Chap. XXI IT.
CENTERING.
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minute to 330, which is considerably increased when the wind is north-west and there areheavy land Hoods. Two chains which pass through the bridge over two cast-iron wheelsare laid across the river, and fastened at the opposite banks ; they are of inch-iron,arid each link is made to a gauge to fit the wheels and prevent its slipping off: the chainssink to the bed of the river when the bridge is alongside either shore, and as it is put inmotion, they form arcs of a circle, the ends on shore, to which weights are attached, risingand falling from 5 to 8 feet in the shafts made to receive them.
Two steam-engines, each with a cylinder of 19 inches diameter, and 2 feet 6 inches stroke,working at a pressure of 3.J pounds per inch, and at an average speed of 35 strokes perminute, is the moving power ; these turn a shaft, at each end of which is a large cast-ironwheel, whereon the guide chains rest. To prevent the chains which cross the Ilamoazefrom becoming tight and impeding the navigation, their ends have heavy weights attachedto them, in shafts 25 feet deep and 16 feet square, at the head of each landing-place, theweights being of cast-iron, each loaded with 5 tons ; these enter the shafts over cast-ironsheaves, 2 feet in diameter, so that when there is a strain upon the chains the weightsrise and fall. The feeding water, which supplies the one boiler common to the twoengines, is obtained on the eastern shore, and the waste steam is made to pass into the tank,where it assists in raising the temperature of the water contained in it.
Centres —Th'e systems laid down for the construction of roofs, particularly those havinga convex form, as domes and cupolas, apply also to that of centres : a pair of principalsmay equally serve for the construction of an ordinary roof, as for the formation of a centre,care being taken to give it strength in the parts where the greatest weight is deposited ;this strength varies according to the nature of the material to be supported : brick, rubble,and freestone, differing in weight, require different degrees of support.
Timber Centres , formed of a series of principals or trusses, are always adopted when thearches are of freestone and strength is required ; in constructing the arches of a bridge ofany considerable diameter, where the form is that of a semicircle, or semi-ellipsis, after thevoussoirs are laid to a certain height, the timbers of the centre are compressed, occa-sioning the crown or upper portion, which is not loaded, to spring, producing a con-siderable change of form; this inconvenience is only to be obviated by loading the summitas the process of laying the voussoirs advances; but this precaution will not prevent thecentre from altering towards the haunches : whenever a tie-beam is omitted in that partwhere the greatest pressure occurs, compression and change of form are the consequence.Centres with tie-beams are always to be preferred to those formed by parts of polygonsinscribed within each other; when constructed without horizontal timbers, in the directionof the greatest compression, they must yield very considerably, occasioning great de-rangement and inconvenience : various principles have been acted upon in later times forsetting out centres, and for the disposal of the principal timbers.
M. Lorgna , of Verona , in a workentitled Saggi di Station e Mecajiieu.applicate alle Arti , gives the dia-gram here introduced : but the tim-bers C, D are too much inclined,as are those at E,G, to have therequisite effect; the pendant timbersA Z, 15 T, are not well placed, andthe small struts /*, f, are not calcu-lated to discharge the several weightsF, A,13,1, resting on the centre, winchare expressed by the balls a,6,c,d.
The art of disposing various lengths of timber, so as to form a perfect centre, capable,without undergoing any change in its form, of carrying the weights of the voussoir throughout,until the key-stone which locks the w'hole is placed, and to determine the scantling forthe various pieces composing it, requires greater knowledge than is usually possessedby the ordinary builder. Kondelet observes that mathematicians, who have writtenlargely on the subject, have not entered sufficiently into detail to make their labourspractically useful ; their theory is sd general that it cannot be applied to particular cases ;and when it is considered that their formula does not embrace every quality that thematerials may possess, or comprise the kind of workmanship adopted in putting the piecestogether, it is not surprising that Che results are so much removed from truth, and that thereshould be such material differences between those who practise the art of construction.
After examining with the greatest attention all that Pitot, Couplet. Freziers, andLorgna of Verona had written, Kondelet found that their theories simply proved, thatall the pieces of timber composing a centre ought to be so combined as to supportin the most advantageous manner possible all the stones or voussoirs, which were supposedto slide in their joints, more or less, according to their inclination : as the surfaces of stones
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