Engineering Wonders of the World
Volume I

326 ENGINEERING WONDERS OF THE WORLD. INCHGARVIE SOUTH-WEST CAISSON ON LAUNCHING WAYS. (Photo, J. Valentine and Sons.) These huge steel structures, which formed the skin of the below-water foundations of the tower piers, were 70 feet in diameter. drills, and charged with dynamite cartridges connected to an electric battery. When the men had withdrawn, the pressing of a button fired all the charges simultaneously. A copious supply of high-pressure air was then blown through the chamber to expel the foul gases, and the gang descended to clear away the débris, which was loaded into skips and sent up through special shafts and air-locks to the upper deck. It was necessary to undercut the edge of the caisson very carefully, and by means of the depth of the cut to counteract any tilt from the perpendicular. The sandbags, which had been piled as a support, were displaced gradually, and pushed down the rock until the cutting edge had worked right through them. Every now and then fish, attracted by the glare of the electric lamps inside the Use of Com- pressed Air, air-chamber, would come to the gaps between the bags and peer in, wondering, no doubt, what strange beings had invaded their haunts. The Inchgarvie caissons were all in their final positions by the beginning of Octobei 1884, and the piers on them completed by February 22, 1885. The Queensferry caissons are the deepest of the eight used. They reached depths of 71, 73, 85, and 89 feet respectively below high-water level. Owing to their very exposed position in The open water, they were pro- Queensferry vided with temporary caissons Caissons, about 20 feet high added to the top of the tapering portion, to exclude the waves while the masonry was built up. All had a double skin, the inner one splayed out at the bottom to meet the other at an acute angle and form a cutting edge. As