angle with sides of 5 inches, and cutting off from each of its three angles smaller equilateral triangles with sides of 14 inches. (See Fig. 53.)

On the Seine, below Paris, the dams are 9 feet 10 inches in height, and the depth provided for navigation is 6 feet 7 inches, the crowns of the masonry of the dams being placed at a low-water line. Whenever the river surface below a dam stands at 2 feet below its top, or at 7 feet 10 inches above the masonry, the movable part is thrown down, and boats can easily cross without striking the sill. In order to prevent the boats from being entirely stopped during low-water, in case of accident to the lock, each dam has a navigation-pass closed by a needledam. As the needles for these passes would have to be so long and heavy that they could not be handled without great labor, it was decided by the General Council of Ponts et Chaussées that a movable sill should be used which could be removed whenever necessary. M. Saint Yves does not approve of this arrangement, as he thinks that in course of time the movable sill would either be destroyed, or so consolidated with the masonry that it could only be removed by tearing it to pieces.

In order to remove this difficulty, M. Marini tried, at the Bezons dam, intermediate supports for long needles, (Figs. 54, 55, 56, 57, and 58,) so that they might have three points of support instead of two. A careful examination into this question shows that for heights of 123 feet, where the needles rest against intermediate horizontal beams, supported against the trestles at a height of 4 feet 7 inches above the sill, they resist well; and this is also the case when the height is 13 feet and the intermediate support is at an elevation of 4 feet 10 inches. With such an arrangement movable sills are no longer necessary, but more skill is required in removing needles thus supported in order to avoid being drawn into the water. The best place for the intermediate beam is a little above the center of pressure of the water. It must be observed, however, that this additional security is only obtained while the needles are in place. They are still weak to resist the forces acting on them when they are being lifted. The best remedy for the difficulty seems to be to have a special apparatus for regulating the level of the pool, so that the needles need only be lifted when the latter ceases to suffice for the discharge, by which time the lower pool will have been raised, and the pressure on the needles will thus have been diminished.

The apparatus recommended is Chanoine wickets on the weirs, but automatic action does not seem desirable, as by dispensing with it a greater swing of the wickets may be obtained. If only a short length of weir be available, Desfontaines wickets should be used, as these can lie down flat, and thus permit a greater discharge.

DAMS ON THE MEUSE.-TRESTLES WITH ESCAPEMENTS.

The arrangements on the Meuse, to open needle-dams by escapements, differ from those on the Yonne. On the latter they were designed with a view to making artificial floods, and therefore rapidity of action was specially desired. As soon as one bay was opened all the others followed, and the movement took place so rapidly that sometimes the dam-tender could not get off the bridge in time, but was precipitated into the river.

The Belgian dams, with escapements, are constructed in accordance with an entirely different order of ideas. Each escapement is put in motion by the application of an exterior force, and the openings take place singly and one after the other. The object

which the Belgian engineers proposed to themselves was to avoid the removal of the needles by hand and one at a time; and by their system they have succeeded in making daily use of needles which square from 4 to 4 inches. The handling of these little pine joists is limited to carrying them on the dam and placing them in the water, which are easy operations that require no manual dexterity, but only the effort necessary to lift a weight and carry it on the shoulder.

By using these heavy needles the height of dams can be increased to 13 feet without using intermediate supports.

GENERAL DESCRIPTION OF TRESTLES WITH ESCAPEMENTS, AS USED ON THE BELGIAN MEUSE.

Their general shape is about the same as that of the trestles used in France. They only differ in the upper part. The cap, which forms the smaller base of the quadrilateral, is surmounted by a rectangular frame-work, which carries the escapement apparatus and the foot-bridge. (See Figs. 59, 60, and 61.)

Of the two vertical parts of this frame, one, that on the up-stream side, is composed of a piece of wrought iron, which is welded to the trestle, and is afterward bored out so as to form a tube, in which turns a vertical iron rod, A. (Fig. 59.)

