three cartridges, 1 inches in diameter, and nine inches long, the charge varying from one to two pounds. One to four holes were fired at one time by the electric battery. No serious damage was done to the walls of the caisson by shattered rock. About 1,500 cubic yards of rock were thus taken out.

The caisson was lighted with electric light by a dynamo capable of running seventy-five incandescent lamps of sixteen candle-power.

During the sinking, the caisson took a slight twist, and at one time the southwest corner was more than a foot lower than the others, causing some cracks in the masonry, then about six feet high upon it. Provision had been made in the dimensions of the base of the pier for small movements of the caisson, and from the time the masonry was brought to normal dimensions, no perceptible movement took place.

For the sinking of the caisson three compressors of different patterns were used, viz.: one single 12 x 18 inch Delamater, one duplex 10 x 16 inch Delamater, and one duplex 9 x 10 inch Clayton compressor. They were operated by two horizontal boilers of sixty and eighty horse-power, respectively.

Sinking was commenced on the 15th of November, 1886, and on the 17th of April, 1887, the caisson reached its final depth 40.6 feet below mean high water. There were some few points where the cutting edge did not rest upon the rock. These were excavated and filled with concrete. It was then sealed and filled two feet deep with concrete formed of Portland cement, with two parts of sand and four of broken stone. The remainder of the concrete filling was made with Rosendale cement. The masonry had been carried to a height of 27.5 feet, the level of mean high water, during the sinking of the caisson. It was continued without intermission, and was completed to the top of the skew-backs, 52.2 feet above high water, by the 17th of July, the cornice and skew-backs being set and ready for the iron.

521,000 feet B.M. of timber were used in the caisson.

Pier III. was located on the east side of Sedgwick Avenue, an important thoroughfare, and at or in the foot of the steep bluff. The hard rock was, in places, as indicated by the borings, at a depth of seventy-five feet below the surface. The excavations were in a partially decomposed gneiss of

unequal hardness. The strata were very much inclined, and had a great tendency to slide. All the sides of the pit were therefore systematically supported by sheathing with four-inch plank, behind longitudinal stringers, these latter being shored with 12 x 12 inch timbers, extending entirely. across the pit in both directions.

At a depth of forty-three feet the rock, when tested with twenty-four and twenty-six tons to the square foot, showed no signs of yielding. The excavations were then suspended, and a bed of concrete six feet in thickness, made with Portland cement, was laid over the entire bottom of the foundation pit, and upon it the masonry was commenced. This, as in the other piers, was made with a facing of heavy granite blocks and an interior of concrete. The masonry of this pier was commenced on the 16th of October, 1886, and it was ready for the iron work early in June following. The ends of all the piers are vertical. piers are vertical. The long faces of main piers below the springing line batter three-quarters of an inch to the foot, or one in sixteen.

The foundations of all the small piers were commenced in the winter and spring of 1887. On the west side of the river they were founded upon the hard gneiss at different depths-sometimes ten or twelve feet below the surface of the ground. The excavations were levelled up with concrete, and the pier was built up to natural surface with rubble masonry faced with gneiss dressed to horizontal beds. Above the surface the piers, which are solid throughout, are faced on the ends with granite; on the long faces with Roxbury gneiss with granite quoins and impost courses. The ends are vertical; the long faces batter one in twenty-four.

On the east side of the river the arch piers are founded upon the partially decomposed gneiss, which becomes harder as we ascend the hill, so that the pier (C) which separates the bridge proper from the approach is built upon the hardest rock. The east abutment of the Undercliff Avenue arch and the approach walls throughout are built upon the solid rock.

The work on the main piers did not stop with the setting of the skewbacks, but was continued without intermission until their completion to the height of the cornice courses.

in granite posts. The rails are of wrought-iron pipe, four and three inches in diameter, covered with bronze one-eighth of an inch thick drawn over the iron.

In the original plans there were two sixty-foot arches, which could have been built at any time during the progress of the work. The contractor, after building one, could have removed his centering and used it for the other. But with seven arches and the entire roadway of the abutments dependent upon them, it became necessary to provide and erect centering for all of them at the same time, that every delay might be prevented. The centres used were strong and unyielding. They contained an excess of material, and the height might have been considerably reduced. As a matter of fact, the contractor, wishing to build the spandril backing to a completed arch did, in several instances, set the arch stones of the adjacent arch to a height of ten or twelve feet above the impost before the centering for the arch was placed.

The centering for the oval arch over Undercliff Avenue was of a cheap and satisfactory form.

