grade to the cities will increase, that of the wagon tracks being much sharper than the longer and easier one of the railroad. The bridge will be 41 or 42 feet in width, with 13 feet in the middle, for trains, one way on either side for cattle and vehicles, and on the outside of these still the passages for foot-passengers. The East River Bridge. - "The work is assuming shape. The caisson for the great tower on the Brooklyn shore has been contracted for. Operations are to be commenced at once. The wood-work at the oil-docks and piers will be torn up and everything down to low-water mark will be removed. The bottom of the river will be excavated to a depth of 22 feet below high tide. The space to be cleared and levelled is 170 feet long by 102 feet, extending out into the river. Divers will be employed to remove the obstructions at the bottom, and blasting will have to be resorted to.

"The caisson is like a large scow, or flat-bottomed boat, turned upside down; nothing more. Then, if one imagines its being sunk to the bottom of the river on a level space prepared for it; that the water is forced out of the boat, or air-chamber,' as it is called, by means of compressed air; that workmen are sent down into the air-chamber in a shaft, cut through the top of the caisson (bottom of the boat), who, with the aid of calcium lights, dig out the material beneath them, which is hoisted up to the world above; that they continue excavating until the proper depth is reached, the caisson sinking, and, of course, on a perfect level as the work progresses, and that the air-chamber' is last of all filled up with cement, a general idea can be formed of the way in which the foundation of the tower will be laid.

"Experiments which have been made on the quicksand bed of the East River while excavating a dry dock prove its bearing power to be 10 tons per square foot. By Mr. Roebling's plan, it is proposed to rest upon this bed a weight of only 4 tons per square foot. The weight of each tower is to be somewhat over 75 tons.

"To distribute this vast weight so that no part of the pressure on the base shall be over 4 tons per foot, it has been decided that the area of the foundation shall be 170 feet long by 102 feet broad. This area will be composed of huge timbers resting on the sand, and bearing the masonry-work of the tower upon it. The timber will be 20 feet thick, and this vast mass of 20 feet by 170 by 102 will be securely bolted into one solid frame, so that the weight of the tower above can never defiect in the slightest degree at any point.

"The board unanimously hold that 300 feet high of a masonry structure could be safely and unyieldingly erected on such a timber foundation as proposed by Mr. Roebling, and that the superstructure thereon, if properly built, would easily bear the weight of the bridge, and all the weight that could be put on the bridge.

"The bridge company have purchased about 4,000,000 feet, broad measure, of yellow Georgia pine, the greater part of which is now on hand. Before the contract with the builders was made,

proposals to construct the caisson were invited from all the shipbuilders in this vicinity, and their bid proved to be the lowest. A more than ordinary depth of water in front of the yard was required, not less than 23 feet, as, when launched, the caisson will draw fully 17 feet of water. It is, to be 170 feet long, 102 feet wide, as already stated, and 15 feet deep, with a top 5 feet thick, and sides of a thickness tapering from 9 feet at the top to a foot below. The time required to build it will be about 4 months. As soon as it has been set afloat it will sink to within 18 inches of the surface of the water; and when the proper time arrives it will be towed down to the ferry and placed in position ready for being submerged. This is to be accomplished by building on the top of the caisson successive layers of timber and concrete to a height of 20 feet. The weight of the caisson, with this 20 feet of timber and cement above the air-chamber,' will be 11,000 tons.

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"The material excavated is hoisted from the air-chamber' through two water-shafts by means of dredges, and as it is raised the caisson sinks, being uniform-undermined round the 4 edges and throughout its whole extent. As the caisson thus gradually sinks, the mason-work, enclosed in a coffer-dam, is in progress on the top of the timber, thus adding the necessary weight. Access is had to the air-chamber' by means of two air-shafts 3 feet in diameter. The depth to which it will be probably necessary to go into the bed of the river will be about 55 feet below high-water mark, so that all the timber of the foundation will be enclosed in the sand and other material through which an excavation has been made."―Journal of Gas Lighting.

Beginning with a span of 822 in 1854, one of 1,057 in 1867, the bridge proposed for the East river by Mr. Roebling, a steelwire suspension bridge, is to have a span of 1,600 feet between the towers, 135 feet above the water; it is calculated to bear six times the strain which can, under any circumstances, be brought to bear on it.

