ON BRIDGES. 131' 



given height of roadway the clearest headway under all its parts, 

 although it is nowhere quite as high as the centre of the arch in the 

 other figures. 



In Figure 58 a small rise of the surface of the stream will much 

 diminish the water-way just at the time when it should be the largest, 

 a. case that will not occur in Figure 57. 



To avoid this the elliptical arch, Figure 59, is often used when the 

 structure must be arched, and it presents the other advantages of in- 

 creased headway and more agreeable lines to the eye. 



When the engineer has determined the upper and lower limits of 

 his intended work, (to do which he must carefully study the banks 

 and surrounding country and collect all the information possible as to 

 the freshets of the stream,) he must examine the ground with a view 

 to the foundations and approaches of his bridge. Should it have but 

 one span or bay, as in the figures, the abutments will be nearly, if not 

 quite all, on dry land, and the difficulties to be met with in their con- 

 struction will probably not be great. Should the width of the stream 

 however be so great as to require several spans, piers must be built 

 in the water, an operation requiring expensive and troublesome ap- 

 pliances. 



The piers of a bridge contract the water-way and cause eddies and 

 currents oftentimes very disturbing to the navigation, and the engi- 

 neer must endeavor to use as few as possible with a due regard to the 

 cost of the structure that he is designing. Modern practice has ex- 

 hibited much fewer and thinner piers than were used by the ancient 

 builders. With them the semicircular arch, with piers almost as 

 wide as the openings, contracting the water-way nearly one-half, was 

 frequently used, while now the spans are increased and consequently 

 the number of intermediate supports diminished, while the piers 

 themselves are made as thin as they can safely be, to support the su- 

 perincumbent weight of the structure. Where the current is swift 

 the piers should be in its direction as nearly as possible, since any 

 obliquity to the current will give rise to violent eddies, which serve 

 still further practically to contract the water-way, as in the case of 

 the Rock Island bridge, on the upper Mississippi, at which the navi- 

 gation, especially for rafts, is made exceedingly dangerous by such 

 an arrangement of the piers. 



Many experiments have been made upon the proper form to be 

 given to piers in order that they may obstruct the passage of the 

 water as little as possible, resulting in the fact that the same form 

 which enables a vessel to move easily through the water, permits the 

 water to flow past a fixed pier with the least commotion. 



When the number and size of the piers are decided upon, the bottom 

 of the river must be examined with a view to their foundations. 

 •Many ways of providing a firm base on which to rest the masonry 

 have been adopted, the simplest being to remove by a dredge the 

 soft mud which generally lies on the bottom and then to drop rough 

 stone on the proper point until the pile appears above the surface, 

 when the masons can commence their work. This method suffices 

 where labor is dear and stone plenty, and where the contraction of 



