IS 47.] 



THE CIVIL ENGINEER AND ARCHITECT'S JOURNAL. 



217 



say two or three feet of low water ; above that, he proposed to construct 

 perp>>ndicular walls, as recommended by Colonel Jones ; observing that if 

 slones were deposited in this manner, and allowed to form their own slope, 

 it would in most situations be the most economical plan. 



He stated that if he had an unlimited command of materials, he would 

 first begin to deposit Ihose materials so as to form a rough mass, and when 

 he had brought his foundations up to that point (nearly low-waler mark) 

 at which the sea would begin to attack him, he would attack the sea, by 

 building with a class of materials that would be its master; adding, that 

 he thought an upright wall in this case might be desirable for a super- 

 structure. 



In Mr. Rendel's examination before the Commission, in November I84.T, 

 his attention was expressly called to his former evidence by several ques- 

 tions, to all of which he replied that he retained the opinions expressed in 

 that evidence; and also stated that he did not know of any instance in 

 which a breakwater with an upright face, of the magnitude now contem- 

 plated, had been constructed in the open sea in seven fathoms water. He 

 added, that so far it is an experimental measure. Mr. Rendel's reasons 

 for adopting the upright wall, in the project which he now proposes, are 

 founded purely on considerations of economy in money and time. He 

 observed, that where there is abundance of masses of stone, fit for con- 

 structing breakwaters, he would form them of rubble stone up to low- 

 water mark, with sloping faces, in themanuer in which he had just finished 

 a design of Holyhead harbour ; hut in order to avoid the expense of 

 bringing stone to Dover, he proposed to adopt, as substitutes for stone, 

 rectangular blocks of brick, set in cement, ten feel long, five wide, and 

 three thick, and with these to build a perfectly upright wall in Dover Bay, 

 by means of powerful machines and the use of the diving-bell. On a for- 

 mer occasion Mr. Reodel objected to the employment of machines, and 

 particularly to the use of the diving-bell. This proposition, therefore, re- 

 solves itself into the question, whether such a project would be economi- 

 cal. 



Mr. Rendel admits that if the execution of the work by means of brick 

 blocks were pressed on so rapidly as to render it necessary to import into 

 Dover bricks, or materials with which to make them, a great part of the 

 economical ad»antage would disappear. He also acknowledges " that the 

 advantages of that mode of construction, namely, the upright wall, over 

 the common sloping-sided breakwaters, is a mere question of economy in 

 money and time," He has further admilled, that if he had unlimited 

 command of materials at Dover, he would adopt the usual mode hitherto 

 observed in constructing breakwaters. Now Mr. Hartley expressly states, 

 that the expense of providing brick blocks made of the materials that he 

 recommends as indispensable in the construction of such a work, would 

 be greater than that at which granite might be procured from the Channel 

 Islands. 



From this and other calculations it appears, that " the mode of con- 

 structing breakwaters hitherto observed," with materials of the best de- 

 scription, is preferable, in an economical sense, to that proposed by Mr, 

 Rendel, and this being so that he would renounce it. We have this reli- 

 ance on Mr. Rendel's discretion and judgment, that he would guard him- 

 self against assuming anything where experience, the only safe guide, can 

 be referred to; and, in a great national work like this, would not propose 

 any new-fangled notions that have nothing but their ingenuity to recom- 

 mend them. 



If then the question, whether the theory of the upright wall, or the 

 established practice of the slope, was to be determined by the opinions of a 

 majority of competitors, the Commission ought to have decided the other 

 way, for, of the eight engineers who gave in plans, four recommended the 

 sloping breakwater; and, of the other four, two propose a combination of 

 the slope below, with a nearly upright superstructure ; and only one pre- 

 lers the upright wall, and this provided his pioposiiion for using brick 

 blocks of 25 tons weight as substitutes for stone, be adopted. 



The following is a list of the persons whose opinions are adduced as 

 advising the construction of the upright wall : — I. Professor .Airy ; 2. Pro- 

 fessor Barlow; 3. Major-general Sir J. Burgoyne; 4. Sir Henry De la 

 Beche ; 5. Mr. Hartley; 6, Major-general Pasley ; 7. Captain Vetch; 

 8. M. Reibell ; 9. Mr. Brunei ; 10. Mr. Bremuer. 



1. Professor Airy 's opinion in matters of science is unquestionably enti- 

 tled to the very highest respect. I have studied with the greatest attention 

 and profit the Astronomer Royal's tract, in «hich the phenomena of tides 

 and waves are investigated by a refined analysis on what is called the 

 *• wave theory." It is assumed that in deep water, the motions of ihe par- 

 ticles are oscillatory, and that the rising and falling of the surlace of the 

 sea depend on the horizontal movements taking place alternately in the 

 same and in contrary directions ; tliat these displacements are represented 

 by a periodical function (ihe sme or cosine of an angle depending on time). 

 The circular or elliptical movement of the particles is shown to lake place 

 only when a wave is transmitted along a channel of uniform breadth and 

 depth ; and the fact, that, as the depth of water becomes less, waves be- 

 come shorter and their fronts steeper, is proved to be in accordance with 

 what may be deduced from the theoretical expressions of the displace- 

 ments. It follows from this, that, as a sea-wave advances into water 

 gradually becoming shallower, it assumes a crested shape, the upper 

 particles moving towards the coast, till at length the top rolls over the 

 base, the wave breaks, and a surf is treated. Keference is made in this 

 article to the special treatises on sea-waves by JIM. de la Coudray and 

 Bremontier. 



