1024

THEORY AND PRACTICE OF ENGINEERING.

Book II.

ornament the portico of the Pantheon, must have required vessels or rafts of considerablesize to have borne them across the Mediterranean : of their construction we have, however,but vague accounts handed down to us, but we have descriptions of many employed in thetransport of the marble from Greece to Rome , and in some instances we learn that theywere not calculated for their destined purpose.

De Cessart's Machine for throwing large stones into the sea consisted of a pontooncarrying an inclined plane made at an angle of 22° ; this deck had a level platform, 6 feetin width, to carry the stone, which was placed on three rollers 6 inches in diameter, and

db cessart’s machine.

Fig. 1563.

which were hooped with iron: a gib moving on a pivot was furnished with pulleys, andkept in its place by two iron rods : two wheels and axles w r orkcd the ropes, the wheels were12 feet, the axles 1 foot in diameter.

Iron chains first liftedthe stone, brought bylighters or other vessels ;the wheels were thenturned till the necessaryheight was obtained, whenthe hook was detached,and the wheels worked,till the gib was in theposition indicated by thedotted lines. It was thenslightly unrolled, and thestones were placed on thethree rollers; two menwith levers pushed themforwards to the inclinedplane, when they wereallowed to slide forwards Fig. 1564. de cessart’s machinb.

into the sea.

The boats used for conveying the stone to the Plymouth breakwater were so arrangedthat they could be easily unloaded by similar tackle, and their contents be thrown into theplaces marked out to receive them. 'Hie transverse and longitudinal section through oneof these boats exhibit the manner in which the stones were placed ; each mounted upon asmall carriage could be easily lowered by the inclined plane at the stern into the hold, outof which they could, by means of a windlass, be drawn up upon deck and lowered.

The arrangement of the ballast and stores of a ship, or its stowage, is of the greatestimportance, as we know that the sailing of a vessel depends on the situation of thecentre of gravity, which is determined by the disposition of the movable weights on board;to this subject Daniel Bernouilli, Euler, Bossuet , and others, directed their studies; theyhave shown us the manner in which such weights affect the position of its centreof gravity, the stability of which is increased or diminished according as it is loweror higher in the ship ; to place the weight, then, as low as possible is necessary ; when the

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