APPENDIX.] NOTE TO CHAP. I. 363 



string, we find that they are very much less in every piece. In 

 fact, there is a total gain of over 10 per cent., and that, too, not- 

 withstanding that the rise of each arc is only Jialf that in the 

 first case. Had we taken a double depth, the saving would 

 have been very great, and as in this case also, for a long span 

 and relatively large dead load, the diagonals would always be in 

 tension, the increased length of these last would be no dis- 

 advantage. 



12. The above construction is worthy of the careful consid- 

 eration of the bridge builder. It peculiarly recommends itself 

 for long spans, and has several important advantages possessed 

 by no other form of truss. For long spans the strains in the 

 flanges are nearly uniform. The diagonals are less strained 

 than in any other system, and are always in tension. Every 

 member acts to support, as well as to strengthen. The height 

 is everywhere proportional to the maximum moment of the ex- 

 terior forces. The load is distributed along the neutral axi$ 

 thus securing the maximum of rigidity ; while the neutral axis 

 itself passes through the points of support. 



This construction is known in Germany, from the name of its 

 inventor, as Paulas Truss. Upon this system are the double 

 track bridge over the Isar at Grossheselohe, 2 spans of 170.6 

 ft. ; a large number of smaller bridges, such as one over the 

 Rodach, 109 ft. span ; over the Main in Schweinfurt, 116.4ft. 

 span ; and especially one over the Rhine at Mayence, of 32 

 spans, 4= of 345 ft., 6 of 116 ft, 20 of 50 ft., and 2 of 82 ft. ; all 

 upon the same system. 



In England, we might notice the famous bridge over the 

 Tamar at Saltash, near Plymouth, whose two principal spans 

 are 455 ft., which is also constructed upon this system. 

 Finally, we may mention the bridge over the Elbe, near 

 Hamburg, the three principal spans of which are 325 ft. each. 



In this latter structure both the upper and lower members 

 are braced or ribbed arches, of a constant depth of about 10 ft., 

 a combination which, for long spans, seems most excellent. A 

 single arch alone, similar, for example, to the steel arch over the 

 Mississippi, by Copt. Eads, would have required heavy abut- 

 ments. 



The same arch inverted would have required equally heavy 

 anchorages. The combination does away with both. The 



