I8i7.] 



THE CIVIL ENGINEER AND ARCHITECT'S JOURiNAL. 



215 



all tend to alTect the stability, and modify the results of theory ; each of 

 which circunislances, acting pro or con, will be made to fulfil its duty to 

 the best advantage, by the skilful engineer. The utmost that theory can 

 do, is to show the conditions of equilibrium, under certain fixed practical 

 data, and there can be no doubt, that the line of equal horizontal thrust in 

 an arch, is analogous to a vertical line drawn through the centre of gravity 

 in a column. That this line should fall within the mass at every joint, and 

 that the position of each joint should be such, that the direction in which 

 the pressure acts should be within the limiting angle of friction, are con- 

 ditions common to both structures. 



[The interesting observations made at the meeting of the Institution, l)y 

 the members, after the reading of the paper, will be given in the uext 

 mouth't Journal. ] 



CONSTRUCTION OF SEA WALLS. 



A Protest against the Decision of the Members of the Harbour of Refuge 

 Commission present at the Sittinij of the 13/A January 1846 ; and Dissent 

 from their Report, on the part of Lieutenant- General 5ir Howard Douglas, 

 one of the Cnmm'Ssion — presented to the House of Commons. (Slightly 

 abridged.) The annexes referred to will be given in the next month's Jour- 

 nal. 



Attaching the greatest importance to the attainment of certainty in the 

 mode of forming, and of durability in that of executing the extensive works 

 about to be undertaken for the proposed harbour of refuge in Dover Bay, I 

 consider it incumbent upon me to express my marked opinion, in opposition 

 to plans which, in my judgment, are founded on modes of construction not 

 resting upon any proved principle, and untried upon any sufficient scale to 

 warrant their present adoption ; which are moreover theoretical in concep- 

 tion, and consequently uncertain in their ultimate result. Such plans are, 

 in my opinion, unfit for the attainment of the great national object which we 

 have in view ; and which it is my most anxious wish to see undertaken in 

 such a manner as will leave no doubt of its being successfully accomplished. 



Considering, then, that the building of an upright wall in the open sea, 

 in seven or eiglit fathoms water, is a proposition novel in theory, and never, 

 in so far as I am aware, proved in practice, on a scale to warrant its adop- 

 tion;' and being of opinion that a breakwater of the proposed elevation 

 and magnitude, rising with an upright face, from the depth of 42 feet at 

 low water, would be far less capable of resisting the violence of seas,'' and 



1 See Observation on Kilnish P er, built in only 9 feet 6 inches depth of water. 



2 The application of the theory of the resistance and impact of fluids Is no doubt at- 

 tended with great difficulties and anomalies in this as in many other cases connected 

 with natur.il philosophy ; but whatever results have been derived, either from theory or 

 observation, they all agree In this, that the horizontal Impulse of a fluid on any resisting 



body is increased, in a very high ratio, 

 as the inclination of its surface (A B) 

 to the direction (B C) of the motion 

 increases. The fundamental theorem 

 is, that this varies as (sin 0)2 (0 re- 

 presenting the inclination), because 

 the height of the breakwatsr con. 

 tinning the same, the quantity of fluid 

 Impinging on it will be the same at all 

 inclinations. If the height of the 

 breakwater were vdriable, the whole 

 Impetus would then be as (sin 0)3, 

 because in (his case, not only is the force of each particle diminished in proportioB to 

 (sin ^ 0)2, but the number ot particles which impinge on the plane varies as sin 0. 



From this resolution of the pressure of a fluid against the surface of a body, whether 

 either be in motion and the other at rest, or both body and fluid be in motion, some of 

 the most useful results of practical science are obtained. By this a ship is impelled for- 

 ward ot'liqueiy to the direction of the wind, even when that direction is before "the 

 beam ;" and a like resolution of pressure gives rise to the propulsive force of the revolv- 

 ing screw, or enables the rudder to retain and guide the vessel in her intended course, 

 Tlie oblique action of the water on the side of the vessel or raft, permits a " flying bridge" 

 to be sheered across a river ; and not only does that of the wind give motion to the sails 

 of a mill, but by a proper variation of the obliquity of these, according to the distance 

 from the axis of motion, the impelling force is rendered equably on every part. 



