]818.] across the Mersey, at Runcorn. 23 



It is the characteristic of this design so to subdivide every 

 strain and weight, as to render them easily subordinate to human 

 power, whether in erecting, adjustment, repair, or renewal. 



II. As to the Stability of the Fnlcriuns. 

 The lofty and seemingly lightly constructed fulcrums, as seen 

 in the elevation, appear to be the most vulnerable parts of this 

 design ; but on turning the eye to the vertical profile of these 

 fulcrums at A, Fig. 2, and its end profile, Fig. '.}, it is observed 

 to be in effect an immense wedge, with its head, or greater end, 

 inserted in the bed of the river, and having the rods distributed 

 over its apex or edge. 



The use of these fulcrums is evidently twofold : to support the 

 mere weight and gravity of the road and rods, and to sustain 

 whatever strains they may be subjected to, by the vibratory mo- 

 tion of the road, when very heavy carriages are passing oyer it. 



In regard to its fitness for supporting the mere weight ot 

 gravity : half the weight of the whole road and rods, supposing 

 the former loaded to its utmost limit, will be only 1500 tons, to 

 be supported by eight columns whose smallest diameters are 18 

 inches, and thickness of metal one inch (less one half: see intro- 

 ductory remarks to the estimate), the area of the smallest 

 section of each column will be 27 inches. Now taking the 

 cohesion of cast-iron at 50,000 lb. per inch,* and doubling this 

 for its resistance to compression, t we have a power equal to 

 the support of 2200 tons in each column, which, however, is 

 only required to carry the eighth part of 1500 tons, i. e. some- ' 

 thing under 200 tons. 



But a column of 180 feet in height, although its base be threa 

 feet diameter, and it be stayed to three-fourths of its height, 

 may possibly be supposed liable to bulge or bend in the middle 

 with a weight of 200 tons. There have been no experiments 

 made on hollow cast-iron cylinders that I know of, from which 

 to calculate the exact weight requisite for this purpose ; but I 

 may venture to assert, that in this design the braces, struts, and 

 tie plates, are so arranged, witli a view to counteracting any 

 tendency of this sort, that the feeling of every practical mechanic 

 will instantly tell him it can never possibly occur under so com- 

 paratively trifling a weight as 200 tons, on a column capable of 

 sustaining 2000, and fixed in a range so braced and stayed as to 

 be in effect a solid cast-iron wedge. 



3. The strain from the vibratory motion of the road, when 

 under the weight and impulse of heavy carriages, does not easily 

 admit of calculation ; but an attempt shall be made to show that 

 for this strain also there is more than sufficient strength. 



As the road does not rest on, or touch, any part of the fulcrums, 



• Dr. Young'> Led. XIII.; and Rees's Cyclopedia, Art. Str. of Mat. 

 + Robison. in third edit, of Lncvrl. Brit. Art, Str. of Mat. 



