feet long, and rise 30 feet above high water, affording support for steel towers 16 feet by 60 at the base, 16 feet by 30 at the top, and 100 feet high to the lower part of the superstructure. The

tower construction is shown in the illustration, consisting, in effect, of two pyramidal structures securely braced in every direction. The

cantilevers are three in number, of 548 feet each, and two connection spans of 525 feet each. It will be noticed that the two end spans and the central span give a greater clear height above the water-namely, 160 feet

- while the connected spans give 130 feet, a plan which facilitated the placing of the canțilevers without staging, and therefore with less obstruction to commerce. The wind-pressures are estimated on the basis of 30 pounds per square foot of surface, including towers, spans, and area of trains. The pressure on the caisson bases is about 3 tons per square foot, and the material upon which they rest is hard gravel.

From end to end, including approaches, the structure is about 11 mile long, and it is an excellent example of the latest ideas in bridge construction.

The Tay Bridge.-Some of the ingenious devices for laying the foundations of this grand bridge were described and illustrated in the article on engineering for 1885. The year 1887 saw the completion of the whole structure, and trains were passed over it early in June. Throughout the whole course of construction the ruins of the old bridge, which fell in 1879, were utilized, and the engineers were thus enabled to dispense with

BRIDGE AT POUGHKEEPSIE.

square foot has been provided for, and every part of the foundation has been tested to one third more than the greatest possible load that can be placed upon it.

Bridge at Taranto, Italy.-A strait separates the old and new towns of Taranto, and connects what is known as the Little Sea (Mare Piccolo) with the Gulf of Taranto, both of classic fame and of considerable maritime importance. The bridge recently finished was constructed with a view to the ready passage of large vessels. It consists of two half-arcs meeting above the middle of the strait. Each half is moved by machinery driven by two turbines of 14-horse power. The halves are raised and rotated, the lifting motion being given by four nuts worked by an endless screw, and the rotation effected through large wheels at the end of the abutment. The turbines are driven by water from a reservoir, and the

distance between abutments is 188 feet. A test-load of 280 tons was left for 24 hours upon the bridge, and caused a deflection of less than 3 inches, or about half what was allowed in the specifications. On the removal of the load, the deflection disappeared altogether.

Stiffened Suspension Bridges.-A peculiar type of suspension bridge, known as Garson's patent, has been adopted by the British authorities in India, which is believed to secure greater stability with less weight of metal, for small bridges, than any previously adopted plan. The usual plan has been to adopt side girders for small suspension bridges, but this involved too much weight and too great cost. The distribution of stress is stated as follows: Upper chain.-Stress at center equal to zero, increases toward piers until it reaches a maximum. Lower horizontal member.-Stress at abutments equal to zero, increases toward center, where it

reaches a maximum. Diagonal braces.-Stress horizontal, almost equalized, and of small amount. The bridge is hinged at the center, so that it can rise and fall with changes of temperature. The pins are of wrought-iron or steel, and are hinged at the base and connected at the summit, so that the stress on them is always axial. The stress on the foundations is purely in the nature of a vertical load. The bridges are built in Glasgow in 70-foot spans complete, and are delivered in Calcutta for $625 each, the weight being 137

Suspension Foot-Bridge at Oak Park, Ill.-Among the feats of noteworthy amateur engineering is a bridge described by the "Scientific American" as the work of amateurs, young men "just in their twenties." It crosses the Desplaines river with a central span of 125 feet. One bank of the river is a bluff, upon which a concrete tower was erected. On the other bank an elm-tree, having double trunks nearly side by side, was used, the bridge passing between them to a concrete anchorage 75 feet distant. The anchorage on the bluff side is afforded by an oak-tree, to which the cables are made fast near its base. The cables are five eighths of an inch in diameter and are four in number, two of them being merely auxiliary. The footway is about four feet wide, and the whole structure weighs only 2,750 pounds. It has borne a test-strain of fifteen men standing together upon it, and is constantly used by footpassengers as means of transit.