In the application of this principle to practical mechanics, great difficulties certainly 

 occur from our imperfect knowledge of the manner In which the forces ol nature are ex- 

 erted ; and the problem concerning the action of the sea against a wall, can no more be 

 solved by the resolution of forces than the trajectory of a shot, in a resisting medium, can 

 be determined by the parabolic theory, or even by any theory founded on the usually as- 

 sumed law 01 resistance ; yet such a theory has its uses for the practical artillerist ; and 

 a knowledge of the mathematical principles of hydrodynamics is essential to an adequate 

 conception of the means to be employed for resisting the actions of waves. 



On the best form for the profile for a " breakwater" a ditference of opinion exists ; and 

 while, on one hand, it is contended that the exterior face of the wall should be vertical, 

 on the other, a face inclined to the horizon is recommended. The advocates of the former 

 construction seem to consider that such a wall is subject only to the hydrostatical pres. 

 sure of a fluid at rest, or that the agitations of the water before it, takes place only in ver- 

 tical dii ections J but neither of these conditions holds good in Dover Bay, or wherever by 

 the force of winds and currents the waves are impelled with violence against the shore. 

 That loose stones constituting a breakwater, when deposited so as to form an inclined 

 plane, should be occasionally displaced by the action of waves is sufficiently obvious; but 

 that, with equal quantities of material, a vertical wall should resist the concussions pro- 

 duced by SHch action, as ef^caciously as one with an exterior slope. Is inconceivable. 



It does not follow, however, that the face cf a breakwater should have one uniform 

 slope from the bottom upwards ; the part lashed by the waves in an open sea requires a 

 longer slope, or a smaller inclination to a horizontal plane, than the part below ; and this 

 deduction trom scientitic priuciplos by many emineotauthorltlei li confirmed by the prac- ' 

 tice uf engineers. 



especially of broken seas, (exposed as it must, nioieover, be to the unre- 

 mitting action of strong tides and currents) than a slopmg breakwater 

 formed in a manner similar to that which has been successfully completed 

 in Plymouth Sound (which is now in a state of perfect repose and stability) 

 (Annex B, D.), as well as similar to others (Delaware Breakwater, Annex 

 L.) constructed on ils model ; having also dissented from the proposition 

 of the upright wall on a former occasion (31st July 1»44, Aunex A.), 1 

 now consider it my duty to oppose myself, decidedly, to the adoption of 

 that mode of construction, and to the employment of any artificial or in- 

 ferior material, as a substitute for stone, Irom mere considerations of pe- 

 cuniary economy. The latter should I tliink have uo place in a great 

 national undertaking of this description, and 1 firmly believe that the me- 

 thod proposed with this view would, in the end, prove by far the most ex- 

 pensive. 



In the more recent minutes and proceedings of this Commission, I find 

 much to confirm me, practically, in these views and opinions (which I 

 brought before the Commission in July 1844), and I perceive that even the 

 highest acknowledged scientitic authorities who adhere or incline to the 

 theory of the upright wall, speak cautiously, diffidently, doubliugly,or am- 

 biguously of the capability of vertical walls to resist the action of waves 

 and seas in all cases and under all circumstances. Some of these main- 

 tain that waves in a breaking state do act percussively ; that a sloping 

 breakwater is therefore best able to resist the action of seas in that stale, 

 and that consequently there should be a sloping breakwater in one part of 

 the proposed harbour of refuge, and a perpendicular wall in others ; whilst 

 other high authorities, who incline to the upright wall, admit that this is 

 merely matter of opinion, quite speculative and experimental as respects 

 themselves, and that there can be no doubt tliat a sloping breakwater 

 would be perfectly secure. Now in my judgment, nothing purely theore- 

 tical can remove the strong objeciions which have been so forcibly ad- 

 vanced by many experienced practical engineers, ' (and I may add other 

 eminent men of high scientific and practical attainments, naval, military, 

 and civil), against the adoption of a mode of construction diliicult if not 

 impracticable, any failure in which would be discreditable to the engineer- 

 ing talent of the country, and in ruining Dover Bay as a natural roadsteatl 

 and anchorage, be productive of evils the most serious to cuuiiuercial ope- 

 ralions in the Channel. 