Bridge over the Schuylkill.-The plans of Mr. Kneats have been adopted for an iron bridge over the Schuylkill, at South Street, Philadelphia. The centre and river piles will be of iron sunk by the pneumatic process; the length of the bridge will be 2,488 feet, and the clear height 32 feet above high water.

The Kansas City Bridge was formally opened July 3d, with appropriate ceremonies; the municipal authorities of Chicago and St. Louis and an immense concourse of people participating.

The bridge consists of combination wood and iron trusses for spans 130 feet, 177 feet, 200 feet, and 250 feet, an iron span 70 feet, and a Linville & Piper patent wrought-iron pivot span 360 feet in length. The superstructure was erected by the Keystone Bridge Company, of Pittsburgh.

Missouri Iron Bridge. The draw spans 363 feet long, and weighs 360 tons. The spans are respectively 200, 250, 200, and 117 feet in length. — Engineering and Mining Journal, July 20. Bridge at Louisville. The largest span of any truss bridge in the United States is that of the great bridge across the Ohio

River at Louisville, which is destined to connect the Kentucky and Indiana shores. The bridge itself will be, when finished (and the engineer in charge expects to turn over his contract for the building some time in November), one of the most splendid structures of the kind in this or any other country. The last span covers 370 feet, and is a marvel of engineering skill.

Bridge over Cape Fear River. -The new iron bridge over the Cape Fear River, to connect all the railroad lines centring in Wilmington, North Carolina, was opened on the 28th of August.

ENGINEERING ITEMS.

Resistance of Roads to Traction. -The following results of the experiments of Sir John McNeill, in regard to traction on roads of different kinds, are pretty generally accepted as accurate: Resistance in pounds per ton on different roads : —

House-lifting.In the work of straightening and widening some of the crooked streets in Boston, Mass., it became necessary to move a huge building known as "Hotel Pelham." This building is of freestone, 96 feet high, and weighs 10,000 tons. It was moved 14 feet in 3 days, by means of rollers and screws, a portion of the sidewalk being also moved with it. So carefully and well was the work done that not a crack was made in the building, and nothing in it was disturbed. The fastest time accomplished was two inches in four minutes. A great number of screws 21 inches long were employed.

The French Cable.-Length between Brest and St. Pierre, 2,595 nautical miles, a length that makes this the longest cable ever laid.

Heavy Blast.-A great blast was lately made at Clitheroe, Eng. A tunnel, 28 yards in length, was bored, and 6,000 lbs. of powder walled in it. The mass of stone, 60 feet in length, was thrown upwards in a vertical direction, and at least 50,000 tons of limestone displaced.

An immense Gasometer. - The Manhattan Gas Company are building, at the foot of Eleventh Street, in this city, a new gasometer of unusually large dimensions. The basin is 225 feet in diameter, and 38 feet deep. The circular wall is 7 feet thick, arranged upon which are 16 elegant guiding columns, each 72 feet high, of wrought iron, united at the top by ornamental girders. This will be one of the largest gasometers in the country. Fog-whistle. -We learn that a fog-whistle, to be worked by a 10-horse power engine, is being constructed for Thatcher's Island,

off Salem, Mass. It will be ready by the first of June. This will be the largest and most powerful fog-whistle in the world.

Forty-two ton Hammer.-In England a huge steam-hammer, weighing 1,000 tons, is being made for the Russian government. The hammer-head weighs 42 tons, the anvil-block 500 tons, and it is to be used in forging steel guns. — Van Nostrand's Eng. Mag., Oct., 1869.

Steel-headed Rails. - Steel-headed rails are made at the Trenton (N. J.) Rolling Mills by the following process: The steel which is to form the head of the rail is first welded to a quite thin piece of iron. The combined bar is then beaten and rolled down until the iron is very thin and the steel reduced to about half its former bulk. After this operation is completed, the whole quantity of iron requisite to complete the bulk of the rail is added to the bottom of the combined bar and welded to the thin layer of iron. This process, it is asserted, doubles the strength of the weld between the iron and the steel, always a difficult operation to perform. The old process consists in welding the relative thickness of iron and steel at one operation, but the new method is reported to furnish better rails.-Van Nostrand's Eng. Mag., Oct., 1869.

