height gives a clear space of twelve feet for the travel underneath. The second abutment on which it rests is composed of iron columns, and from this point to the Old Colony Railroad (about 440 feet) the structure is supported by three rows of hollow, iron columns, placed at intervals of 11 feet lengthwise, and 20 feet apart, across, thus making a 40-foot roadway. From each of the outside columns, spring cast-iron brackets, projecting 10 feet and supporting the sidewalks. The foundation for the columns, which are of wrought iron, of the Phoenix Company's pattern, was made by driving three piles under each, on which was placed a stone capping, bringing it to within four feet of the surface. The rest of the space was boxed up, and as soon as the column was set, filled with concrete, as was also the inside of the column itself. Extending crosswise, from the top of these supports are bars of 12-inch I iron, and from these also extend similar bars of 6-inch dimension, four feet apart, on which rest the four feet square cast-iron plates which form the surface of the superstructure.

The crown of the roadway has been formed of concrete, four inches in thickness in the centre and one inch at the outer edge, where it meets the sidewalk. ment, of the Nicolson pattern. pers, 2×8 inches, about fifty feet allow the surface water to escape. ed to the height of the centre of the roadway, is of cast-iron, in keeping with the rest of the structure, while the walk itself, which is of concrete, is protected on its outer edge by an ornamental iron railing about three and one-half feet in height.

On this rests the wooden pave-
At each side of the road, scup-
apart, have been placed, to
The curb of the walk, rais-

This column section reaches to the Old Colony Railroad, which is spanned by a hundred foot bridge, one end of which rests on columns of larger dimensions than the others, while the other extremity is placed upon a pier formed by five cast-iron screw piles, placed in the channel, where they are driven through the muddy bed of the stream, and screwed into the clay under

neath to a distance of from eight to ten feet.

These supports

are hollow, 27 inches in diameter on the exterior and 24 inches inside, consisting of twelve-foot sections, firmly joined together. They are coated on the outside with a preparation to preserve from the action of the water, and when once placed, like the columns, were filled with concrete.

The bridge is supported by two twelve-foot Moseley trusses, extending the whole length of the span, at each side of the road. From the trusses, reach at proper intervals bars of nine-inch I iron, in pairs, further supported by two-inch truss rods, extending across the structure. On these beams the floor is laid, consisting of three-inch yellow pine plank, lengthwise, above which, and laid the other way, is a surface of three-inch white oak planking. The clear space underneath this bridge is fourteen feet.

The pier on which the water end of the structure rests is protected from any floating articles, by a pier formed of sheet piling, six inches in thickness.

We next come to the draw. The foundation for it was made by driving nineteen screw piles, of the dimensions before mentioned. Sixteen of them are placed in a circle forty feet in diameter, while one is placed in the centre, and flanked by two others in the direction of the current, from each of which the centre (or pivot) is braced by beams. Radial beams also extend from this point to each pile in the outside circle, which are also bound together by boiler plate three-quarters of an inch in thickness, and braced by diagonal rods at the top and bottom of the upper twelve-foot section of these supports. This piling has been protected by a low, wooden pier, about three hundred feet in length, made of six-inch sheet piling, and extending to the Old Colony pier on one hand. The upper portion of the circular piling has also been enclosed by a circular wooden building forty feet in diameter, forming a room in which are placed two caloric engines of three-horse power each, for use in

turning the draw. The roadway on which the draw turns is formed of fifteen-inch curved I iron beams, joined together and forming a circle, crowning the piling underneath, and having a steel rail on top. On this rail, run fifteen-inch cast iron friction wheels, held in place by a spider frame which turns on a pivot. Above them is another circular railing also connected with a spider frame, and having an inverted steel rail, on which the draw rests. On the outside of this railing is a large gear which connects with a smaller gear on an upright shaft, which, propelled by the engine, turns the draw.

