16 Report of a Plan for building 



carry 10| tons, and about one half of that weight before it be- 

 gins to stretch. For the Menai bridge, I have taken a section 

 of 192 square inches, which at 5| tons to each square inch, will 

 support 1,008 tons, being a surplus of 666 tons above the real 

 weight of the bridge, and there would be required a further weight 

 of 1,008 tons to break down the bridge: this I conceive is ma- 

 king ample provision against any probable trial to which such 

 a bridge can be exposed. From the elevation it vvill be seen, 

 that the cables attain their curvature by passing over cast-iron 

 frames, part of which are of a pyramidal form, and the other 

 parts are connected with the top of the masonry; from thence it 

 vvill be seen, by dotted lines, that these cables pass down the 

 masonry to another cast-iron frame, laid horizontally along the 

 top of the arches, and connected with their springers by means 

 of perpendicular rods, thereby embracing the whole mass of ma- 

 sonry and spandrels, making in all about 12,000 tons at each 

 end of the bridge, and this exclusive of the great pvramids. As 

 the weight of the bridge between the two points of suspension, 

 including the cables, is 489 tons, there is not much reason to 

 expect undulation from any weight which will be laid on any par- 

 ticular part ; but to guard against any effect of that sort, 1 pro- 

 pose making the four sides of the roadways of framed iron-work 

 firmly bound together for seven feet in height, and similar work 

 for five feet in depth below the cables, which when they meet 

 towards the middle of the bridge will constitute a frame-work of 

 twelve feet deep. 



" With a bridge 30 feet in breadth, and 532 feet in length, 

 there is not much to be apprehended from side vibration ; but 

 in order to provide against this operation, I have in the plan 

 placed two horizontal cables, crossing the bridge diagonally: each 

 laying hold of the middle of its length, and passing round a cast 

 iron projecting frame, at the opposite sides of the great pyramids, 

 is from thence carried to the masonry of the abutments ; thus 

 creating a diagonal stay upon 70 feet in breadth. 



" When it is considered that from four to five tons are re- 

 quired to crush a cube of one quarter of an inch of good cast- 

 iron, there can be no doubt of the sufficiency of the cast-iron 

 frames over which the cables will pass. 



"These cables are continued to the cast-iron frames which con- 

 nect the masonry of the abutments. The weight of the bridge 

 is 489 tons, upon which if 300 tons additional are placed, they 

 make 789 tons. The pull of this weight at the abutments, upon 

 a cunature of one-fifteenth, is found by mv experiments over a 

 pulley, with a perpendicular weight, equal to about two and a 

 half times the weight on the other side, or 1,972 tons. To 

 counteract this, the cables are, as has already been observed, 



continued 



