LARY BRIDGE. 165 



it involves, when the intrados are semi-circular or semi- 

 elliptical, renders it almost useless to the bridge builder. 

 But that an arch of equilibrium, with any description 

 of curve for the intrados of the arch, can be obtained, 

 is a fact, which I hope satisfactorily to prove in some 

 future communication ; it being inconsistent with the 

 present design to enter more fully into the discussion ; 

 I would however remark, that the arches of the Lary 

 Bridge are constructed on this principle ; and that the 

 piers are made sufficiently thick to include the line of 

 thrust, which has this advantage, that the destruction 

 of one or more of the arches would not affect the stabi- 

 lity of the remainder; a circumstance which in the con- 

 struction of bridges of several arches, claims particular 

 attention. 



The works commenced by inclosing the areas of the 

 intended foundations with sheeting piles of beech plank, 

 four inches thick, and of convenient width ; say from 

 nine inches to fourteen inches. Their edges being pre- 

 pared thus 



and being driven quite close, effectually inclosed the 

 areas of the foundations. These piles were driven to 

 a depth of fourteen feet, in guide-frames, (to keep them 

 perpendicular) by a pile engine, in which worked a cast 

 iron weight of 4501bs. The spaces thus inclosed were 

 excavated by means of what were termed ' sand spoons', 

 to a depth of from six to seven feet ; they were worked 

 by three men on stages, fixed about four feet above low 

 water, the excavated sand being discharged over the 

 sides of the piles. In appearance, these spoons re- 

 sembled the ballast-dredge used on the Thames. 



As the excavations were completed, the ground 

 was piled with whole Memel or Norway timbers, the 

 piles being arranged in rows, about four feet six inches 

 apart from centre to centre, in each direction. These 

 bearing piles, having to carry the whole weight, or 

 nearly so, of the superstructure, were driven, not as 



