ON BRIDGES. 133 



off to a level, heavy timbers are bolted to them to receive the plat- 

 form. 



Whenever there is any doubt of the foundations, piling will be 

 found to insure the greatest safety and permanence, although it is 

 frequently costly; it is only interfered with by natural springs in the 

 ground which loosen the material around the piles and cause them to 

 settle or to rise. Piles may alsp be used to advantage to prevent the 

 spreading out of a loose, yielding material, such as quicksand. For 

 this purpose piles sawed out square, so that they will fit tightly to- 

 gether, are driven around the material to be enclosed, touching each 

 other, and connected at the top by string-pieces; the soft material 

 thus confined will present a good bearing mass, after being covered 

 with a wooden platform. 



Solid rock is generally the most certain and satisfactory foundation, 

 but the labor of preparing its surface under water to receive the first 

 courses is considerable; hard, gravelly clay and cemented sands are 

 nearly as good; sand, which makes the best of foundations on the 

 land, must be narrowly watched under water, since it is easily moved 

 by currents by the obstruction in the channel, which may wash it 

 away and undermine the structure. 



The abutments or land supports of th.e bridge may, of course, be 

 founded in the same manner as the piers; but being mainly on dry 

 land the difficulties attending them will usually be less. 



Piers have been sometimes founded in a stream by building them 

 on the bottoms of flat scows or caissons and permitting them gradually 

 to sink, as the weight of masonry is increased, until the whole mass 

 rested upon the bottom, which has been dredged level to receive it; 

 the sides being detached, the bottom is left under the pier as a plat- 

 form. 



Cast-iron piles, terminating in an auger-shaped flange, which en- 

 ables them to be screwed into the soil, invented by an Englishman 

 named Mitchell, have been extensively used with good results, es- 

 pecially in the foundation of light-houses and beacons along the coral 

 reefs of the Florida coast. They are easily screwed down in places 

 where ordinary piles are difficult to manage, and their resistance to a 

 vertical pressure is enormous, owing to the surface exposed by the 

 wings of the screw. 



Within a few years since the general introduction of iron into en- 

 gineering practice, a system of foundations by means of cast or 

 wrought-iron cylinders sunk into the bottom of the river has come 

 into favor. These cylinders or pneumatic piles, as they are called, 

 were first proposed and used by M. Triger, an engineer of the north 

 of France, for sinking a shaft through a quicksand; and are known in 

 England as Pott's pneumatic pile, although he is really not the in- 

 ventor. The cylinders, from six to ten feet in diameter, prepared in 

 the proper lengths, are bolted together and sunk vertically until the 

 lower edge rests on the earth; the top is then closed by a plate, to 

 which is fitted an air-lock or chamber furnished with double doors, 

 like the vestibule of a house, so that a person can enter the lock and 



