209 

 LARY BRIDGE. 



Concluded from Page 166. 



THE next operations were of more difficulty than 

 any of the preceding, and required considerable care 

 and accuracy in the execution. I allude to the level- 

 ling of the pile-heads, which were to be cut off at least 

 nine feet below low water of spring tides ; and the 

 paving of the spaces between them with masonry. 



From the nature of the bed of the river, it will be 

 evident, that to exhaust the foundations of water would 

 have been impossible ; since, however secure the sides 

 of the dam might have been, the surrounding water 

 would have forced itself below the dam, and made its 

 appearance through the bottom. * 



A saw of a similar description to those used by 

 Mr. Milne in the construction of Blackfriars bridge, 

 would have been sufficient to cut of the pile-heads : 

 but to pave the spaces between them, it was necessary 

 that the work should be laid open to the hands and eyes 

 of the labourer. It was therefore proposed, that both 

 operations should be performed by the assistance of a 

 Diving Bell ; and economy being important, it occurred 

 to me, that instead of a cast-iron bell, which, with its 

 apparatus, would cost from 700. to 800. one of wood 

 might be made for one-fifth of the amount. The ex- 

 periment was accordingly tried, and the result more 

 than answered our expectations. I am induced to give 

 a description of this bell, as its economy may render it 

 useful to others. Our first care was to procure well 

 seasoned elm wood, in boards of 1J inch thick, and 

 these were put together in two thicknesses, crossing- 

 each other and treenailed. The joints were dowelled 

 and tongued ; and, to render them air tight, flannel, 



* So exceedingly loose is the bed of the river, that I feel convin- 

 ced had it been possible to exhaust the water from the foundations 

 instantly, the whole of the inclosed space would have been blown 

 up, notwithstanding the increased density of the ground occasioned 

 ly the piling. 



VOL. i.. 1833. 2 c 



