APPENDIX.] TIIE BRACED AROH. 407 



also we have end moments, and here also for long spans the 

 advantage over the simple girder is marked. 



In the hinged arch it is, indeed, perfectly true that, " when 

 one segment is loaded, any weight whatever in any other posi- 

 tion on the other segment will lessen the tension on the lower 

 arc of the loaded segment." In the arch without hinges the 

 case is altogether different, owing to the influence of the end 

 moments, which the paper referred to so persistently ignores. 



The two cases have, indeed, nothing whatever in common, 

 and from the strains in one no conclusion whatever can be 

 drawn as to what should be the strains in the other. With the 

 same propriety might one comparing the strains in the same 

 girder fixed at ends and free at ends, as given in Art. 17 of 

 this Appendix, infer that the strains in the .first were unduly 

 small. The only legitimate conclusion from such comparison 

 is the one there drawn, viz., that the one in which the strains 

 are least is the one most economical of material. In thia 

 respect, and in this only, the results of the paper are valuable, 

 and we are indebted to it for having saved us the labor of 

 making the comparison for ourselves. 



As a case in point bearing out our conclusion above, we may 

 instance the Coblentz bridge, which, as originally constructed, 

 was continuous at the crown, but pivoted at the ends of the 

 centre line, as in our example, Art. 20. But, unlike that 

 example, owing to the length of span being much greater, and 

 the rise and depth much less in proportion, it was found advan- 

 tageous to block up the ends after erection, and thus fix it at 

 the ends. 



If Mr. Shreve's deductions, as given in his paper, are to be 

 believed, this was a very dangerous thing to do ; but, as ex- 

 perience has proved, greater rigidity has thereby been secured, 

 and no evil effects have as yet been perceptible. It is, how- 

 ever, quite possible that before thus blocking the ends, the 

 effect of the end moments thus brought into play was duly con- 

 sidered ; and in view of the result, it would appear as if they 

 really had some influence upon the character and distribution 

 of the strains. 



It would seem, therefore, that, for the present at least, the 

 " strengthening" of the arches of the St. Lonis bridge by hinging 

 them (!) at crown and ends may be safely postponed until it 

 can be satisfactorily shown in what manner, for rise of teni- 



