274 



ENGINEERING. 



with 30 feet of roadway, and two 8-feet walks, 

 at San Jose, Cal., and a 256-feet span, at Pres- 

 ton, W. Va. All the details in their patent 

 truss-bridges are in wrought-iron. 



Two spans of the great bridge across the 

 Mississippi, at Council Bluff's, were swept away 

 on August 25th, but were replaced, and trains 

 were run across on September 16th. Although 

 the spans were 250 feet long, and of 4 trusses, 

 the actual work was accomplished in a single 

 fortnight. Another still more remarkable ex- 

 ample of quick work in repairing was the clear- 

 ing away of the broken parts of a fallen span in 

 the Chicago and Northwestern bridge over the 

 Des Moines, and putting up the new span of 170 

 feet, and laying the ties and rails ready for the 

 passage of trains, in the time of 17 hours, the 

 materials having arrived at the spot from Clin- 

 ton within 20 hours of the accident. 



The great Tay bridge, crossing the estuary 

 of the Tay, near Dundee, in Scotland, was 

 opened to traffic on September 25, 1877. Its 

 length is almost fully 2 miles, thus making it 

 the longest railroad bridge in the world. It 

 has been built for the North British Eailway, 

 which has hitherto been obliged to transfer 

 goods and passengers across the Tay by means 

 of a ferry. The plans of the bridge were pre- 

 pared by Thomas Bouch. The foundation- 

 stone was laid on July 22, 1871. The south 

 end of the bridge is 1^ mile above Newport, 

 where the depth of the water at spring tide is 

 45 feet, and its velocity sometimes 5 knots an 

 hour. The original plan was for a bridge of 

 89 spans, with double cylindrical supports. 

 The bridge, as it was afterward constructed, 

 consists of 85 spans, varying from 67 to 245 feet 

 in length. The longest, to the number of 13, 

 overhang the navigable part of the river ; and 

 shipping must pass under them to reach the 

 ports of Perth and Newburg. In these widest 

 spans, over the centre of the river, the floor 

 has a height of 88 feet clear above high water, 

 from which elevation it descends in a gradient 

 of 1 in 356 to the Fife side, and of 1 in 73 to 

 the Dundee side ; in the latter part it takes a 

 bend to the eastward, so as to meet the land 

 line. The earliest laid piers consisted of cylin- 

 ders of iron, 9 feet in diameter, lined with 

 brickwork set in Portland cement, with a hol- 

 low shaft through the centre, of 3 feet diame- 

 ter, for the workmen to descend. From the 

 bottom of these cylinders, after the water had 

 been expelled by compressed air, by the bell- 

 chamber process (mentioned below in the ac- 

 count of the Severn bridge), the laborers ex- 

 cavated the mud and sand underneath until 

 they sank to the rock, in which a level bed 

 was cut for them. They were placed in cou- 

 ples, 12 feet apart from centre to centre. An 

 intermediate wall of brickwork, about 5 feet 

 wide, braced them at the top. They were 

 floated out to their posTRolr-between barges, 

 and lowered by means of hydraulic machinery. 

 These double piers weighed about 120 tons. 

 They were made of such length that their tops 



would stand a little above low-water mark 

 after they had sunk to their beds. The plan 

 was originally to have all the piers made of 

 this description ; but after a good number of 

 the double piers had been laid, a point was 

 reached where the rock suddenly declined to a 

 great depth, under beds of clay, sand, and 

 gravel. It being impracticable to construct 

 cylinders of such great length as to rest on the 

 rock beyond this declivity, the form and di- 

 mensions were so changed as to give them a 

 firm seat on a gravel foundation. Single cylin- 

 ders of 31 feet diameter replaced the coupled 

 ones. Wrought-iron was used in the shell, 

 instead of the cast-iron of which the double 

 cylinders were composed. The mode in which 

 these huge masses, weighing some 200 tons, 

 were carried out to their positions was by fast- 

 ening to them a framework of iron girders, 

 under which the barges were floated at low 

 water; when the tide rose they were lifted 

 clear into the water, and were towed out to 

 their places while hanging perpendicularly be- 

 tween the barges. The brickwork was only 

 filled in at their bases, which held them plum, 

 iron columns taking the place of the brick 

 lining in the upper portion. A new invention, 

 the idea of one of the engineers of the bridge, 

 was used to excavate under the base of the 

 cylinder. It is a kind of pump which makes a 

 cavity at the bottom from the inside, expelling 

 the sand or mud with great rapidity, the great 

 pier sinking by its own weight into the hole 

 thus formed. This invention may hereafter 

 prove of great service in dredging operations, 

 as well as in bridge construction. The suitable 

 foundation was found about 18 feet below the 

 river-bed. All the subsequent piers were built 

 in this way ; 142 of these caissons were sunk 

 in all with Reeves's excavator. The cylin- 

 ders, after they were set, were filled to the top 

 with concrete, and then the iron casing was 

 stripped off down to the river bottom. On 

 this base of artificial stone a hexagonal pier of 

 brickwork, of oblong form, was superimposed. 

 This brick superstructure, 20 feet long by 10 

 wide, and about 20 feet high, was also built on 

 shore, and floated out between barges. When 

 in place on the concrete basis it rose to a point 

 just above low-water mark. The brickwork 

 was then raised by masons at low tide, and 

 capped at the high-water mark with 4 or 5 feet 

 of stone-work, laid in four courses. The piers 

 of 14 spans of the bridge were built in this man- 

 ner. They were so disposed that their narrow 

 sides cut the tide. The portion of the piers 

 above the high-tide mark is composed of iron 

 columns of 12 and 15 inches diameter. The 

 245-feet girders, weighing 190 tons for each 

 span, were towed out and deposited on the piers, 

 and then raised by hydraulic machines to their 

 places, in lifts of 20 feet at a time. Each span 

 has 2 girders, of depths varying according to 

 the length of the span, which are joined by 

 transverse braces. The smallest girders are 

 12 feet deep. On either side of the 13 high 



