114

THE PRINCIPLES OF BRIDGES.

TRACT I.

Salient Angle, of a pier, is the. projection of the endagainst the stream, to divide it. The right-lined angle bestdivides the stream, and the more acute the better for thatpurpose ; but the right angle is generally used, as makingthe best masonry. A semicircular end, though it does notdivide the stream so well, is sometimes better in large navi-gable rivers, as it carries the craft the better off, or bears theirshocks the better.

Siioot, of an'arch, is the same as drift, thrust, &c.

Span, of an arch, is the extent or width at the bottom, oron the level at its springing.

Spandrels, or Spandrils, are the spaces about the flanksor haunches of the arch, above the curve or intrados.

Springers, are the first or lowest stones of an arch, beingthose at its feet, bearing immediately on the impost.

Sterlings, or Jettees, a kind of case, made of stilts, &c,about a pier, to secure it. It is particularly described underthe next word Stilts.

Stilts, a set of piles driven into the space intended forthe pier, whose tops being sawed level off about low-watermark, the pier is then raised on them. This method wasformerly used, when the bottom of the river could not belaid dry ; and these stilts were surrounded, at a few feet dis-tance, by a row of piles and planks, &c, close to them like acoffer-dam, and called a sterling or jettee ; after which, loosestones, &c, are thrown or poured down into the space, till itbe filled up to the top, by that means forming a kind of pierof rubble or loose work, which is kept together by the sidesof the sterlings : this is then paved level at the top, and thearches turned upon it. This method was formerly much used,most of the large old bridges in England being constructedin that way; such as London-bridge, Newcastle-bridge, Ifo-chester-bridge, &c. But the inconveniencies attending it areso great, that it is now quite exploded and disused : for, be-cause of the loose composition of the piers, they must be madevery large or broad, otherwise the arch would push them over,and rush down as soon as the centre should be drawn: which