CHAP. XTV.] THE BRACED AECH. 273 



a positive result, giving tension, a negative, compression in the 

 outer flanges. 



166. Effects of Temperature. We are now able to solve 

 accurately and thoroughly any class of braced arch, both for 

 variable loading and changes of temperature, and here the fol- 

 lowing remarks upon the latter subject may not be without 

 interest. We quote from Culmann Die Graphische Statik, 

 p. 487 : 



" The question arises whether the fears which the additional 

 strains, due to variations of temperature, have given rise to, are 

 jWell founded. Before the construction of the Arcole Bridge 

 in Paris the Engineer Oudry made various experiments with 

 a rib of about the same span as the bridge itself, of which the 

 following seems decisive as regards the present question. By 

 driving in the wedges upon which the rib rested above and be- 

 low, he could raise and lower the crown much more than the 

 distance due to variation of temperature without diminishing 



its supporting capacity Oudry, having thus assured 



himself of the harmlessness of temperature variations, decided 

 upon broad and firm bearing surfaces. 



" Interesting observations have also been made upon the 

 changes of form of the cast-iron arch of 60 metres span over 

 the Rhone at Tarascon, published in the Annales des Fonts et 

 Chemins, 1854, from which, however, it only appeared that the 

 changes of form followed slowly the temperature; that they 

 were less than the received coefficients would have led us to 

 expect, and were nowhere found to be prejudicial. 



" Since, then, this question appears to have been settled more 

 than ten years ago, may we not fear that those who still wish to 

 pivot iron may some day seize upon the idea of pivoting stone 

 arches also ! 



" Stone, as is well known, expands not much less than iron 

 for equal changes of temperature, and, moreover, its modulus of 

 elasticity is much less. The expanded stone arch cannot accom- 

 modate itself to the given span, therefore, as easily as the iron 

 arch, and it would then be clearly more advantageous to pivot 

 the stone arch ! As, however, such a clumsy contrivance would 

 give no great impression of stability, we feel justified in recom- 

 mending a- broad and solid bearing surface for all arches." 



As the opinion of an eminent engineer, the above may not be 

 without interest. We would only add that, according to the 

 18 