The clamp-bar, which connects one trestle with the next, is provided at one end with a collar which envelopes the tube, around which the clamp-bar turns. This collar is hollowed out on the larger part of its circumference so as to admit, in all its positions, (it can only describe a quadrant,) the head of the adjoining clamp-bar, B. The collar is held, so as to prevent any vertical movement, by a ring, C, which is fastened to the tube by two screws. The head of each clamp-bar is forged in the shape of a T, and enters the cavities cut out of the collar of the adjoining clamp-bar, those cut out of the tube, and lastly those cut out of the iron rod. Finally, on the under side of each clamp-bar and near its head is a vertical projection, D. (Fig. 60,) which, after the bar is turned, strikes against the cap of the trestle proper and limits its movement of rotation.

The iron rod is prevented from moving vertically by a screw, E, (Fig. 59,) which accompanies it in its quarter of a revolution, moving in a groove cut out of the tube. The down-stream post of the top frame is arranged so as to receive a horizontal bar for supporting the iron plate, which serves to make a foot-bridge, and to bind together the trestles by means of claws. This bar has a socket on its up-stream side, which surrounds the tube.

The iron rod works in the tube by friction, and it is turned by nippers shaped like a wrench. The escapement cannot, therefore, act spontaneously.

The dams on the Meuse are divided into two passes by a pier, which is hollowed out so as to have chambers on each side to receive the end trestles. There are no niches in the abutments, and the trestles are always dropped in the same direction.

The trestles are connected by chains, but the necessity of leaving room for the clamp-bars to turn and the needles to escape compels the division of the chains into two parts, one of which is fastened to a trestle, and the other to the flooring of the adjoining trestle. When the trestles are lowered these chains are connected, but they are separated when the bridge is up.

The trestles are bedded by giving them a slight push after raising the floor, so as to disengage the claws. They are raised by using a light portable windlass. The needles of each bay are connected by a rope which passes under the flooring, and is fastened to the down-stream post of the trestle. When it is necessary to increase the discharge, a certain number of bays are opened by turning their clamp-bars. In this case the end of the rope which holds the needles of each bay is tied to a cable which is fastened to the pier or abutment, so that the needles float to them and are picked up at leisure.

From the above the author concludes that it is quite practicable to use the Poirée system, with a regulating weir provided with Chanoine wickets up to a height of 13 feet; but the semi-regular hexagonal needles should be preferred in order to prevent excessive leakage.

The best exemplification of the present practice of foreign engineers in improving a river on a plan similar to that proposed for the Ohio is to be found in the works recently executed by the French government in the Upper Seine and Yonne.

IMPROVEMENT OF THE YONNE AND THE UPPER SEINE.

The following description is taken from the March number for 1873, of the Annales des ponts et chaussées. This article is so valuable in itself, and so pertinent to our present investigation, that but little of it can be omitted. Unfortunately the illustrative plates do not accompany the text, but are promised in some future number. It is believed, however, that the plates that accompany the other reports sufficiently explain this one. The author is M. Cambuzat, chief engineer des ponts et chaussées.

The great water route connecting Havre, Rouen, and Paris with Lyons and Marseilles, by the Seine and Yonne Rivers, the Burgundy Canal, and the rivers Saône and Rhone, (see map on plate 6,) had, until September, 1871, a very defective section; in fact, a veritable gap, 118 miles long, between Paris and Laroche, where the Burgundy Canal enters the Yonne. In fact, for eight or nine months of the year, from March to November, the descent of loaded boats was only possible, especially on the Yonne, twice a week, by the aid of artificial floods or waves from the Upper Yonne, and the draught of water available varied from 24 feet to 3 feet 4 inches, and 3 feet 7 inches at most; so that boats from the Burgundy Canal, drawing from 3 feet 7 inches to 4 feet 7 inches, were obliged to break bulk at Laroche. As to ascending craft, they were generally empty, or only carried a few tons of merchandise. This costly, slow, and altogether insufficient navigation, was accompanied by great fatigue, by danger, and by numerous accidents. Although it was somewhat less difficult on the Seine, navigation was much hindered, and often stopped, between Montereau and Paris. Since the 1st of September, 1871, there has been a continuous navigation between Paris and Laroche, thanks to 17 movable dams constructed on the Yonne, and to 2 cut-offs aud 12 movable dams constructed on the Seine. The minimum depth of water in the pools is 5 feet 3 inches, and therefore boats can move up and down with perfect safety, drawing from 4 to 5 feet. At present, while the works are yet incomplete, the greater part of the loads, especially those that come from the Nivernais Canal, do not draw over 4 feet; but when the works of the same kind (that is, 8 movable dams and one cut-off, now being built between Laroche and Auxerre, where the Nivernais Canal enters) are finished, which will be by the end of this year, (1873,) the great advantages will be happily realized which the government had in view, and which were looked for with impatience, but with confidence, by the boating, commercial, industrial, and agricultural interests.

