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HISTORY OF ENGINEERING.

Book I.

basin, 525 fathoms in length, and 150 in breadth ; the depth at the entrance is 18 feet, andin the harbour from 12 to 24 feet, which is maintained by the constant employment ofthe dredging machines.

The lighthouse is situated on the Fort St. Jean, which is on the north side of theentrance to the port, and at some distance from the north of the city is the lazaretto.

On the island de Planier is another lighthouse, 131 feet in height, which from the excel-lence of its revolving lights may be seen in clear weather at a distance of seven leagues.

There is a slip for ship-building constructed of stone, the length of which is sufficient toadmit a man-of-war, and the width at top 47 feet. The whole is ingeniously roofed in,with flights of steps descending to the various levels.

Rochefort, a fine maritime city, had its port greatly improved in 1664, under the orders ofLouis XIV . The slips for ship-building were the finest in France ; they were connectedwith each other and had but one entrance, with a pair of lock gates to shut out the sea.The first entered was calculated from its depth to receive vessels of the first rate ; thesecond had its platform 7 feet higher, two vessels could enter, and the water at low tidebeing suffered to run out, the gates were closed, and the two vessels were in a situation tobe caulked at the same time. In the side walls were small aqueducts, by which the watercould be admitted, when it was required to float them again out to sea.

To use the upper slip for the construction of a vessel, without being deprived of thelower one, grooves were contrived in the portion where the separation took place, intowhich planks were dropped, which kept the water from entering, as the intervals betweenthe two ranges of planks were filled with clay in the manner of a cofferdam.

In the construction of this double slip, as great inconvenience was experienced from theworking of the springs, a well was sunk in the bottom of the upper slip, which, by meansof drains, collected all the water that would otherwise pass under and around the works;from the well it was pumped out by an hydraulic machine made for the express purpose.It is undoubtedly most important to keep slips perfectly free both from the action of thesprings, and from the water intended to pass through the locks, but this should be accom-plished without the aid of machines.

At low water they should be perfectly dry, to effect which it is necessary that thebottom should be laid a foot higher than the ordinary level of low water in the port towhich they belong. Vessels drawing from twenty to thirty feet of water when loadedrequire less when their freights are discharged, and the height to which the tide rises mustdetermine the level to be given to the slip.

To render the entrance gates at Marseilles water-tight, they are stopped at all thecrevices and joints by caulking, and the water is thus effectually shut out. When thewooden platform is laid too low, it is liable to be covered with mud, and to prevent theopening of the gates ; this was guarded against in the present instance. The length andwidth were also made proportionate to the vessels they were intended to receive.

In the slips at Rochefort the entrance lock was first formed with sluices at the bottom,which allowed the water from the springs to drain off.

To form the foundations the same precautions were adopted for laying the timberplatform as for that of a lock of a canal; planking and piling were made use of throughout,and the masonry fixed with great care after the heads of the piles were cut off. When allthe intervals between the heads of the piles were filled in, a mass of brickwork or masonry3 feet in thickness was laid in hydraulic mortar or cement over the whole area; thencross timbers were so placed upon it, that they formed sleepers to an inclined plane, madeby the longitudinal timbers which rest upon them. All the intervals were again filled upwith masonry, and an inclined floor was then laid, having a fall of 8 or 9 inches fromthe upper end.

The piling was carefully executed, and every precaution taken to render the platformfirm and secure.

Toulon , which takes its name from Telo Martius, is a very considerable port, on theshores of the Mediterranean, and distant from Paris 220 leagues. The arsenal andmagazines are the finest in France , and there is every requisite for a maritime establish-ment ; a quay wall and slip were built in deep water by means of a caissoon, which Belidor thus describes : “ The quay being too narrow, the walls were advanced 40 feet or moreinto the sea, where it had 20 feet depth of water. The lines being set out, two machineswere employed to raise the mud between them until a good bottom was obtained, whichwas perfectly levelled; the foundations were then formed, 18 inches in width, of a bed ofgravel, and chippings of stone, spread level with great care.”

Caissoons, or cases of timber, upwards of 60 feet in length, 13 in width, and 25 in depth,well caulked and pitched, were made use of in deep water ; these were afterwards takento pieces, and all except the bottom again served for other caissoons to lengthen out thework ; about 200 feet in length being operated upon at one time.

After these caissoons had been put together on shore, they were launched, floated totheir right situation, and then maintained in an upright position by ropes which passed