APPLICATIONS OP TIMBER. 197 



The driving of the piles is in some degree a simple 

 matter ; though there be a good deal of ingenuity in 

 the engines by which they are driven, and some 

 judgment 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 afford scope 

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

 haps among the happiest applications of timber; as 

 without them no stone bridge could be constructed. 



Coffer-dams are certainly the best means of get- 

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

 Two circles of piles are driven into the bed of the 

 river, round the part intended for the foundation of 

 the pier, and the external surfaces being planked, the 

 space between the two circles is filled up with clay. 

 The whole is made water-tight; and the inner water 

 being removed, the foundation is laid as for any other 

 building. 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 



VOT.. II. 17* 



