TRANSACTIONS OF THE SECTIONS. 99 



stated that government documents, even when trustworthy, presented difficulties in 

 practical use, from their being constructed on various bases, published in different 

 forms, and calculated on systems which did not admit of immediate comparison. 

 He offered, on the part of the new Commission, to send copies of their publications 

 to the statistical societies of Great Britain, from whom he requested communications 

 in ret\irn. 



MECHANICS. 



On the Plymouth Breakwater. By Wm. Stuart, C. E., Superintendent of 



the Work. 

 The importance of a breakwater at Plymouth attracted the attention of the Ad- 

 miralty in 1806, and in February of that year Mr. Rennie and Mr. Whidbey, the 

 Master Attendant of Woolwich Dockyard, were directed to survey the Sound. As 

 the results of their survey, they submitted a plan for a stone breakwater, and gave 

 their opinions upon several plans previously proposed. The stone breakwater was to 

 be 1700 yards in length, at the top of which the middle was to be straight for 1000 

 yards, and each end, 350 yards in length, was to incline at an angle of about 20° to 

 the straight part : it was to be ten yards in width at the level often feet above the low 

 water of an ordinary spring tide, with a slope of three to one on the south or sea side, 

 and one and a half to one on the north or land side ; and to be constructed by blocks 

 of limestone thrown promiscuously into the sea on the intended line, with a cut stone 

 pier on the top. This plan was favourably received, and an Order in Council was issued 

 in June 1811, for the execution of the work, and in August 1812 the first stone was 

 deposited. Mr. Stuart then described in detail the progress of the work, and the va- 

 rious alterations found advisable. The south slope is regularly formed with squared 

 blocks of limestone and dove-tailed granite, from the level of low water spring tides, 

 with a slope of five to one, and the north side with rough blocks of limestone, with 

 a slope of two to one. A lighthouse is now in course of construction at the western 

 end, and a buttress for the protection of the lighthouse, and securing the front 

 of the south slope. The force of the sea is so great, that stones of fifteen or 

 even twenty tons have been taken from low water and carried over the top of the 

 work. According to the original calculation of Messrs. Rennie and Whidbey, 

 2,000,000 tons would be required for th^ work, but owing to the various exten- 

 sions, the quantity is much increased , and between the 12th of August 1812, and 

 the 31st of July 1841, 3,377,068 tons had been deposited. The estimated cost of the 

 original breakwater was 1,013,900/.; but owing to the alterations in the work, and 

 an increase in the materials, the whole outlay to the present moment is 1,111,700/., 

 and the cost of the breakwater when completed, including the lighthouse, will not 

 exceed 1,300,000. Various other breakwaters have been proposed to the Admiralty : 

 one of cast iron in 1804 ; two of stone, and one of wood, by Mr. Bentham, in 1811. 

 The wooden breakwater was to consist of 117 floats of wood, of a triangular or 

 prismatic forai ; each float thirty feet in breadth and depth, forty feet in length, to be 

 moored by iron chains, at a cost of 201,805/. ; but the Admiralty resolved on a stone 

 breakwater, and the thirty years' experience since elapsed have confirmed the author 

 in his opinion of the wisdom of the choice. The stone breakwater is said to have 

 occasioned an accumulation of mud and silt within the harbour, and a consequent 

 diminution in depth of the water to the extent of five feet. In the original report of 

 Messrs. Rennie and Whidbey, is contained the following statement : — " From con- 

 versing with pilots and various other intelligent men whom we met at Plymouth, we 

 have reason to believe, that the depth of water in the Sound is on the decrease, by 

 the settlement of mud and silt brought down by the rivers from the interior country, 

 and also by the embankment of the mud lands within, thus diminishing the ancient 

 receptacles of the water of the tide, which both in its flux and reflux occasions a 

 powerful scour in its passage through the Sound." The fact is, that a recent inclosure 

 of 275 acres of the backwater of the Catwater above the Lara Bridge had just taken 

 place ; it seems evident that mud and silt were then in the Sound. On a considera- 

 tion of the whole question, Messrs. Rennie and Whidbev were of opinion that there 



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