Mr. Alan Stevenson states, that to build an upright wall in seven or 

 eight fathoms water, so far as his experience goes, would be entirely au 

 experimental measure ; that to attempt this in an open sea-way like Dover 

 Bay, would be a work of the utmost difficulty, if not wholly impracticable ; 

 and that so far from recommending the trial of such a work, he would 

 humbly, but decidedly, dissuade the Government from making an attempt 

 which he was sure would end in failure; and, in reply to cross-questions 

 put to him with a view to shake his testimony against the upright wall, he 

 denies the theory on which that mode of construction is founded. He as- 

 serts, on his own experience, that waves are not purely oscillatory, but 

 have onward motion, and consequently percussive force, such, in his con- 

 viction, that any attempt to check their force by means of a vertical wall, 

 will prove a signal failure ; for that a force would be developed by the 

 collision of the wave with the wall, whose amount will be found to sur- 

 pass any which has ever been experienced on the face of a sloping break- 

 water. 



In the course of the protracted discussions to which my opposition gave 

 rise, the danger was demonstrated of using, in such a work, a material 

 (concrete) to which I had always objected, as deficient in tenacity, and in- 

 capable of resisting mechanical action of water. An opinion of the efficiency 

 of this material was, however, strongly supported by the reference made lu 

 an official report, and in a leading question to the use of blocks of con- 

 crete for the completion of the breakwater in Cherbourg Bay, which was 

 described as a successful experiment, and one deserving of bein;; adopted 

 by us, as a precedent; but the contrary of both was soon evident; for 

 within the period to which, happdy, the proceedings of the Commission 

 were thus extended, an important failure occurred in the works at that 

 place ; and the employment of concrete, as a substitute for stone iu this 

 climate, has been atjandoned by the French engineers. 



This failure, and the opinion of Sir R. Sinirke against the adoption o( 

 blocks of concrete as an artificial stone, which he thought would fail, d s. 

 posed of this proposition; and it will not be conducive to the public inte. 

 rests, in ray opinion, that the other description of artificial material reconi. 

 meuded by Mr. Kendel (namely, brick set in cement), which Mr. Corderoy 



Poisson, Prony, Charles Dupin, Girard, Cachin: Professor Cape, Military Seminar^-, 

 Addiscombe; Professor Narrien, Royal Military College ; General Bernard, United States' 

 Kngineers; Commodore Rogers, United States' Navy; Mr. William Strickland. See 

 Annex. (L.), on the Delaware Breakwater, by which it appears that Colonel Jones was 

 misinformed, — See his Report of l64tl, Appendix, No, 1, p. 72. by the Unit*^d States' En- 

 gineer, who told him that the long slope was not approved ot by American engineers. 



3 1, Sir John Rennie, Annex (B,) See also his Report and Opinion, No. 4, of plans 

 Bent in. 



2, Mr, George Rennie, Annex (C.) See also his plan, No. 2, of the Reports; his model 

 and recent examination, Q, 430 to 4^4, 445 to 447. 



3, Mr, Cubitt, Annex (D.) See his Report and plan, No, 6, and recent examination, 

 Q, 210 to 213. 234 to 238; and letter to the Chairman, Appendix, No, 11, Second Report. 



4, Mr, William Stuart, superintendent of Plymouth Breakwater, from the commence- 

 ment of the work in 1811 to the present time. Annex (F.), evidence of June 20, 1844. 



An important evideuce, showuig th.tt the damages which tliat work had sustained aro^e 

 from the slope or foreshore not being long enough ; stating his practical objections to a 

 more upright slope, and his conviction that it could not stand ; and that, if that break- 

 water had been constructed upright from the bottom of the lis, it would have been in- 

 capable of resisting the force of the waves. 



