12§ 



THE ARCH. 



may be observed a dotted line showing the natural line of 

 thrust, under the inclined roadway, which passes most 

 nearly to the present arch— it will be seen that this line 

 of thrust passes oompletely and largely outside the arch in 

 two places ; it is certain that the line of thrust must follow 

 some such curve, and therefore that it goes for a long 

 distance through the earth filling on the top of the brick 

 arch, and that this earth has been jammed together so 

 tightly that it forms that portion of the arch of itself ; that 

 it then passes into the brickwork of the arch again, and 

 then again that it passes out of the brickwork onco more 

 into the earth backing at the point of greatest thrust- 

 namely, the lower springing, the earth backing forming 

 that abutment of the bridge. It may bo added, that where 

 the old arch at one part had become nearly a straight line, 

 an iron plate has been added underneath, to keep the bricks 

 from tumbling out, proving that there was no thrust in that 

 part of the arch itself. The Canal Company is very poor, or 

 they would probably have rebuilt the arch ; but it is worth 

 preserving as a great curiosity. 



Under a sloping roadway an arch ought to be built with a 

 corresponding variation of the curvature, as shown in the 

 drawing of a bridge (page 12G) which I designed and built 

 at that time over the railway a few miles higher up the 

 valley. The roadline there is steeper than 1 in 3, and a 

 square arch would certainly have fallen. The span is 50 

 feet, and the crown thickaess 4 feet; but one abutment is 

 12 foot higher tlian the other. The bridge is built of Jiisley 

 cammon stone, and is in the truly equilibrated form. 



It may have been observed how very unsightly a square 

 arch always looks under a sloping roadway. 



A word more about friction. In Molesworth's well-known 

 Engineers' Fochet-hoolt, rules are given for determining the 