The two arms of the draw are supported by separate trusses, 12 feet in height and 80 in length, of the same character as those in the long bridges, of which these arms are counterparts. They are connected in the centre by two ornamental towers, forty-five feet in height, from each of which extend three cables on either hand, to the extreme ends of the draw, thus giving them support when they are open. When they are not open, however, the ends will rest on the piers before mentioned, and the strain be thus removed from the cables. The two openings for vessels are 44 feet wide, each.

Passing over the space occupied by the draw of 160 feet, another bridge of the same length and general dimensions as the one before described as spanning the O. C. & N. R. R., is reached, while further on is another column section, 212 fect in length, only differing from the one first mentioned in having a part of the way -- where it crosses the water small screw piles instead of columns. Then another 50 foot span, crossing Lehigh street, reaches the Albany street abutment, the Boston proper end of the work. Cost of the whole extension to April 30, 1870, $887,982.48.

DEER ISLAND SEA WALL. A sea wall has been built during the past season at Deer Island, extending southwardly from the steamboat wharf at a point about 240 feet from the shore, a distance of 300 feet, with a return of 210 feet to the shore. The

wall is 12 feet wide at the bottom, 4 feet wide at the top, averages 21 feet high, with a batter on the front of one in twelve. The water on the line of the wall averaged 3 feet in depth at mean low water, and a trench was excavated to hard bottom, about 2 feet. This trench was filled with large stone, and upon this foundation the wall was built.

The contract for building the wall was taken by Clapp & Ballou, and amounted to $11,873,00. The space enclosed by the wall is to be filled with earth from the island, and when. filled will be improved by the erection thereon of coal sheds.

BERKELEY AND PROVIDENCE STREET RETAINING WALL. — A retaining wall, built of rubble masonry, has been constructed from the northerly abutment of the Berkeley street bridge and extending along the easterly side of said street to Providence street; thence along the southerly side of Providence street, a total distance of about 685 feet. The wall was built by contract, and was completed about the first of September at a cost of $12,206 60.

SHAWMUT AVENUE EXTENSION Bridge This bridge is being built over Orange street and the Boston & Albany Railroad, on the extension of Shawmut avenue. The two abutments are placed, one on the south side of Orange street and the other on the site of the former retaining wall on Indiana place, with a central pier about on a line with the southerly abutment of the Washington street bridge. The superstructure consists of a system of wrought iron girders placed four feet apart under the roadway, and five feet apart under the sidewalks, to be built continuous over the central pier, and to have a depth of three feet at the centre of the spans and two feet and six inches at the abutments. The whole distance from abutment to abutment is about 113 feet, and the width of the roadway 60 feet. There are no main girders or trusses projecting above the level of the

roadway as in most bridges, and the roadway is entirely clear of obstruction. The bridge is to be completed by the first of July, provided the abutments and pier are ready June 1st, and is to cost above the abutments, $33,600, being under contract to G. W. & F. Smith.

This bridge has been

MT. WASHINGTON AVENUE BRIDGE. almost entirely rebuilt during the past year. The old wooden drawbridge has been replaced by one built of iron. This new drawbridge is 136 feet long and 40 feet in width between the sidewalk railings, and 29 feet and 8 inches in width between the girders. It consists of two wrought iron girders, supported at their centres upon the ends of a wrought iron box girder, which carries the entire weight of the drawbridge and transmits it to a centre pivot upon which the drawbridge turns. There are besides, a track and wheels to steady and sustain the drawbridge in case of necessity, but the bulk of the weight is intended to be carried and to swing upon the centre pivot. The original contract price for doing the work was $56,000. The Committee on Bridges were subsequently authorized to expend $3,200 additional, for Burnettizing plank and substituting stone pavement for wood. Ross & Lord, the contractors, have completed the work in a very thorough and substantial manner.

NEW FERRY DROP. (East Boston Ferry.) Plans and specifications for a new ferry drop and tank were prepared in the summer by the City Engineer, at the request of the directors, and the drop has been built in conformity with the plans. The trusses are of the style known as the "Pratt Truss," whereas all the other drops have trusses built upon the "Howe" plan. It is a stronger and better structure than any built hitherto, and gives good satisfaction.