262

HISTORY OF ENGINEERING.

Boor I.

hold the main upright in its place was withdrawn, permitted it to descend, and the wedgepiece by its side to be drawn up ; when this was done the whole side was easily re-moved.

This caissoon was constructed in a convenient situation, and commenced by drivingthree parallel rows of piles 3 feet from centre to centre, forming twenty-four piles, unitedby cross pieces 22 feet in length ; the first row was cut off level with the water line, thesecond and third 3 feet higher, in order that the inclined plane might facilitate the floatingof the caissoon. The whole was made level to receive the platform by blocking up withtimbers, 30 feet in length, and of a scantling 15 by 12 inches. In the middle was a pro-jecting bracket and other contrivances, so that when the bottom of the caissoon was raisedto an angle of 20 degrees, it would slide easily into the water.

In laying out the bottom, care was taken to place its centre of gravity 6 inches within onthe land side, and after the caissoon was constructed, it was filled with water by means of apump, to prove the caulking.

To fix the upright timbers which were attached to the horizontal layers that formed thesides of the caissoon, four iron bolts, 20 inches in length and an inch in diameter, were passedthrough them, with their heads on the outside and the nuts within, that they might beeasily unscrewed; these were well caulked round with moss to keep the water from pene-

Fig. 271.

CAISSOON.

trating, and eight chains were attached to them to facilitate their drawing. To prevent the-caissoon from collapsing when placed in the water, five diagonal struts were introduced, whichcould be easily moved: after all was prepared, the wedges towards the land were withdrawn,and the centre of gravity being 6 inches nearer that side, an inclination to move was given toit; sixteen men at eight jacks raised it a trifle, and then allowed it to slide gently towardsthe river ; when afloat it drew 24 inches of water ; the bottom was forced up about 3 inchesm the centre for every 1000 feet superficial of base, 3£ inches for 2080 feet, and 5 inchesfor 5084 feet. It was towed to its position on the Loire by six rowers, and before theheads of the piles were cut off, the lower course of masonry was commenced, 14 inchesIn thickness, over the lines previously drawn for the position of each stone. After thisk was found that the caissoon drew 41 inches of water, and remained perfectly level. Ailinclined plane was formed of two pieces of timber, on which ran a small carriage, thatbrought down the blocks of stone, and facilitated the operation of the masons.

When the heads of the piles were all cut off, the caissoon was towed to its place, everyprecaution being taken to moor it in its exact situation ; the second course of stone was thenlaid, 20 inches in height, formed of 30 blocks ; the caissoon then drew Cl inches of water, itsposition was again verified by stretching a piece of timber across the intended span, andwhen half the third course was laid, it was settled on the heads of 116 piles, driven toreceive it; a verification was then made that it had taken up its exact distance from thepiers already constructed on the land.

The fourth, fifth, sixth, and seventh courses of stone were then laid and cramped, and inthe eighth course, which formed the springing of the arches, were introduced three mooringrings on each side, tied at their ends by irons 14 feet in length.

All the masonry being completed, the joints well dressed, and covered with powderedlime and fine sand, which hardens in water like puzzolana, the sides of the caissoon were