When Mr. Airy was Professor of Natural Philosophy at Cambridge, he 



explained, with success, that waves in a fluid at rest, such as we may con 

 ceive to arise from throwing a stone into a pond, or the ordinary waves in 

 a close lake, are more or less superficial undulations, and that in reality ao 

 current, or onward motion of the fluid, appears to take place. I well re- 

 member, also, that he invented an ingenious machine by which he illustra- 

 ted this oscillatory motion. But admitting this to be true, to a considerable 

 extent, in a pond or a small lake, it is totally inapplicable to the sea, the 

 open sea, in Dover Bay, where an immense body of water is in constant 

 motion, by tides rising and falling fifteen or twenty feet in the course of 

 two or three hours, and where the surface is liable to be acted upon by 

 heavy gales, which drive in rolling seas in succession with rapid onward 

 motion, and therefore producing percussive force in the direction of the 

 wind. Without however entering here on Professor Airy's theory of waves 

 in deep open sea, but confining myself to deductions from that theory, as 

 to the practical effect of waves in gales of wind on erections in the sea, of 

 a limited depth, it will be seen, that instead of his theory (that the upright 

 wall is in all cases preferable to the slope) being absolute, this eminent 

 authority allows that waves in a breaking or broken state do act percus- 

 sively and powerfully as hydraulic rams, and not by hydrostatic pressure. 

 How then can that hydraulic action cease and become merely hydrostatical 

 pressure unless it has first exerted a force of impact upon the wall which 

 arrests its motion? Even if the wall should stand after having received 

 the shuck, the concussion must be more severe on an upright wall, in Ihe 

 ratio above mentioned, than that which would lake place on a sloping wall 

 of equal height. 



The question of construction, then, resolves itself into this : in what 

 depths of water do waves assume that form and acquire that percussive 

 force? Where, according to this, should the slope cease and the upright 

 wall commence ? The professor says, practical opinion, that of the pilots, 

 can best determine this. 



Those whom I have questioned on that subject say, that this will be 

 found to take place, in heavy gales of south-west and southerly winds, 

 throughout nearly the whole of Dover Bay at low water. 



However this may be, it is clear from the Astronomer Royal's deductions 

 from his own theory, that there should be a sloping breakwater in the 

 shallower parts of the space to be enclosed, and an upright wall in the 

 deeper. 



But with respect to the practical question. Professor Airy states, in re- 

 ply to question 59.5, whatever theory may say, " that building an upright 

 wall in the open sea, in seven fathoms water, is so far an experimental 

 measure, that no such work has ever been executed." 



With every respect, then, for the theoretical opinion of this high autho- 

 rity, I cannot consider that it would justify the Government in sanctioning 

 the mode of construction recommended by a majority of the Commission ; 

 it may rather be inferred that this is contrary to the deduction of science, 

 and that, if the difliculties of constructing such a wall in deep water could 

 be overcome, it would be incapable of resisting the action of the sea where 

 waves assume that shape, and possess that percussive power, which Mr. 

 Airy admits. 



2. Professor Barlow has most usefully applied mathematical investigatioa 

 to practical purposes, and knows well the difference between theoretical 

 views and practical effects upon a proposition of this description. In his 

 letter of the 5th January 1846, written in reply to the question referred to 

 him, he stales, that theory cannot safely settle that question ; he avows that 

 he has not sufficient practical knowledge or experience to enable him to 

 speak confidently on the subject ; expresses himself diffidently, cautiously, 

 and even ambiguously, as to theory ; and recommends that the question be 

 referred to practical men for their opinion, made upon results obtained 

 from actual experience and observation. The learned professor therefore 

 rather declined and disclaimed giving a decided opinion in favour of the 

 upright wall ; and I think he will be surprised to find that his letter has 

 been adduced, by a majority of the Commission, rather as conflicting with, 

 than as deferring to the opinion of practical men. Further it appears, by 

 the professor's letter, that he is decidedly opposed to the theory of the up- 

 right wall ; for he denies the assumption on which it is based : namely, 

 " that waves have no onward motion." He states, " there can be no doubt 

 that waves when acted upon by tempestuous winds, will beat with great 

 violence against any obstacle opposed to their progress ; that what we want , 

 in breakwaters is, to resist that force ; to withstand that momentum ; and 

 that much of this direct violence would be avoided, by receiving that actioa 



on an inclined surface." 



3. I refer with the greatest deference and respect to any practical opinion 

 of so eminent a man as Major-General Sir John Burgoyne; but I do not 

 read his letter on the comparative merits and capabilities of the upright 

 wall and the slope, as containing any very positive or confident preference 

 of the former; and in that letter it is admitted that there can be no doubt 

 as to the security of the slope. This distinguished military engineer says, 



" The effort against the upright wall I conceive would be far less. 



" In deep water, the action of the wave is, I apprehend, an up and down 

 undulation, the water having very little, if any, forward motion, except 

 where it breaks. A flat piece of wood, floating on the surface, and present- 

 ing no hold to the wind, would progress very slowly before the heaviest 

 gale ; therefore I consider that there would be no blovv or impulse generally 

 on the upright wall, but merely the weight of water from the top of the 

 wave to its mean level, to be supported. 



" I should not expect that the wall itself would cause the waves to break, 

 and even those that accidenlall; did so at that particular place would have 



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