The greatest flood ever known on the Yonne, that of January, 1802, rose 16 feet at Montereau. The low-water discharge between Auxerre and Laroche is 459 cubic feet per second, and the average slope 33 feet per mile; between Laroche and Montereau the discharge is 600 cubic feet per second, and the slope 1 foot 10 inches per mile.

The first effort to improve the navigation of the Yonne was directed toward checking the flow of the artificial floods by movable dams, in order to prevent their too speedy absorption. On this plan, descending boats and rafts were stopped at each dam, and after all had arrived a new flood carried them to the next one. But the increasing demands of commerce for a continuous navigation caused the adoption of the present system. Between Laroche and Montereau (the mouth of the Youne) there are now 17 movable dams, of which 6 belonged to the old system of navigation by temporary floods. These had each a navigable pass varying in width from 194 to 230 feet. One of these dams was replaced by a new one, but the others were retained, with some modifications. Each had a lock attached for use during low-water. Eleven new dams were constructed, all of which have locks attached, except those at the heads of cut-offs, whose locks are generally at the lower end of the cut-off. navigation passes of these new dems are 115 feet in width, and the weir has a minimum length of 164 feet. The sill of the navigation pass is placed at 2 feet below the low-water line. The sill of the weir is placed 20 inches above the same line, and therefore is 3 feet 8 inches higher than the sill of the pass. Two cut-offs across difficult bends

The

shorten the river a little more than 4 miles. At the head of each cutoff is a guard-lock and at the lower end a lift lock. At the Courlon cutoff the lift of the lower lock is 12 feet 9 inches. The bottom width of each cut-off is 52 feet.

Wherever dredging was necessary a channel was made 100 feet wide and 5 feet deep at low water.

The works for improving the navigation between Montereau and Laroche, authorized in 1861, were nearly completed in 1868, when a continuation up the Yonne to Auxerre was ordered. These new works were commenced in 1869, and although retarded by the war with Germany, will probably be completed by the end of the present year. These new works consist of eight movable dams, (of which seven have locks,) one cut-off, dredging, bank-protection, and minor works. The dam of La Chaînette, just below Auxerre, has a navigation-pass 138 feet wide, closed by a Poirée needle dam, and a permanent weir 656 feet long, whose top is even with the surface of the pool. The sole of the navigation-pass is 20 inches below low water. The chamber of the lock attached to the dam is 27 feet wide, and has an available length of 305 feet. This dam was built under the old system of creating temporary floods. The seven new dams have navigation-passes varying in width from 98 to 115 feet, closed by Chanoine wickets, with their soles 2 feet below low water. The weirs, whose soles are 20 inches above low water, are from 82 to 131 feet in length. Six of these weirs have needle-dams on the Poirée system, and one has large Girard shutters 11 feet wide, with a vertical height of 63 feet.

Five principal conditions controlled the decision on the position and lift of each dam: 1. The preservation of existing dams.

2. The horizontal plane passing through the tops of the wickets of the navigationpass (whose height fixed the lift of the dam) should be 5 feet above the lower mitersill of the lock next above, and the same distance above the intermediate bars, excepting those which were to be dredged.

3. The height of the natural banks should be at least from 16 to 20 inches above the level of the pool, excepting that certain parts near the dams were to be raised.

4. The space under the keystones of arched bridges, and under the bottom chords of truss-bridges, should be at least 18 feet above the surface of the pools, because the greatest space required for boats is seldom over 16 feet.

5. Finally, a maximum vertical height of from 9 feet 10 inches to 10 feet 2 inches to be given to the wickets of the navigation-pass.

On account of the very small flow in low-water, the surfaces of the pools were always assumed as horizontal.

When the project for movable dams on the Yonne was approved, it was not thought that wickets could be built on the Chanoine system as high as 10 feet, but recently a sluice for navigation has been constructed on the Seine, through the Port á l'Anglais Dam, whose wickets have a vertical height of 124 feet.