HISTORY OF ENGINEERING.

230

Book I.

The casks were 12 feet in length, 7 feet in diameter, and it was calculated that each wascapable of buoying up 28,000 pounds.

Fig. 241.

PLAN OP CASKS.

Fig. 240. SECTION OF CONE.

When all the casks were fastened to thebottom, the cones were towed to the positionwhere the boats were to be attached: it had beenpreviously ascertained by experiment, that fourmen rowing could tow one cask from 15 to 20toises per minute against tide, and twice as fastwhen the tide was in their favour. Upon thiscalculation 250 men would be required in calmweather to tow out the cone, weighing, it wassupposed, upwards of 2,000,000 pounds; butDe Valon’s capstan being employed, forty-twomen only were required to do the samework, and they could in five or six hours moveit a distance of 3000 toises; after which itwould be in a position to float at all times ofthe tide, and could be then towed by sailingboats or by rowing.

By the 6th of June, 1781, all was in readinesswith the first cone; a circuit of strong cableswas placed immediately above the line of im-mersion, and another above the line of floatation,crossing the bottom with a grillage of ropestending to the centre, to resist the force of thewater, and to prevent the timbers spreading.

For cutting away the casks after the cone hadbeen floated to its destined position, there werea number of hatchets or cutting instrumentsattached, one over the rope which held eachcask; they were made to move upwards bya rope, and, from being loaded with lead, droppedby their own weight, like a pile-driving machine,and severed the rope upon which they fell.Thirty-five casks were attached to the inside,and forty-nine outside. Thirty-one of a smallerkind were added to increase the buoyancy, andladders were provided for the numerous work-men employed. It would have been practicable

to have floated the cone after it was detached from the platform upon which it had beenframed and put together, by allowing the tide to rise around it, but the following arrange-ment was preferred.

In proper situations anchors were fixed, provided with tackle and blocks, that were