1398

THEORY AND PRACTICE OF ENGINEERING.

Book II.

When the plane of the joint becomes so much inclined that a vertical line passingthrough the centre of gravity of the voussoir does not fall within the lower bed of thestone, the whole weight of the voussoir may be said to rest upon the centre. If it berequired to determine the weight of any voussoir, we must take from the table thedecimals opposite the degrees at which it is laid ; for example, to obtain the pressureupon the centre of the twenty degrees which commence at the joint: making an angle of32° with the horizon, or from 32° to 52°, we must add them together, and multiply thissum by the weight of that portion of the voussoirs comprehended between 2°, and the pro-duct will be the pressure of 20° of the arch upon the centre. Supposing the frames of thecentre to be 5 feet apart from the middle of each, the depth of the voussoirs 4 feet, and thespace comprehended between 2° of the arch, measured at the middle of the depth of thestone, 1 *5 feet, the solid content of which is 30 cube feet, the weight of 1 foot being 150pounds ; the weight of 2° will be 30 x 150 = 4500 pounds : adding the decimals togetherfrom 32° to 52°, the sum is 2-26, which multiplied by 4500, gives 10*170 pounds.for thepressure of 20 degrees.

The two middle arches of the Bridge of Cher , near Tours , one 62 feet, the other 59 feet,has centres formed upon this construction.

>1 I

Fig. 2326.

BRIDGE OF CHER,

At the Bridge of the Assise , near Tours, where the arches had 64 feet span, and the thicknessof the arch was 3 feet 2 inches, this sort of centre was also used: the stone employed wasof the ordinary weight, and the principals which formed the centering were placed 6 feetapart.

Fig. 2327.

BRIDGE OF L'ASSISE, NEAR TOCRR.