ENGINEERING. 



255 



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 de- 

 livered in Calcutta for $625 

 each, the weight being 137 

 cwt. 



Other notable bridges are 

 finished or in course of con- 

 struction, namely, across 

 the Harlem river at 181st 

 Street, New York city, 

 across the Missouri river, 

 near Kansas City, and the 

 Rulo Bridge over the same 

 river. The Hawkesbtiry 

 river bridge in New South 

 Wales is well under way in 

 charge of American con- 

 STIFFENED SUSPENSION tractors, and it may be said, 



BRIDGE. CROSS SEC- . 7 . , . T . 



TION . in conclusion, that Ameri- 



can systems of bridge con- 

 struction are commanding the respectful atten- 

 tion of engineers the world over. 



Suspension Foot-Bridge at Oak Park, HI. Among 

 the feats of noteworthy amateur engineering 

 is a bridge described by the " Scientific Ameri- 

 can " as the work of amateurs, young men u 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 con- 

 crete 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 foot- 

 way is about four feet wide, and the whole 

 structure weighs only 2,750 pounds. It has 

 borne a test- strain of fifteen men standing to- 

 gether upon it, and is constantly used by foot- 

 passengers as means of transit. 



Dams. The near completion of the Vyrnwy 

 dam in Wales, for the water-supply of Liver- 

 pool, 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 connec- 

 tion 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 cir- 

 cumference 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 Bern, in Hindostan. Mo^t 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 circum- 

 ference. That of Horra-Bera is from 50 to 70 

 feet high, between 3 and 4 miles long, and con- 

 trolled 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 perfec- 

 tion. 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 Pueutes Dam, No. 5, is in Spain, and is 

 almost identical in its elements with the Ali- 

 cante 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 bot- 

 tom 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 over- 

 flows, 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 



