Mr Sang on the Conslruction of Oblique Arches. 311 



be the horizontal thrust of the bridge, and the other against 

 DE, equal to the weight of all the mason-M-ork between 

 the crown of the arch and the vertical line through B. Now, 

 since all the stones of the piers are squared, no change (ex- 

 cept by improper straining) can take place in the direc- 

 tions in which these pressures are propagated. The pres- 

 sure against CD is communicated along the abutment course 

 to the spring of the next arch, or to the corresponding breadth 

 of the final abutment ; while the pressure against CD is trans- 

 mitted through the facing stones of the pier to the outer row 

 of piles. It will, indeed, be said, that the cohesion of the mor- 

 tar, and the alternate jointing of the courses, render the pier 

 one mass, and that, therefore, such niceties are not worthy of 

 attention. But, indeed ! is the final disposal of the entire 

 strain of a bridge, such a trifle ? Then let us fit our arch-stones 

 by guess, and sweep the span in any fancy. It is at this very 

 corner that all the care of the engineer is required ; and I do 

 maintain, that the method in common use outrages the doc- 

 trines of equilibrivim, and renders om* arches less secure th?n 

 they ought to be. It is a piece of bad engineering to throw 

 the whole weight of a bridge upon one row of its supports, and 

 to give the others scai'cely any strain ; especially when it is 

 considered that that row is most liable to decay. The alter- 

 nate jointing of the stones calls into action that species of re- 

 sistance which ordinary building-material is least capable of 

 exhibiting : one end of a stone is pressed downwards, while 

 its other end is engaged between two blocks ; the consequence 

 Is a tendency to break the stone over, to distend its upper sur- 

 face ; and it is notorious that the strength of stones in this way 

 is much inferior to their power of resisting a simple crush. 

 The alternate jointing and the mortar are useful enough in cor- 

 recting the bad eftects of unavoidable inaccuracy : there is no 

 need for deliberate error to put them to a severer use. 



The best possible arrangement is to give to each square foot 

 of the foundation its fair .share of the whole burden. In order 

 to do this, it becomes neces.sary to lay a counter arch, of a 

 parabolic form (its convexity downwards), upon the pier-head. 

 Such an abutment course would carry the horizontal thrust to 

 the spring of the next arch, precisely as a flat course would ; 



