pass, and accordingly it is only 39 feet wide. The weir is 207 feet in length. The wickets are 3 feet 7 inches high, and their tops are 8 feet 2 inches above the bottom of the Saint Maur Canal. The lift of the dam in low water is 7 feet.

Experience on the upper dams of the Marne having shown that the Chamoine wickets were liable to many accidents, and the Marne not being a river subject to sudden heavy floods, and there being, on account of the special peculiarities of the situation, but seldom occasion for opening the pass of this dam, it was thought best to close the latter by a needle-dam, and to use the Desfontaines wickets on the weir.

The Desfontaines system has been in operation at the Damery Dam since 1857, and at the Courcelles Dam since 1861, but is still not well known, and M. Malęzieux gives a description of it, from which we only extract such matter as has not been mentioned already.

The weir is contained between two rows of sheet-piling, 26 feet apart. The up-stream capping pieces are even with the permanent sill, at 3 feet 8 inches below the level of the pool; the down-stream caps are at the level of low water. Before the piles were driven, the area to be covered with beton was dredged. The least thickness of beton below the level of the low water, below the dam, was taken at 4 feet, but this depth was occasionally exceeded.

The first thing is to construct the main body of masonry, which contains a cavity 7 feet wide, with a cross-section composed of a quadrant adjoining a rectangle, and extending from one end of the weir to the other. The two edges of the opening have cut-stone borders. When this is finished it is divided into sections, or drums, each 5 feet long, separated by large transverse diaphragms of cast iron. These plates enter the masonry to a depth of 3 inches, and each one has two openings cut through it; one is on the up-stream side near the top of the quadrant, and is wider than it is high, making a horizontal opening; the other is on the down-stream side, is higher than it is wide, and forms a vertical opening. The latter is made of such a size that a man can without much difficulty pass through it from one end of the weir to the other.

Each drum now received a large plate-iron wicket, which has near the middle a horizontal hinge. This hinge is a cast-iron tube, which contains a wrought-iron spindle from one end to the other. Its extremities rest on the two diaphragms, at the level of the upper border but nearer the lower. The wicket itself is formed of three arms, each 7 feet 11 inches long, and of two pieces of sheet-iron two-tenths of an inch in thickness.

The arms pass through the hinge, and inside of it are shaped as collars, in which the axis turns; they completely fill the annular space between the latter and the inside surface of the hinge.

The counter-wicket, starting from the hinge, is bent backward 1 foot 4 inches, and then becomes parallel to the wicket. The object of this is to prevent the counter-wicket from extending above the liorizontal opening when the wicket is down. When the wicket is up the down-stream side