LONGEST BRIDGE IN THE WORLD. 611 



five feet wide, thus placing a space of twelve feet between their centers. 

 The whole being then made fast to a sj^stem of strong iron girders, barges 

 were introduced at low water underneath the girders so that when the tide 

 rose, the ponderous mass, weighing, it might be, as much as 120 tons, was 

 lifted bodily, and quietly floated out into the river. Having been towed 

 out to the site of the intended pier, the cylinders were lowered by hydraulic 

 apparatus till they rested on the river bottom. Workmen then descended 

 the internal shafts, excavated the material from beneath the cylinders, and 

 so caused them gradually to sink until they reached the rock, in which a 

 level bed was cut for their permanent resting place ; but after the work had 

 been in progress for some time it was found that the rock suddenly shelved 

 away to a great depth under beds of clay, gravel and sand. It therefore 

 became impracticable to sink the piers to that foundation, and a new meth- 

 od had to be introduced. The weight of the pier was lightened by substi- 

 tuting in the upper works iron columns for solid brick, while the adoption 

 lor each pier of a single oval cylinder measuring 23^ feet by 13^ feet se- 

 cured a larger bearing than had previously been obtained with two smaller 

 ones. The outer casing was of malleable instead of cast iron. When the 

 large masses were prepared they were floated out and lowered to the river 

 bottom, the sand at the base being removed by a pump invented by one of 

 the assistant engineers. By the working of the pump a large cavity was 

 speedily formed under the cylinder, and the huge mass of metal sunk into it by 

 its own weight, reaching to a depth of about 18 feet below the bed of the river. 

 The interior of the cylinder was afterwards filled to the top with concrete, 

 and the upper part, so far as it stood aboVe the bottom of the river, was 

 next removed, thus leaving an ample platform of artificial rock for the re- 

 ception of the superstructure, which consisted in the first instance of brick- 

 work in the form of an elongated hexagon, measuring 20 feet by 10 feet, 

 and placed with its greater length in the direction of the current. This 

 part of the pier was likewise put together on shore, and floated out be- 

 tween barges in a length of about 20 feet, being sufficient, when placed 

 upon the cylinder, to reach above low-water taark. From the low- water 

 level the pier was formed of solid brickwork, built in the ordinary way by 

 workmen brought alongside as the state of the tide would permit; and at 

 high- water mark four courses of stone, of an aggregate thickness of four 

 or five feet, finished off this portion of the work. The piers of fourteen 

 spans were founded in this way, the upper works being formed of iron col- 

 umns of twelve and fifteen inches diameter. 



When the piers had been brought to the necessary height, the girders, 

 measuring 245 feet in length and weighing 190 tons for each span, were 

 towed out and deposited. The raising of the girders when the piers had 

 been completed was carried on in lifts of 20 feet at a time by hydraulic ap- 

 paratus. Two girders, connected by tranverse braces, go to each span, 

 the depths varying according to the width of the space to be crossed. The 

 length of the span diminishes in going toward the shores, on either side of 



