214 RENNIE'S DOCKS AND HARBOURS. PART VII. 



of stout oak) to be fixed. It was in the course of execut- 

 ing the Hull Harbour works that Mr. Rennie invented 

 the dredging-machine, as it is now used, for the purpose 

 of clearing the basins of mud and silt. Various unsuc- 

 cessful attempts had before been made to contrive an 

 apparatus with this object. A series of rollers, armed 

 with spikes to rake up the deposit, followed by buckets 

 and spoons to lift it from the bottom, worked by means 

 of a walking wheel between two barges, was the most 

 common practice ; but it was clumsy, tedious, and 

 inefficient. Other machines for a similar purpose were 

 driven by tread-wheels. At length the idea was taken 

 up of fixing a series of buckets to an endless chain, 

 worked by horse power. Mr. Rennie carefully investi- 

 gated all that had previously been attempted in this 

 direction, and then proceeded to plan and construct a 

 complete dredging-machine, with improved cast iron 

 machinery, to which he yoked the power of the steam- 

 engine. He was thereby enabled to raise as much as 

 300 tons of mud and gravel in a day from a depth 

 of 22 feet; and the expedient proved completely suc- 

 cessful. The same kind of machine was extensively 

 used by Mr. Rennie in executing his various harbour 

 works. One of these, constructed for the excavation of 

 the Perry Dock at Blackwall in 1802, was even fur- 

 nished with a powerful apparatus for splintering rocks 

 and large stones which could not otherwise be removed ; 

 and it answered the purpose most effectually. In the 

 Clyde, the Thames, the Mersey, and the Witham, as 

 well as in various foreign rivers, the dredging-machine, 

 as contrived by Mr. Rennie, has been found invaluable ; 

 and there is scarcely a port or harbour in the United 

 Kingdom in which it has not been most beneficially 

 employed. 



In addition to these docks and harbours, Mr. Rennie 

 furnished the plans of the new quays and docks at Green- 

 ock on the Clyde in 1802 ; and those at Leith, the port 



