20 Design for a Bridge [Jan. 



into use, the load being regularly and equably placed on the 

 rods so that each bears no more than its due proportion. 



3. That less altitude is required for the fulcrums. 



If it be asked why this construction would not suit a span of 

 1000 feet, it is answered, that it would require the two fulcrums 

 to be nearly 130 feet above high water-mark ; and that as the 

 road must pass over the tops of these fulcrums, and should not 

 any where be steeper than 1 in 12, * it would require a mound 

 or inclined plane of 1000 feet on each shore ; thus doubling the 

 length of the bridge, as it would be either by Figs. 1 and 17. 

 Were it not for this objection, three openings, arranged in 

 the style of Fig. 17, may, without hesitation, be affirmed 

 preferable to Fig l.f 



Fi°\ 18, diagram to show the ratio of strain in the fulcrum, 

 according to the depth of the catenarian segment. Thus, when 

 it depends 50 feet from a chord line of 1000, the strain is as tin* 

 horizontal line at 50, or about 2^ times its weight on each 

 fulcrum ; where 100, as at the line 100 ; where 150, as at the 

 line 150, &c. 



Estimate. 



The dimensions and calculations on which the following 

 estimate is founded were taken from the sketch herewith sent, 

 which is on too small a scale to admit of perfect exactitude. All 

 fractional parts have, therefore, been omitted, though they are 

 80 amply allowed for in the totals, that the gross amount of 

 metal may be considered as exceeding, rather than falling short, 

 of what the castings would actually weigh. 



Cast-iron Fulcrums. 



It may be premised that the whole of these fulcrums, except- 

 ing the ridge piece, are proposed to be cast riddled, or covered 

 with circular holes or blanks, equal in contents to, from one-third 

 to one-half the solid contents of the figures. The advantage of 

 this mode is, that sufficient strength is obtained, with a saving 

 of from one-third to one-half of metal : 



Cast-iron fulcrums 808 tons, at 10/. per ton £ 8080 



Labour in fixing at 1000 per fulcrum 2000 



Suspending rods, 2080 cwt. at 50s 5100 



* Smeaton. 



\ A design on this principle would be suitable for crossing the Forth at the 

 Queen's Ferry ; and if any of your Scotch renders will favour me with a section 

 of that part of the river, I will send yon a design raisonee, and an approximation 

 to the cost of such a work. If an idea shall ever be entertained of constructing 

 a bridge across the Irish Channel, this design (Fig. 17), or something very near it, 

 will be resorted to. 



