THE ARCH. 



127 



presentljr. But it is neither scientific nor safe to trust to it, 

 particularly in arches of wide span, in which the thrust iS 

 great and in direct proportion to the radius of curvature. 

 At the same time, when an arch is truly built and with 

 sufficient abutments, the flatter the arch is, the stronger it 

 will be and the less likely to be disturbed, so long as the 

 thrust is well within the strength of the materials. 



A "skew" arch has always to depend largely upon 

 friction ; and it is doubtful whether there is any advantage 

 obtained in building thorn in what are called "spiral" 

 courses. Nature does not altogether accept the " skew," 

 but the thrust always comes heaviest upon the obtuse 

 corners, as is proved by the constant tendency of the acute, 

 long corners, to droop ; and if two skew arches spring from 

 a centre pier, they will have a tendency to twist the pier 

 round nearer to the square. 



The finest proof of the power of fridiou that I have met 

 with, is shown by a small drawing recently made of a small 

 bridge over the Stroudwater Canal, built about the begin- 

 ning of this century. At the time, in 1844, when I was 

 looking after the construction of the Stroud Valley Railway, 

 which passes near it, I used often to stop and gaze with 

 wonder upon this arch— wonder that it should still be 

 standing. The arch was, when built, of 24 feet span and 

 4^ feet rise (the dotted black line shows the original 

 form) with an arch-thickness of 9 inches ; but, note here, 

 it was built with headers; forming a vortical bond, and that 

 chiefly gave it the strength to be still standing. It was 

 built under a steeply-sloping roadway, and under this load 

 the arch was very far from being in equilibration ; the 

 heavier hauncli shoved up the lighter, and the arch must, 

 by degrees, have assumed its present form, for it has not al- 

 tered during the last fifty years. On the drawing (page 126) 



