26 Design for a Bridge -[Jan. 



balanced by what is on the other. Where the road has been 

 hoisted too high, it can always be lowered a little ; but at all 

 events the length of the suspending rods is supposed to be so 

 adjusted beforehand as that, when left to themselves, they will 

 retain the road in its proper place, without requiring any altera- 

 tion. 



The suspending arch of chains may now be removed, and the 

 remainder of the bridge filled in at pleasure. 



I make no mention of machines to be made use of for drawing 

 the rods into place, hoisting the plates, &c. nor of various other 

 details, perfectly known to practical men, and which always 

 occur to the mind with the occasion of using them. 



V. As to the comparative Merits of this Design with that of a 

 Road suspended from a catenarian Arch. 



1 . The first advantage of the design submitted is, that it requires 

 much less weight and strain in proportion to the weight of the 

 road to be supported. The weight of the centre part of the road, 

 1000 feet in length, is 900 tons. With the strain taken by 

 measurement of Fig. 11, it amounts only to 1320 tons, or a 

 strain of 660 tons on each fulcrum. 



Supposing the road weighing 900 tons, to be supported by a 

 catenarian arch, of which 13, 13, should be tangents ; that is, by 

 an arch or segment which should depend from the chord line 

 about 70 feet: then (by admeasurement of A, C, Fig. 11, A, 

 D, B, representing 900 tons,) the total strain on the fulcrums 

 would be 3960 tons, or 1980 tons on each fulcrum, instead of 

 660, according to mu design. 



The great difference in the amount of strain between these two 

 modes arises from this, that in the arch every separate strain is 

 communicated to the fulcrum, through the same oblique lever, or 

 curve of the arch, as is the centre strain ; whereas in the design 

 submitted every separate strain is communicated in direct lines 

 to the fulcrum, and the nearer the fulcrum such strains approach, 

 the more acute is the angle formed with the fulcrum by the line 

 through which their influence is communicated, and of course 

 that influence is proportionally diminished. 



Hence it appears that the strain on the rods of an inverted 

 arch, however much it may depend from the chord line, will 

 ever be much greater than by the construction submitted, and as 

 near as possible in the above ratio of one to three. 



For in Fig. 18, where the strain on an inverted arch is shown 

 at three different depths of segment, viz. at 50, 100, and 150, 

 from the chord line, ihe line under the figures shows the strain 

 by the construction submitted (Fig. 1), which in all the three 

 cases amounts to about one-third of the other. When it is con- 

 sidered that in an inverted arch this strain must be borne by 

 four, eight, or 16 rods ; while in the construction submitted it is 

 borne by 50, the advantage of the latter must be further obvious. 