Dams.-The near completion of the Vyrnwy dam in Wales, for the water-supply of Liverpool, England, and the beginning of work on the great dam at Quaker Bridge, N. Y., for the supply of New York city, are among the larg est engineering works of the day. In connection with them it may be well to consider the other great dams of the world, ancient and modern, for the construction of reservoirs dates back to prehistoric times, and bore a conspicuous part in the oldest civilizations. Herodotus describes the lake of Moeris as formed by the Egyptians for husbanding the surplus of the Nile floods, and within a year or

two there has been some talk of reconstructing the ancient canals and restoring the lake to its former usefulness. The same writer mentions the reservoir of Nebuchadnezzar at Sippara, which is said to have been 140 miles in circumference-a statement that must be taken with some grains of allowance. Certain it is that in Egypt, Asia, India, Ceylon, and China, vast works were executed for the retention of the surplus rain-fall of the winter months. Some of these ancient earthworks and masonry have wholly disappeared, but traces of others still remain. Conspicuous among them are the reservoirs of Cummum, Kala-Weva, and Horra Bera, in Hindostan. Most of the dams are in ruins now, but have been surveyed, and evince a very creditable degree of engineering skill. The first named, though perhaps the oldest, is still serviceable. The embankment is 102 feet high, with a breadth on top of 76 feet, and a base of about 300 feet. The lake that it created, when perfect, was about 15 square miles in area. The ruins of the dam of Kala-Weva are 12 miles long, and the lake, when full, must have been 40 miles in circumference. That of Horra-Bera is from 50 to 70 feet high, between 3 and 4 miles long, and controlled a lake 8 or 10 miles long and 3 or 4 miles wide.

The advance from earthwork to masonry marks a long step toward theoretical perfection. Most of the great masonry dams have been constructed within the present century. Sections of several of them are shown on the next page in outline, resting upon a common base for ease of comparison, and having a scale in feet at the left.

The Puentes Dam, No. 5, is in Spain, and is almost identical in its elements with the Alicante dam in the same country. Its height is 164 feet, and its width 65 feet at crest. It was built about three centuries ago. The sides of the valley at Puentes were rock, but the bottom was untrustworthy, and a heavy arch of masonry was thrown across, springing from solid rock, and upon this the dam was built, the under space being filled in with walling. The locality was liable to sudden and violent floods, and probably the great width at top was provided in view of unavoidable overflows, covering the entire extent of the dam and calling for great weight, the elements of pressures not being fully understood at that time. It was not practicable to construct side overflows. A very large amount of sand and silt is brought down these streams, and to get rid of it a somewhat primitive method was adopted. Two openings were provided at the base of the dam, the upper end being stopped with loose timber, while the lower end was closed by iron doors. When the accumulation of silt necessitated flushing the dam the iron gates were opened, and workmen sent in to break out the timber screen. If they had good luck it was hoped that the silt would keep back the rush of water long enough for them

Champs between the years 1859 and 1866, and was intended partly to protect the town from freshets, and partly to afford a perennial watersupply. Its cross section is shown in the engraving. Its plan is a curve on a radius of 828 feet from a center on the down-stream side. It is founded on compact granite, a trench 3 feet deep having been quarried out to prevent slipping. The material is rubble masonry, laid in courses of 5 feet, and carried up to a height of 184 feet. At the base it is 110 feet thick, and 9 feet 8 inches at the crest. The calculations aimed for a pressure of about 95 pounds to the square inch. The dam contains about 52,000 cubic yards of masonry, and the cost of erection was $180,400. The capacity of the reservoir is 352,000,000 gallons. The reservoir discharges its surplus water through two tunnels, leading through a hill into an adjacent valley, where such power as is constant is usefully employed.

CROTON

dations are carried down into solid rock. Its total height is 154 feet, and the width of the crest, which carries a roadway, is 49 feet. The base is 216 feet thick. The outlet pipes, always a source of danger, are carried through a hill at some distance from the end of the dam.

Another great Spanish dam is on the river Lozoyers, and supplies water to the city of Madrid. It is known as the Villar Dam. The capacity of its reservoir is 4,400,000,000 gallons, nearly thirteen times that of Furens. It is built on a sharp curve, the radius being 440 feet, and the length of the dam, on the crest, 546 feet. A curve like this probably adds considerably to the strength of a short dam. The material is rubble masonry in hydraulic mortar, costing $402,780.

The Vyrnwy Dam, shown in section in No. 4. crosses the Vyrnwy river in North Wales. The area that will be flooded is, or was, a charming region, largely occupied by villas and country-seats, and, of course, involving a large amount of outlay in property rights. The dam will impound an area of 1,115 acres. It is 1,255 feet long, built of Cyclopean rubble set in mortar, and with the interspaces filled with cement-concrete. The individual masses weigh from 2 to 8 tons each, and it is calculated that this method of construction will give exceptional solidity to the wall. The upper face of the dam is coated with cement. The height is 146 feet, and the breadth at base 117 feet 9 inches.

