84 ANNUAL OF SCIENTIFIC DISCOVERY. 



ment. Tho arch was left to set 4 months ; the centring- was then 

 eased by allowing the sand to flow rep^ularly from the cylinders. 

 When the arch and centring were separate, it was found that the 

 crown had come down .6 of an inch, and the joints of the skew- 

 back had opened on the built abutment side .007 of an inch ; after 

 3 days the arch had come down .07 of an inch more. It was then 

 loaded with a weight of 360 tons, disposed over the whole surface 

 of the roadway, the loading occupying 13 daj's; when complete 

 the crown was found to have come down .3 of an inch. Since 

 then nothing has stirred. The arch was afterward tested by a 

 weight of 5 tons being allowed to fall on the roadvvay vertically 

 over the keystone, from a height of 1^ feet; but no joint opened, 

 nor did the ridge sustain the slightest injur}^ This proves that 

 the relative pi'oportions of rise to span in large masonry arches to 

 which engineers have hitherto limited themselves may be largely 

 modified ; though such structures as the above will require very 

 accurate work and the greatest care in execution. — Civ. Eng. and 

 Arch. Jour.y Jan.., 1868. 



NEW METHOD OF STRIKING CENTRING. 



A method frequently adopted by French engineers* for easing 

 large centrings from arches on completion of works, is by resting 

 the principals of the centring upon sand contained in iron cylin- 

 ders, from the bottom of which the sand is allowed slowly to 

 escape. Each principal is supported upon round props, fitting as 

 a piston into a cylinder containing fine, dry sand. The cylinders 

 are of sheet iron one-thirty-second of an inch thick, 1 foot high, and 

 1 foot in diameter ; about 2 inches from the bottom they are pierced 

 with holes about three-fourths of an inch in diameter, which are 

 stopped with common corks. To ease the centring the corks are 

 removed ; the sand then escapes through the holes, until a cone of 

 sand is formed at the base of the cylinder; the formation of this 

 cone arrests the further escape of the sand, and therel'ore the de- 

 scent of the piston, until the cone is swept away, when it re-forms. 

 The sweeping is repeated until the piston has descended sufficiently 

 to detach the centring from the masonry. By taking care to sweep 

 away the same cones simultaneously, the lowering of the centring 

 can be performed with perfect evenness, and as gradually as may 

 be desired, by one-millionth of an inch, if necessary ; whilst, as no 

 force is required to be used, the arch is not subjected to the slight- 

 est shock during the operation. This system was originated by 

 M. Beaudemoulin. — Civ. Eng. and Arch. Jour., Jan., 1868. 



PRESSURE OF THE WIND. 



In the case of roofs and similar structures a pressure of from 28 

 to 30 pounds per square foot is generally considered b\' architects 

 an ample margin to allow for, and in exposed situations, as in 

 bridges, 40 to 50 pounds. Many of the earlier suspension bridges 



