MECHANICS AND USEFUL ARTS. 33 



structure. A point -which bears somewhat favorably upon the 

 long-span system is, that the cost of sinking a pier, or even a 

 number of piles, is always an unknown quantity, whereas the 

 erection of the superstructure can be estimated to a few pounds. 

 It is rarely the latter item that delays the progress of a structure, 

 but the former is a common source of vexation and expense, 

 whenever the foundations have to be got in under water. The 

 argument that t^lls most strongly against the long or single span 

 princii^le is, that the cost of constructing a girder increases in a 

 far higher ratio than that of the mere augmentation of its span. 

 With piers it is different; for, ceteris paribus, the cost of any 

 number of piers in the same situation will, upon the average, be 

 in simple arithmetical progression. 



Across an impassable gorge, a precipitous ravine, or a furious 

 torrent, it may be impossible to emplo}'' any other than a bridge 

 of single span, and the importance attached to the connection of 

 the opposite sides must determine whether the necessary outlay is 

 justitiable or otherwise. For the future it may be asserted, judg- 

 ing from the data, that short spans will be the rule, long ones the 

 exception. — Mechanics'^ Magazine. 



FLAT ARCHES IN ENGINEERING. 



At the Paris Exposition was exhibited the model of a novel 

 masonry arch, designed and built by M. Vaudray, preliminary 

 to the construction of a bridge over the Seine, in Paris. The 

 bridge had to span the river exactly over the locks of the Canal 

 de la Monnaie ; and the necessity, on the one hand, of l^eeping 

 the springing of the arch above the lock walls, and, on the other, 

 of keeping the level of the roadwa}'^ down to the existing level of 

 the streets leading to it, confined the rise of the arch to a height 

 not exceeding 7 feet, while the span had to be nearly 125 feet. The 

 possibility of building so flat an arch with this span of stone 

 being questioned, an experimental one fulfilling the required con- 

 ditions was built. 



The description and general dimensions of the arch are as fol- 

 lows : its form is a segment of a circle, of which the chord is 

 124 feet, the versed sine 6 feet 11 inches. It is built entirely of 

 cut stone; the number of the voussoirs in each ring is 77, diminish- 

 ing in depth from 3 feet 7 inches at the springing, to 2 feet 8 inches 

 at the keystone ; the beds and joints of the voussoirs arc dressed 

 with the greatest care, and laid in Portland cement; the thickness 

 allowed to the mortar joints was three-eighths of an inch. The joints 

 next the skewback were not flushed until after the completion of the 

 ring, having been meantime kept open with fir wedges. The artifi- 

 cial abutment is 27 feet in height, 49 feet in mean thickness, and 12 

 feet wide, — this also being the width of the'arch, — built of rubble 

 masonr}', well bonded together and laid in Portland cement, — 1 

 part cement to 3 parts of sand. The arrangement for striking the 

 centring was by means of dry sand contained in iron cylinders, 

 — a method peculiarly well adapted for such a critical experi- 



