ENGINEERING. 



275 



245-feet spans, the lengths of the spans are 

 gradually less toward the shores. The girders 

 are so fastened to the piers as to allow a cer- 

 tain amount of free play to compensate for 

 expansion by heat; the whole range of con- 

 traction and expansion amounts to about 7 

 feet for the whole length of the bridge. The 

 strain calculated for is 21 ton to the square 

 inch, or a rolling load of 1$ ton to the foot 

 run ; it would bear an endless train of loaded 

 freight-cars. It will not be subjected to a 

 greater strain than 4 tons to the square inch. 

 The stock company which has built the bridge 

 is separate from the railway corporation. The 

 capital stock is $1,750,000. The allowance of 

 Parliament was obtained in 1870, and the con- 

 tract given out in the succeeding year. 



There are, besides the 13 great 245-feet 

 spans, 1 with a bowstring girder of 170 feet, 

 1 of 162 feet, 13 of 146 feet, 21 of 129 feet, 2 

 of 88 feet, 14 of 70$ feet, 14 of 67i feet, 6 of 

 27 feet. The girders are of the lattice kind, 

 continuous in sets of 4, with sliding beds on 

 the end ones of every set. They have the 

 double triangulation, and trough booms at top 

 and bottom, from 15 to 24 inches wide. The 

 rule for the depth of the girders, that it should 

 be equal to -J- of the span, was decided upon 

 after long experimentation. A vertical tie binds 

 the top boom to the crossing of the struts 

 and ties at every alternate crossing. The 

 cross-girders, of pitch-pine, bear the rails, 

 laid on longitudinal timbers, and the whole is 

 planked with 3-inch Memel covered with as- 

 phalt. Over the 13 long spans the rail-plat- 

 form rests on the lower booms, the trains run- 

 ning between the girders; but in the other 

 portions of the bridge it is supported by the 

 top booms ; by this arrangement the grade was 

 made more equal, while the cost of construc- 

 tion was diminished. During the nights of 

 the winter of 1876 the work was continued ; 

 two of Serrin's electrical lamps connected with 

 Gramme machines, each furnishing a light of 

 1,000 candles, illuminating the works. 



The system of pneumatic excavation, em- 

 ployed in sinking the great piers, was devised 

 by the assistant engineer Reeves. The appa- 

 ratus consisted of 4 wrought-iron tanks, placed 

 upon a barge. They were connected with 2 

 air-pumps. The tanks were cylindrical, with 

 funnel-shaped bottoms, through which the ex- 

 cavated material was discharged through the 

 bottom of the barge. The air-pump could be 

 changed from one of each pair of tanks, when 

 that was full of the mud, gravel, or other ex- 

 cavated matter, and connected with the other. 

 Air being then admitted into the full tank by 

 an inlet-cock, a valve closes over the pipe con- 

 nected with the suction hose, while a similar 

 valve opens in the companion pipe, connecting 

 that with the suction hose. The action of the 

 air entering the one tank and being exhausted 

 in the other also opens a discharge-valve over 

 the conical bottom of the full tank" or drum, 

 allowing its contents to empty, while it closes 



a similar valve in the other tank, creating a 

 vacuum, into which the nioist matter is pumped 

 from the bottom of the caisson, so that the 

 one drum is filling while the other is voiding 

 the excavated material. The mouth of the 

 suction hose could be moved to any part of the 

 caisson by a man at the bottom. The attend- 

 ance of 2 men and a boy was required for each 

 barge ; the quantity of material excavated was 

 over 400 tons per day of 10 hours, at a cost of 

 about 8d. per yard. 



The Severn bridge, another English struc- 

 ture, nearly three-quarters of a mile in length, 

 will connect the Great Western and the Severn 

 & Wye Railways with the Midland. The en- 

 gineers are G. W. Owen and G. W. Keeling. 

 It consists of 22 spans, bridged by bowstring 

 girders of from 134 to 827 feet in length, and 

 12 arches 70 feet high in the northern ap- 

 proaches. One of the spans at the southern 

 end is crossed by a swing-bridge. The coupled 

 iron cylinders which form the piers are sunk 

 through 28 feet of sand to the rocky founda- 

 tion, and bedded in the rock to the depth of 4 

 feet. The excavation under the cylinders was, 

 in the beginning, accomplished from the inte- 

 rior by a proceeding similar to that at first 

 used on the Tay bridge. A bell-shaped cham- 

 ber is fixed to the cylinder a few feet above 

 high-water mark. Compressed air is driven 

 in, and the water expelled, allowing the work- 

 men to descend to the bottom. Later the 

 Reeves excavator was applied, with which the 

 piers could be sunk much more rapidly. The 

 piers, when bedded, are filled with concrete. 

 The height of the piers above high water is 60 

 feet. Between the concrete and the iron cyl- 

 inders is a felt lining, to guard against unequal 

 expansion or contraction of the materials. 



The Ilkeston viaduct, in the Derbyshire ex- 

 tension of the Great Northern Railway, is a 

 structure of extraordinary lightness, the honey- 

 combed ground underneath being unable to 

 bear a great weight. It crosses the Erewash 

 Valley in 19 spans, each 77 feet in length. The 

 piers are mainly composed of hollow columns 

 of malleable iron, riveted together in 4 sec- 

 tions and joined by cross-braces and ties. The 

 floor is formed by ridges and furrows of plate 

 iron, riveted at the angles and ends, which 

 answer for cross-girders and stays as well as 

 flooring. The weight of this floor is 784 Ibs. 

 per foot run. The columns rest on brick ma- 

 sonry, which has a broad concrete base, and 

 the weight of the whole on the foundation is 

 but 1,200 Ibs. per square foot. The peculiar 

 corrugated floor is very elastic. The furrows 

 are filled in with ballast, in which the sleepers 

 are dropped. The height of the bridge is 42 

 feet. 



The Marguerite Bridge at Bnda-Pesth, com- 

 menced in 1873, was opened in April, 1877. 

 The total length is 570 metres, the width 17 

 metres. The spans are 6 2 each with 74, 

 88, and 88 metres opening. Over 15 million 

 pounds of iron were used in the construction, 



