THE CIVIL ENGINEER AND AUCIIITJECT'S JOURNAL. 



1839.] ' 



the axis, the edges of which planes aie represented by the lines LH, 

 MK. 



Hence it will he seen, that to ascertain the position of any point H, 

 when the line DE is wrapped round the cylinder, we have only to 

 wrap round the line LH ; this may easily be done by drawings an end 

 view G, of the cylinder, and taking NP eijinl to LH, finding NH' the 

 length of the curve equal to NP,* and projecting the point H' to H", 

 we obtain the position that H will occupy upon the cylinder. In the 

 same manner all the points in the curve may be found. 



We now come to the practical application. 



165 



Fig. 2. — Let ABCn denote the outlines of the plan of the soffit of a 

 skew arch, and let EFG be drawn making the proper anglef with the 

 face. 



Then by the plan I have just described, the line EG may be wrapped 

 round the cylinder, and E' FG' the curve generated, will represent one 

 of the spiral courses. Now each of the courses of a skew arch would, 

 if produced, wrap itself round the cylinder, and present a curve similar 

 to E' FG', hence every one of the courses of the arch will be a portion 

 of this curve ; if, then, a mould be cut to the curve E' FG', it is evi- 

 dent that by setting on the proper distances, along the lines AG' E'C, 

 and applying the mould to the corresponding points, all the courses 

 may be drawn, as shown on the figure, with little trouble. 



I believe the common practice is to project each of these joints on 

 the soffit separately ; where the arch is brick, and each course shoftn, 

 tliis is a work of much laliour. 



The same principle, with a little modification, is applicable to the 

 other views of the arch, more particularly to the outline of the deve- 

 lopement of the soffit, only that in this case the operation is unrolling 

 instead of rolling the line. 



In case any may not understand the preceding explanation?, I 

 would recommend those who feel interested in the matter to try the 

 experiments with the paper and wooden roller, and ihey will quickly 

 perceive the priuci[)le. 



S. ^- G. Railtcay-office, Your's obediently, 



Worcester. H. Spencer. 



PLYMOUTH BREAKWATER. 



A communicntion frnm Sir John Rennie to T. L. Donaldson, Esq., 

 Hon. Sec., read befure the Institute uf British Architects, April 

 11th, 1839. 



Dkab Sir, — 1 have the greatest pleasure in complying with your 

 request, relative to the construction of the Breakwater in Plymouth 

 Sound, and I shall always be extremely happy to forward the views 

 of the the Royal Institute of British Architects in any way which 

 may be considered most conducive to the welfare of that valuable 

 tnciety, in which you take such a laudable interest, and only regret 

 that your application of yesterday had not been communicated to me 

 earlier, in order that I might have been enabled to render the following 

 account more complete, and more worthy of the society ; and as I have 

 written this chiefly from memory as far as the shortness of the time 

 would allow, I trust that every allowance will be made for any 

 omission, as I am about publishing a work upon Harbours, in which 

 the subject will be more fully explained hereafter: and I entirely 

 agree with you, that although strictly speaking, a work like the 

 Breakwater may not generally come within the range of civil 

 architecture as now practised ; nevertheless it is to be hoped that the 

 study of it may be found not altogether unprofitable ; the more so, as 

 Vitruvius, the earliest founder of the art, includes a knowledge of these 

 kind of works among the necessary acquirements of the archiiects. 

 I shall now, therefore, proceed to tlie description of the work in 

 question. 



* This may be done by calculation, but measurement by compasses is near enough 

 ior practical purposes. 



T This line wuu'rl be at ri^'ht angles to the face, on the surface halfway bet\reen 

 She soffit anrlcrowQ. (See Mr. Fox's I'amphlet on Skew Arches.) 



