MECHANICS AND USEFUL ARTS. 19 



covering securely adjusted, their final connection is effected as fol- 

 lows. A " rivet-boy " and a " holder-up " crawl into one of these 

 flues, and having got arranged in there, at a signal red-hot rivets are 

 passed through holes, made for the purpose, to the boy, who delivers 

 them to the holder-up, and he in turn drives them through rivet-holes 

 to the outside, where they are also pounded, so as to form a head 

 there. It is extraordinary how any person can work from morning 

 till night, as these do, in a space hardly large enough to lie down in. 

 The plates, having been thus adjusted in the positions best suited to 

 resist the strains they will have to bear, are finally connected together 

 by small ribs riveted to them. The quantity of angle-iron thus 

 worked up, through the top, bottom, and sides of all the tubes, 

 amounts to 65 miles. 



The Britannia tower in the centre of the strait is at the base 62 

 feet by 52, and rises to a height of 230 feet. This enormous struc- 

 ture, which weighs over 20,000 tons, contains 148,625 cubic feet of 

 Anglesey marble for the exterior, 144,625 cubic feet of sandstone for 

 the interior ; and 387 tons of cast-iron beams and girders, worked into it, 

 give strength and security to the mass. The province of this tower is 

 to sustain the four ends of the four long iron tubes which will span 

 the strait from shore to shore. The total quantity of stone contained 

 in the bridge is 1,500,000 cubic feet. The side or land towers are 

 each 62 feet by 52 in the base, and 190 feet high. They contain 210 

 tons of cast iron. 



III. The Floating of tlie Tube. The props on which the tube 

 rested having been removed, so that it was supported only at the ends, 

 it was found that now the slightly circular form of the bottom, be- 

 came, as was intended, perfectly straight. The pontoons, eight in 

 number, each 98 feet long, 25 wide, and 11 deep, were built with 

 valves in the bottom to let in or keep out the water at pleasure, and 

 were capable of bearing a weight of 3,200 tons, though the tube 

 weighed but 1,800. From these pontoons, hawsers, whose united 

 length was over two miles, were passed to capstans on the two 

 shores, and on the Britannia tower, and when, at the signal, the Caer- 

 narvon ropes were cut, the tube at once slid on to the pontoons. It 

 was then slowly floated by the tide down to the position from which 

 it was to be raised, wfiere it was securely fastened. 



IV. Raising the Tube. The tube was raised by means of an hy- 

 draulic press of immense power. The cylinder or large tube of the 

 syphon of the press, which is 9 feet 4 inches long, 4 feet 10 inches in 

 diameter, and which is made of cast iron 11 inches thick, weighs 16 

 tons. The whole machine, complete, weighs over 40 tons. Its lift- 

 ing power is 2,622 tons, and it has force enough to throw water 5,000 

 feet higher than Mont Blanc. The manner in which tin's immense 

 machine works is as follows. Its position on the Britannia tower is 

 148 feet above the level of the water, and about 45 feet above that to 

 which the tube must be raised. Around the neck of the iron ram or 

 piston is affixed a strong horizontal iron beam, from the extremities 

 of which hang two enormous chains, composed of eight or nine flat 

 links or plates, 7 inches broad, 1 inch thick, and 6 feet long, firmly 



