The preliminary step was the fabrication of a caisson. The first part was more like a raft with sides. It was about 40 feet square, 5 feet thick, and with walls 7 feet deep. It was built of timber, the staves being 12 inches square, and upside down--that is, with the floor uppermost--on a building-slip, as if it were a ship, and was launched into the water upon similar lines. The sides and top were lined, so as to secure water-tightness. In the centre there was a circular space 5 feet in diameter to form the air-shaft. As the structure was built upside down, the rim was brought to the lowermost position, and this formed the cutting edge, which was to be sunk into the sand. On this floating platform a circular iron cylinder was erected. This tube was 35 feet in diameter, and was built up in plates, 6 feet in width by 1½ inches thick. When three rings of iron were set up the cylinder was 18 feet in height. In order to sink it to a depth of 15½ feet into the water for towing purposes, it was charged with a layer of concrete, 9 inches in thickness, to serve as ballast, and in this condition the caisson weighed 400 tons. This huge barrel was built at Lewes, Delaware, and when it was launched two powerful steam-tugs set out to drag it to the shoal, twenty miles away. As the tide rises and falls a matter of 6 feet in these waters, and the currents are somewhat wicked, the engineers displayed no undue haste. They waited for the first favourable opportunity, and seized it. But it took the two tugs some six hours to reach the site; an average speed of about three and a half miles per hour cannot be construed into fast travelling.