304 NOTE TO CHAP. I. [APPENDIX 



thrust of the upright arch is opposed by the pull of the inverted 

 one, all the advantages of Pauli's system are obtained, and 

 ' are no temperature strains such as occur in the single 

 airh, while the bracing is reduced to a minimum. At the same 

 time all the rigidity due to the arch is obtained.* 



13. In the construction of the diagrams, care should be exer- 

 cised in the selection of the scales, that the frame diagram may 

 be large enough to secure the desired accuracy. Lines should 

 bo drawn very fine with a hard, sharp-pointed pencil, so as to be 

 scarcely discernable, and their intersections accurately marked 

 by needle point. 



With an accurate scale and good instruments, strains can be 

 taken off in nearly every practical case to hundredths of a ton 



* Compare Long and Short Span Railway Bridges, by John A. Roebling, C.E. 

 In this work, Mr. Boebling proposes a system in principle essentially the 

 lame as the above, to which he gives the name of "Parabolic Truss." He, 

 however, constructs the arch of channel irons bolted to the sides of a straight 

 truss, the sole office of which is to give rigidity to the system. Also, claiming 

 that iron in the shape of wire will safely sustain three times as much as in the 

 shape of bars or rods, he introduces a wire cable in place of the inverted 

 braced arch. 



It will thus be seen that for rigidity the system is wholly dependent upon 

 extraneous members, such as the auxiliary truss and the tower stays, which are 

 liberally introduced. By dividing the material composing the upright arch 

 into two portions, bracing between them, and thus forming a braced arch sim- 

 ilar to Capt. Eads, the stays and stiffening truss might be entirely dispensed 

 with, the construction greatly simplified in the number of its members, and 

 the bracing reduced to a minimum. If, also, as claimed by Capt. Eads, the 

 conditions for cast sted are just the reverse of iron, and it is most advantageous 

 to use it in compression, then it seems that such a modification of Mr. Roeb- 

 ling' s design with wire cable and a cast-steel braced arch would better sustain 

 the thesis with which his work, above quoted, opens, viz. : that u the greatest 

 economy in bridging is only to be obtained by a judicious application of the Para~ 

 bolic Truss." 



Bach a combination of the suspension and upright arch would seem to avoid 

 the principal objections urged against each separately. The anchorages and 

 abutments are dispensed with, the greatest rigidity is secured with the mini- 

 mum of bracing, and the material is used in the most advantageous way. In 

 addition to the advantages of Pauli's system being secured, we have the ease 

 of erection of the suspension system combined with the rigidity of the arch. 

 The system is self -balancing, and practically unaffected by changes of temper- 

 ature. 



For the practical details of construction of such a system, the reader can 

 with profit consult Mr. Roebling's work, above quoted. They will be found 

 to be neither expensive nor difficult of execution. 



