198 VEGETABLE SUBSTANCES. 



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 

 when the removal of old London Bridge admits of 

 a more rapid current, and consequently a greater 

 scouring in the river. But though the foundations 

 of Westminster Bridge are thus scarcely deep and 

 secure enough, there is much excellence, as well as 

 ingenuity, in the structure; and it is to be regretted 

 that Mr. Labelye, the very able engineer of it, did not 

 publish his plans, as the science of bridge-building 

 might have derived much advantage from a complete 

 knowledge of the principles upon which he worked. 



Blackfriars Bridge is another instance of building 

 in caissons; only Mr. Mylne, the engineer of that 

 work, took the precaution of piling round the foun- 

 dations. 



Old London Bridge is under still greater obliga- 

 tions to timber than any of those that have been 

 mentioned; being founded wholly on piles, the tops 

 of which are as high as the low water mark, and 

 thus neither caissons nor coffer-dams were necessary. 

 Those piles are of various timbers, but chiefly of oak 

 and elm, oak being most abundant in the more 

 ancient part of the structure. They have stood 

 uncommonly well; for though the sap wood had de- 

 cayed, those which were recently taken up, even in 

 the most ancient parts of the foundations, which had 

 probably remained in water for about six hundred 

 years, were undecayed in the central parts. 



Foundations in coffer-dams are accounted the most 

 secure, and therefore they are now most generally 

 adopted in large works. From the depth of water, 

 the size of the piers, and the great weight they have 