The voussoirs or arch stones were set by derricks, generally placed on the crown of the arch centres.

Great care was taken to set each stone accurately to its place, blocking up where necessary on the lagging of the centres. The centres were not struck; they were simply allowed to stand until, by the action of the weather, they were found to be free. No perceptible settlements took place.

half-inch joints. They were The courses near the crown,

The stones were cut to plane beds to lay laid in a good mortar of Portland cement. the beds of which were too steep to take a bed of mortar, were filled with thin mortar worked in with a steel blade.

1,500,000 feet B.M., spruce and hemlock, were used in trestles for landing materials, and in centres for masonry arches.

The roadway is of Trinidad asphalt on a bituminous base. Upon the fine concrete, hereafter described, on the metal part, and upon the coated concrete covering of the brick arches upon the masonry, the roadbed is made up to within three and a half inches of the top, with broken stone in

layers four to six inches deep, each layer well rolled by a steam roller of about two hundred and fifty pounds to the linear inch; and over the broken stone hot coal-tar distillate is poured. Upon this is placed a binder course made of very fine broken stone and coal-tar distillate, mixed by machinery, spread two inches thick and well rolled; the asphalt wearingsurface, one and a half inches in thickness, is laid upon the binder course. The roadway is crowned nine inches in its width of fifty feet. The drainage of the roadway over the metal structure is to the abutments. The slope is very slight from the centre pier to the centre of the arches; being in hot summer weather not over four inches in two hundred and fifty-five feet, half of which is formed in the gutter and the remainder by the grade of the roadway. In cooler weather, when the fall in temperature has lowered the crown of the arch, the slope is greater. From the centre to the abutments it is from eight to twelve inches. Considering the smooth surface of the roadway and gutters, this slope was believed to be sufficient. On the abutments, which are level, the gutters are given a slope of three inches toward the basin heads of the drain-pipes, which are spaced from seventy to one hundred and forty feet.

The drain-pipes, eight and twelve inches in diameter, descend vertically, secured to the interior wall surfaces, and discharge into transverse underground drains which are connected with a twelve-inch and fifteeninch terra cotta pipe, extending the entire length of the bridge on each side of the river.

The roadway, commenced in August, 1888, was not completed until November.

The sidewalks, about 75,000 square feet in area, are composed of large flags of blue-stone six and nine feet long, not less than five feet wide, and from three to five inches thick, planed on the top surface and axed on the heads, which rest upon the granite curb. They were laid in August, September, and October, 1888.

A channel is left in the concrete under the flags of the masonry portion, in which the gas-pipes are laid. They would be more accessible if placed under the longitudinal arches of the roadway, but a leak would fill the chambers with gas and might be the cause of an explosion, with possi

ble loss of life. Pipes for electric conductors are laid alongside the gaspipes.

METAL WORK.

Each of the steel arches is composed of six ribs with free ends resting upon pins. The span between pin-centres is five hundred and eight feet eight and a half inches, the rise to neutral axis at crown eighty-nine feet ten inches. Clear span between piers, five hundred and ten feet; rise from apparent springing line to soffit, ninety-one and a quarter feet. The ribs are of a nearly uniform depth of thirteen feet (with slight variations caused by varying thickness of flange-plates), and are composed each of a web-plate of mild steel three-eighths of an inch in thickness (except for the end-panels, which are three-quarters of an inch thick), with double flanges at top and bottom, and stiffeners of angle iron at intervals of about five feet. The outer flange-plates are twenty inches wide, and vary in thickness from two and one-eighth inches to three and one-sixteenth inches. The inner flange-plates-two in each flange-are twelve inches wide by three-quarters of an inch thick. The six angle irons in each double flange are six by six by five-eighths inches.

Each rib is formed of thirty-four segments of such length that the horizontal projection of the extrados of each segment is fourteen feet eleven and a half inches-the distance between centres of the posts which stand upon the joints and carry the roadway.

The ends of the segments are planed and stiffened with angles, through which they are riveted together. The joints of the flanges are covered with splice-plates.

Each rib rests at its ends upon pins of forged steel, thirty-four inches long and eighteen inches in diameter. The pins lie in steel bearings, which are carried by pedestals made of plates and angles upon a base of two superimposed three-quarter inch steel plates, thirteen feet by four feet four inches, resting upon and bolted to the granite skew-backs. To secure uniformity of bearing and close contact, wool felt filled with asphalt is interposed between the pedestal and the granite. The ribs are connected by lateral bracing on both top and bottom flanges, and by sway-bracing on the segment joints-all of steel latticed beams and angles.