Plymouth Sound, from its extent, its depth of water, the numerous 

 creeks and rivers which discharge themselves intrf its bosom, and its 

 proximity to the Atlantic Ocean, had long pointed itself out as one 

 ol the principal stations for the British navy ; and at an early period 

 of our liislory, Devonport was selected as the chief port for fitting out 

 vessels of war for the western portion of the kingdom ; and although 

 on account of the exposed nature of Plymouth Sound, a certain degree 

 of inconvenience was more or less experienced during the infancy of 

 the British navy, nevertheless, it never became so apparent until the 

 tremendous conflict of the hist war, when it became absolutely neces- 

 sary for self defence to increase our fleet to that extent, that more 

 than ordinary accommodation became necessary ; it was then that the 

 danger and inconvenience of the Sound became most apparent, and 

 imperiously called for a remedy ; various ideas had been suggested, 

 and numerous plans proposed to obviate the existing evils, until at 

 last the Admiralty applied to tlie late Mr. Rennie, my fattier, who 

 proposed to erect a detached or iso'ated breakwater in the centre of 

 the Sound, which, after a great deal of discussion and opposition, 

 was finally approved of, adopted, and ordered to be carried into 

 effect ; and the first stone was accordingly deposited in the summer of 

 1812. The reasons for adopting the isolated or detached Break- 

 water were; first, because this part of the Sound was already obstructed, 

 and to a considerable extent rendered unnavigable by the Panther, 

 Shovel, and Tucker rocks : secondly, by placing it in the centre of 

 the Sound, there would be an entrance at each end available for vesseh 

 to take advantage of either an easterly or westerly wind, whioli 

 would considerably laeilitate their entrance or departure to or from 

 port : thirdly, by having two entrances there would he a greater 

 facility for the reception, and discharge of the tidal and fresh waters, 

 and thus maintain a more general and equal circulation of the current 

 throughout the estuary and its various receptacles, and prevent the 

 dejjosit and accumulation of sedimentary matter, with which the 

 waters are more or less charged ; and thus, whilst protection again<t 

 storms would be gained, the depth would still be raaintaipeJ, which 

 constitutes one, if not the most difficult problem in the construction 

 of haibours. The correctness of Mr. Bennie's decision has been 

 fully corroborated by the result, for, contrary to general anticipation, 

 thedepth of the Sound has not been found to decrease, but rather to 

 augment ; as otherwise, although the embankment of several marshes 

 over which the tide used formerly to flow, and thus to contribute and 

 augment the scour or backwater, might reasonably have been sup- 

 posed to have produced a different result, and should if possible have 

 been avoided. 



Fig. 1. — Plan of Breakwater. 



Scale of yards, each divison 100 yards, 



t 'll" I I ■ T 1 I 1 ' ' ■ ' -J 



The span between A 13, or eastern arm, is about 10 feet above low water 

 spriiig-liHe : B to C is finished with rubble ; — C' to I> is to be finished with 

 masonry ; — D to E is finished with masonry. 



Fig. 2. — Transverse section of the finished part of the Breakwater. 



A _ y ^i_ 



Scale of feet, each division 10 feel, 

 r 1 T I 1 1 I I I ■ ' T • t I I , I t j_i 



A A. Hijih water sprinytiiles ; — BB, Low water spring-tides; — CC. Orl- 

 j»inal bottom, varvhig from 40 to 4-5 feet below low water mark ; — D, The 

 Ftrestiore ; — E, Sea slope ; — F, Top 45 feel wide. 



The first stone wai deposited Aiisust 12, 1819, 

 Quantity of stone deposited to April 3. 1S30 . 2.0(13.979 Tons. 

 Ditto from do. to January 22. 1833 . . 0ll,'l23 



DiltQ from do. to March 31, 1838 . 68,311 



Total 



3,283,713 



Ry referring to the plan, it will be seen that the Breakw,Tter is 

 composed of three arms, the centre being 1,000 yards, and the two 

 outer arms ,360 yards each, and inclines at an angle of "0 dcfi-i-oes to 

 llie main arm, and comprehending a total length of 1,700 yards at 



