252 



THE CIVIL ENGIVEER AND ARCHITECTS JOURNAL. 



[August, 



in my opinion it is better fo take that thing which we thoroughly iiniler- 

 sland and know, than lu take ii thing which is in the least degree cxperi- 

 nienlal on sucli an ininiense scale. 



Vou arc, no doiiht, aware of the tremendous disasters that at'ended the 

 long slope at Plymouth durint; its progresa of ronslruclion. AVilh such 

 facts as these l)efore os, do you think tlie long slope can be safely adoptid 

 at llover? — Uuile so, and for very good reasons. It is not because a 

 casually happened from a great storm arising whilst the breakwater was 

 building there, or because another happened afterwards, in which they 

 tiink up 200,000 Ions of stone, which is not a great <iuantity, that we are 

 lo conclude ihut the plan was not a good one. 1 believe thai that break- 

 water was set about in an imperfect manner. The makers of Fl) mouth 

 breakwater prided themselves upon taking all large stones for il, the 

 breakwater consequently was too hollow, and when a heavy sea came it 

 curled over llie surface of the breakwater, coveiing it with the water, and 

 the compression of the air inside drove up the sione*. Now, if all llie 

 sliili' that was raised in the quarries, large and small, had been put into 

 Ihat breakwaler, 1 do nut think that that quantity of stone would have 

 tumbled out of it; and again, when a breakwaler is fairly trimmed over, 

 and trimmed into form, there is no dillicully in paving that wilh heavy 

 stones on ibe edge, an<l giving it as perfect and smooth a face, after a little 

 time, as can be, at any angle you please, so that no sea can break into it at 

 all. It is because the thing is imperfectly done that it is not good. And 

 wilh regard to casualties happening, 1 am sure I do not myself suppose, 

 and I do not think any paily in l■ea^on will suppose, that we can build an 

 upright wall without having casualties ; things will be tumbled bead over 

 heels, and we cannot work half the time, or sometimes a quarter of the 

 time. We must have the benept of experience in a great work of this 

 I. ind ; and, I think, there is not that experience for an upright breakwater 

 yet to justify us in entering upon it lo this extent in the first instance. 



1 will ask y<iur attention to the dillerenl aspects of the two works, the 

 I'ljmoulh hreakwa'er and that proposed at Dover. What will be tlie 

 eflect of its sirdiing in that way upon those loose stones; will it throw 

 them into the passage? — 1 would have none come out. But in completing 

 the end of the rubble, if there were a thousand stones driven out lliey 

 wc.uld be taken out, ami it would be cleaie<l out again in completing the 

 ends with the divingbeli, exactly as they are now completing the walls of 

 Kingstown harbour. 



You are aware thai at Plj mouth tlie entrances of the breakwater are at 

 each end, but at Dover the entrances must be directly through the slope? 

 — Kxactly so. So it is at Kiugslown harbour; and ihougb the seas there 

 are not equal lo those at Plymouth Sound, they are at times uncommonly 

 heavy seas, and I have seen a perfeclly clear passage in the seven or eight 

 fathom water. There is no difliculty about that. 



Without going into the question of the principle of the action of waves 

 on upright faces or slopes, you think that to build an upright wall in 

 Dover Bay in the open sea, in seven fathoms at low water, exposed to the 

 action of currents and tides, would be altogether a very difiicult under- 

 taking? — Upon any plan whatever. 



And having bestowed upon Ibis important question all the science and 

 all the attention which you have devoted to il, and wilh all your experi- 

 ence, you come at length to this practical conclusion, lliat upon the whole, 

 after the most careful consideration of the subject, your recommendation 

 would be to form the harbour at Dover Bay by depositing blocks or masses 

 of stone, as shown in the plan which accompanies your report, using the 

 largest blocks either of granite, Portland stone, or limeslone, so as to form 

 a section, having a long lore-slope ? — 1 do ; but I not only do that, I give 

 a statement at the same time of the dimensions of the thing, the quantity 

 measured, and show what addition it would make in the cost of the whole, 

 and what saving might be made by using other nialerials in its construc- 

 tion. 



Then you think that any attempt to erect an upright wall from the bot- 

 tom in Dover Hay, at the depth of seven falhoMis at low water, and to form 

 that wall of blocks of concrete, or any other arliliciai material would par- 

 take very consideiably of an experinnntal undertaking? — Quite so. 



And Ihat we should not be juslilied in recommending or approving any- 

 thing which is of an esperinienlal character for this national work ? — I 

 think there would be great danger in undertaking a woik of that kind 

 upon an experiinenlal or new plan. We are not sullicienlly experienced 

 beforehand, having no previous knowledge. Tli>ie are very weeks in the 

 year in which il is at all comlortalde, being oil' Dover one mile or two 

 miles at sea. It is all blind woik at the bolloin of the si a. People may 

 think Ihat they can put those down with scalluldings, but what is to be 

 done at 72 feet dicp, with a diving bell, in the open sea, is very didicult 

 to say. 



Annex (E)._Mr. Alan SxrvENSON's .4Hsiccri lo Qiieslions proposed 



to him 0)1 the JShnic oj ConstmetUm. 

 You are considered to have bestowed much study and observation, and 

 to have great practical experience as to the action of waves on erections in 

 the sea? — I have had considerable experience in the erection of works 

 exposed to the action of the sea, in piers, lighthouses, harbours, and bea- 

 cons ; and I believe 1 have enjoyed good opportunities, more especially at 

 theSkerryvore Kocks, of observing the action of the waves. 