A large dam is building at San Mateo, Cal.,

BRIDGE DAM

M

HUNTERSVILLE

Another famous structure is the Gileppe Dam at Verviers, Belgium, No. 3. It was finished in 1875, under the supervision of M. Bodson. It differs largely from the Furens section, and, indeed, from the best theories of dam-construction. This was rendered necessary by the anxiety of down-stream residents, who strongly opposed the construction of the dam, on the ground of danger. It is laid on an arc, described by a radius of 1,640 feet, with a length of 771 feet. The reservoir contains 2,701,687,000 gallons, nearly eight times as much as the reservoir at Furens. The foun

CROTON WATER-SHED.

designed for the water-supply of San Francisco. It is 170 feet high, 176 feet thick at the base, and 20 feet wide, being in shape very much like a truncated pyramid.

No. 6 represents another variety of what may be termed the square-block type of dam, which appears to have been a favorite with Moorish engineers. It is known as the Val d'Inferno, and is in the province of Sarco, Spain. The dam at Quaker Bridge, N. Y., shown in the diagram, towers above all other structures of the kind, actual or prospective. The lake that will be created will include the present reservoirs, covering the existing dams to a depth of many feet. It will contain something like 40,000,000,000 gallons of water, more or less a matter of a few million gallons is of small moment where such amounts are concerned. It is estimated that the dam will impound the whole rainfall of the Croton watershed, and will afford an ample supply for the city, even if no rain at all falls for a period of several months. The plans and calculations for this vast structure were drawn by Benjamin S. Church, chief engineer. The estimated strength of the wall is about double what it will ever be called upon to bear. The Croton water-shed, as may be seen from the map, is an irregular valley about 25 miles long by 12 or 15 miles wide.

The dam proper will be 1,350 feet long on the crest, rather more than of a mile. It will rest in a ditch, quarried out of the solid rock, 216 feet wide at the widest part, and will have an extreme height of 277 feet, decreasing to nothing at the wings. The dam will not be laid on the arc of a circle, that plan being regarded as obsolete for a structure of this size, however it may add to the stability of a short span like that at Furens. The dam is planned to resist by sheer weight any possible pressure that can result from the accumulation of water. The idea that the extent of area increases pressure is wholly erroneous. The pressure of water is due to its depth, not to its extent, as may be readily seen by reference to any authority on hydrostatics. The danger from a large impounded body of water arises, not from the increased pressure upon the restraining dam, but from the cumulative rush of water after a break has occurred. In an earthwork dam, a trifling leak may spread with disastrous rapidity and carry away the whole structure; but a masonry dam, if properly constructed, might be split from crest to base, and would remain in position while the water trickled through the crack.

sources it is unlikely that serious interruptions will occur.

The system of construction is simple. The first work is to lay bare the bed-rock of syenitic gneiss that underlies the whole valley, at a depth of about 90 feet below the river-bed, the width of the trench corresponding with the base of the dam. Two smaller trenches will be made, each 10 feet wide and as many deep, running lengthwise of the dam, and, after all natural fissures in the bed-rock have been filled with hydraulic cement, the trenches will be built in with Cyclopean rubble, as it is termed— namely, large, rough, irregular blocks of stone, laid so as to break joints and filled in with cement, so that the mass becomes as solid as natural rock. All unevenness will be used to anchor the dam beyond the possibility of slipping. Gates and weirs of the best construction will be provided to carry off any overflow during exceptionally wet seasons, and the whole mass of water can be drawn off in case of necessity.

The estimates for the actual cost of construction are $3,000,000, and the contracts contemplate its completion in 1891.

Floating Dock. An off-shore floating dock for Cardiff, Wales, was constructed at Gray's, near Tilbury, and towed to Cardiff in June. In end elevation the structure resembles the letter L, the horizontal limb, which is in fact the pontoon, being wider than the vertical limb is high. The upright part of the dock is attached to vertical columns on shore by means of booms arranged in pairs, so as to insure parallel motion with the rising or sinking of the dock.

The pontoon, being filled with water, sinks to the desired depth, and the vessel is floated over it with obvious ease, since it can be warped into position broadside on. The keelblocks and bilge-blocks are all worked by machinery from the top deck, as in all the best modern docks. The pontoon can be held at a level or given an inclination, if a vessel with a considerable list has to be taken up.

Maritime Engineering.-Among the great feats of launching should be noted that of an enormous lumber-raft at Joggins, Nova Scotia. The construction was begun in 1885, the design being to tow the completed raft to New York, and thereby save expense, and at the same time bring to market larger logs than can be handled on ordinary coasting-vessels. The