Centrifugal Pumps.-The great Appold centrifugal pump to be worked in connection with Mr. Hawkshaw's important work, the Amsterdam Ship Canal, is to lift 2,000 cubic metres, or, say, 440,000 gallons of water per minute. The lift is not great, but for each foot of lift, the actual duty, irrespective of all losses of effect, is 133 horse-power. Engineering.

Inverted Siphon. An iron-pipe, 11 inches in diameter, and 8,800 feet (one and two-thirds miles) long, has been laid in Tuolumne County, California. It runs down a mountain, under a creek, and up the ascent on the opposite side, under a perpendicular pressure at the lowest point of 684 feet. — Journal Franklin Inst. A rapid Change of Gauge.-In Missouri, the Missouri Pacific Railway-a road nearly 200 miles long-changed its line from the broad to the narrow gauge. Nearly 1,400 men were engaged in the work; and they labored with such celerity, that the task was accomplished in 12 hours, and without interrupting the business of the road.

Large Blast. The operation of blasting off the rocky headland of Lime Point, opposite Fort Point, and forming the northern entrance to St. Francisco Bay, for a heavy water-battery, has been conducted under the direction of Col. G. H. Mendell, U. S. Engineers corps. Two blasts have already been made; one with about 10,000 lbs. of powder and a second with 24,000. This second blast is supposed to be the largest ever used in military engineering, and moved about 80,000 lbs. of rock. At the point a tunnel had been run in a north-westerly direction into the base of the hill, a distance of about 30 feet, where a chamber was formed on the right to contain 3,000 lbs. of powder; thence the tunnel ran in a direction south of west 31 feet, where a chamber was formed on the left for 6,000 lbs. of powder, thence on the same line 45 feet, where the third chamber was formed to contain 7,500 lbs. These chambers were about 5 feet by 7 feet, to contain

from 125 to 130 cubic feet. When all were chambered out, a board partition was put up in front of each chamber to hold the powder. The greatest care was used in placing the powder in the chambers; the men wore the French sabots, or bandaged their feet in bagging; the barrel of powder was opened at the mouth of the tunnel, and carried into the chamber in sacks, the men groping their way into the dark tunnel, and delivering their dangerous burden to the foreman, who emptied it into one immense bin in the chamber. At a certain stage of the filling up, 8 cartridges were distributed at different points in the mass, each cartridge having an electric wire leading to the central wire connected with the machine outside. As fast as these chambers were filled, they were sealed up with clay and the tunnel tamped with the same material, the wires for firing the mass leading through a small box at the bottom of the tunnel. These wires, two in number, were of copper, one an insulated wire to convey the electricity to the mass of powder, and the other a plain wire for the return current; one connected with the positive, and the other with the negative pole of a powerful " Beardslee " magnetico-electric machine, located in a secure place outside, and several feet distant. On connecting the poles, the explosion took place with a heavy, dull sound, and an immense mass of earth and rock was thrown into the air about 70 feet, and the whole face of the cliff came crashing down to the base and tumbled into the sea. The cliff has been blasted off for about 200 feet along its base and tumbled into the sea, and about 175 feet in height with an average depth of about 60 feet. - San Francisco paper.

THE NEW THAMES TUNNEL.

The way the tube tunnel is built is by means of 3 segments of a circle of cast iron, each weighing 4 cwt., with a centre keypiece at the top, weighing one cwt. Each segment or ring when bolted together is only 18 inches long, but no fewer than 6 of these rings are bolted on in every 24 hours, so the tunnel is advancing at the rate of 9 feet a day. As the shield, which is 7 feet 3 inches in diameter, is pushed on for a length of 18 inches, it leaves within the tube or rim a space one inch greater all round than that occupied by its own tube on the outside. This, therefore, leaves ample room to fit in the segments of the tunnel-tube easily. This is done very rapidly. The bottom segment is laid in its place, and the two side segments above it, and between these at the top the key-piece is slid in. Between the long horizontal flanges a layer of white pine is placed before they are screwed close up. The spaces between the circular flanges of each segment are regularly calked in with tow and cement. Still, the shield on the cap is one inch wider all round than the diameter of the tunnel tube within it, which comes afterward to occupy it, leaving an opening of that space between the clay and iron. This interstice, when the segment ring is fixed, is closed by pumping in blue lias cement, which, as it quickly sets, forms a ring of stone-work, preventing the action of the