All my experience, observation, and consideration, lead me to believe 

 that a sloping face is belter calculated to resist the action of the waves 

 than a perpendicular one, and the force expended against the perpendicu- 

 lar plane seems by concentration to become more intense, for the seas rite 



to a greater height than thise which strike a sloping face. Such nn 

 opinion is in accordance with the phenomena which characterise almost 

 every part of the coast, where it is found that the angle of the shore 

 varies wiih the force of the waves. I cannot imagine that the waves of 

 Ibe sea exert no percussive energy, when I ob>erve iheir power in forcing 

 forward a vessel which has neither wind nor tide lo help her, or a vessel 

 at anchor, an elfect which I have (elt in \'i and 18 latlioms water; or when 

 1 consider the height lo which spray rises in deep water by striking a 

 vessel at anchor. 



That waves driven in by gales of wind are destitute of percussive elTect 

 I cannot conceive to be possible. 



The force of the waves will be greatly diminished when they act 

 obliquely on a sloping surface. I'roni the effect of the slope to increase 

 the surface of the wall opposed to a given perpendicular surface of llie 

 wave, the eneigy of the wave will necessarily be decreased in proportion 

 as the impinging particles are spread over a greater surface. But the 

 force of impact thus diminished in the ratio of the sine of the inclination 

 of the surface to the direction of the fluid's motion, must, in order lo esti- 

 mate its tendency lo displace the wall horizontally, be resolved, first, per- 

 jienilicularly to the surface, and again in a horizontal direction, so as lo 

 be thus finally diminished in the ratio of the cube ot Ihe sine of ibe inch- 

 nation of the surface of the wall lo Ihe directum of the wave. Kxperi- 

 ments upon the action of Uuids or surfaces conlirm ihe view thus theoreti- 

 cally assumed. 



I cannot perceive any material difference, in so far as Ihe result is con- 

 cerned, between Ihe case of the breaking wave and that of an unbroken 

 wave, except thai I should expect i.iore force in an unbroken wave which 

 has not encounteied an obstacle. Admitting that both have an onward 

 movement (which I lake to be the case with all waves which are acted on 

 by the wind), it would appear lo me that the direction of the force in a 

 broken wave must be more diffused in different directions, and would thus 

 seem to possess less of a '* rara-like power." 



What do you think of the theory which assumes Ihat waves have no 

 other action than statical pressure upon a perfectly upright wall, although 

 it is admitted that waves in a broken or breaking slate have a percussive 

 force, which an upright plane is not so capable of resisting as a slope, 

 according to the well-known hydraulic theorem to which 1 have adverted? 

 — My opinion of that theory is that it is not sound, and 1 found my con- 

 clusion on observation, and on reasoning, which both conduct me to the 

 same result. 



How can Ihe hydraulic action or percussion of the wave, in the direc- 

 tion of its motion, cease when it comes in coniact with the wall, and 

 become hydrostatic pressure, without acting by impact on the wall which 

 stops that motion, and which consequently, if it stands, resists that im- 

 |iulse? — I see no reason, as slated in the last answer, for supposing that 

 the purely vertical or undulalory movement, which Ihe above theory 

 ascribes to all unbroken waves, should not produce, in the process of its 

 neulralization by a vertical wall, effects similar in kind lo Ihose produced 

 by its neutralization by an inclined plane. In both cases the undulation 

 is checked ; and whether this is done by reflecting the vertical or undula- 

 lory motion in one direction or another, seems in no way to change the 

 measure of the whole shock, which such a concussion and final extinction 

 of the force seems to imply. 



Are 50U aware of any case in which a perfeclly upright wall has 

 been bnill in the open sea, in a depth of seven or eight fathoms ? — I never 

 heard of any upright wall being built in any such depth as seven or eight 

 fathoms. 



Would not such a mode of construction, applied to Dover Bay, be 

 es=entially an experimental measure? — Certainly, so far as my experience 

 goes. 



W hat is your opinion of the difliculty, facility, or practicability of 

 building an uprig.it wall in the open sea iu such a deptii of water? and 

 how sliould jou proceed to execute such a woik? — I should consider 

 building an upright wall from the bottom in seven or eight fathoms in an 

 open seaway, like that at Dover, as a work of the utmost difliculty, if not 

 indeed wholly impracticable. 



Are you prepared, as a practical and experienced engineer, to recom- 

 mend til It such an experimental mode of construction should be tried iu 

 such a place, on such a scale, at an enormous cost, and for such permanent 

 national objects as those lo which these proceedings relate? — So far from 

 recommending the trial of such a work, I should hnmlily, but decidedly, 

 dissuade the government from such au attempt, which I am sure would 

 end in failure. 



What, upon the whole then, is the mode of construction which you 

 would propose for executing this great work, in the most certain, solid, 

 and enduring manner? — I'aking into account the lornis of the natural 

 shore, and the tendency of the foregoing views, I see nothing to warrant a 

 depailiire, in any material degree, from Ihe existing practice of engineers 

 in the construction of breakwaters. 



What is your opinion of the action of the wave upon a perpendicular 

 wall, with a smooth surface in deep water ?—l cannot conceive that the 

 unbroken waves caused by wind have less percussive force than when they 

 are broken and diffused; I must concluiie that Ihe sudden check of this 

 force by a vertical barrier will produce a greater single effect than the 

 gradual expenditure of force over a larger space caused by meeting the 

 successive surfaces presented by a sloping wall. 'Ihe tendency, therefore, 

 appears lo me to be towards a more certain and rapid destruction of the 

 vertical barrier than of the sloping one. The detlruclion of the sloping 



