APPLICATIONS OF TIMBER. Wf 



■ The drivinpf of the piles is in some degree a simple 

 matter ; th()Uij;h there be a cjood deal ol" ing-eiiuity in 

 the enfriiies by which they are driven, and some 

 judg-ment necessary in order to ascertain that the 

 piling- is equally firm throughout the whole founda- 

 tion : — because if one pier, or portion of a pier, were 

 to settle down much more than the others, the beauty 

 of the superstructure would be spoiled, and its sta- 

 bility would be endangered. The other uses of tim- 

 ber in the construction of stone bridges atford scope 

 for the display of much more ingenuity, and are per- 

 haps among the happiest applications of timber ; as 

 without theui no stone bridge could be constructed. 



Cotfer-dams are certainly the best means of get- 

 ting a foundation for the piers of bridges in a river. 

 Sometimes a different mode is resorted to : the pier 

 is built to a certain height, — the surface of the water 

 in a running river, and the low water mark, at least 

 in a tideway, — in a caisson, or case composed of a 

 strong bottom and sides of timber. These are made 

 water-tight, sunk at first partly by water, and partly 

 by a portion of the materials ; and by pumping out 

 the water as high as the surface of the work, that 

 work is continued until it be brought above the sur- 

 face ; then the sides of the caisson are removed 

 while the bottom or platform remains, and becomes 

 the foundation of the pier. In order that building 

 in a caisson may be secure, two circumstances are 

 essential : first, that the platform shall be constructed 

 of the kind and quality of timber least liable to decay 

 in water ; and, secondly, that the bottom on which it 

 is laid shall be of a firm consistency, levelled through- 

 out, and not liable to be washed or scoured away by 

 currents. 



Westminster Bridge is an example of founding in 

 caissons; and some of the piers of it are already un- 

 dermined by scouring, — a danger which will increase 



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