1846.] 



THE CIVIL ENGINEER AND ARCHITECTS JOURNAL. 



135 



design depends so mainly upon them, that to erect an insulated structure 

 of the kind, without other windows on its sides than those of the cleres- 

 tory, might be deemed improper and perfectly arbitrary; not that I myself 

 should despair of something good as well as striking being produced with- 

 out any windows below. But did I in what I said, go to the extent of 

 recommending the banishment of side windows, except del eslory ones, 

 from churches? I think not; at least, I did not intend to do so, and 

 tlierefore, I snppose, expressed myself very muddy-headedly. But 

 what Mr. Wightwick has done is— oh horrible '.—not according to 

 precedent. Now ten thousand maledictions upon that 'said " prece- 

 dent"— or rather the servile doctrine founded upon a blindly super- 

 stitious reverence for it,— doctrine that entirely denies to art the power 

 of further production ; or if it does chance at any time to throw out 

 or begin to throw oat a fresh shoot, it must be extirpated at once. 

 Mhat is good requires no precedent to justify it, and what is bad is not to 

 be justified by a thousand precedents in support of it, for if the latter be 

 iu themselves good, they rather convict the thing that appeals to them of 

 departing from iheir spirit. Your precedent-mongers do not even know 

 whether they ought to admire or censure what is shown them, until they 

 refer to authorilies. Tell them that such or such idea is quite new, and 

 they will instantly begin to frown, although they may just before have 

 seemed disposed to relish ; or, on the contrary, they will, on hearing that 

 there is valid authority for what appears to be quite a novelty, be instantly 

 propitiated in its favour, yea, even though an energetic " Damnable !" 

 were actually hovering upon their lips. The present overweening rever- 

 ence for precedent in some quarters, is likely to prove the Dry rot of Art. 



PRESSURE ON RETAINING WALLS. 



Sir— In your remarks, at Page 109 of the Civil Engineer and Archi- 

 tect's Journal, on a paper of mine on the pressure to which retaining walls 

 may be subjected, a necessary preliminary investigation has, apparently, 

 been mistaken for one on retaining walls. The subject was pressure, not 

 retaining walls ; and you may have seen from the conclusion of the paper, 

 and other parts of it, that it was merely introductory. 



It is best to calculate the pressure free from all considerations of mo- 

 mentum at first, because when once this pressure is determined, it can be 

 readily applied to the determination of the strength, &c., required in a 

 retaining wall, so as it may resist being overthrown, being moved forward, 

 or being fractured, by the resultant of its own weight and this pressure. 



Mere formulae without calculations from them are, to the practical man, 

 nearly useless. I have therefore given tables of pressure ; but tables of 

 momentum would, in my judgment be useless. Telford gives pressure, not 

 momentum, very properly ; and gives separately the leverage, or the po- 

 sition of the centre of pressure ; I have not done so, because it was not 

 necessary to do so in this part of my subject, and because, also, any ana- 

 lyticiil determination of the position of the centre pressure must depend a 

 good deal on an hypothesis, which may be considerably modified by the 

 manner in which a wall is brought up and the filling behind it. Friction 

 at the back of the wall belongs also, most properly, to the next part of my 

 subject. 



The formula / p x dx is a correct representation of the momeatam, but 



^o make it useful, take p=w (A — x), a fraction of x, the height k, and a 

 known quantity w: we find then, by integration, &c., the momentum 



w =tcl — — — 1 = ^when x=h) — — x - and, as it is easy to show that 



IV h^ h . 



— ;r — is the pressure, - is the leverage ; but there are many circumstances 



in practice which may alter the position of the centre of pressure, whilst 

 the pressure itself remains unchanged. 



With respect to the conclusion of your remarks, I believe yoa will find 

 on a re-examination, that I am not mistaken here. I have benefitted a 

 good deal by the study of Telford's works, and if I have pointed out some 

 of his mistakes— which every investigator is subject to — it was for the 

 purpose of warning those who, not being disposed to investigate for them- 

 selves, may adopt them without consideratiua. The mistakes of 6ucb men 



as Telford are doubly dangerous, from the position which bis works justly 

 hold. 



May I request insertion for this in your next number. 

 I am Sir, 



Your obedient servant, 

 Dundalk, April Zrd, 1846. John Neville. 



[Jlr. Neville having stated his intention of completing his memoir in the 

 particular to which we alluded, there now remains but slight dilTereuce of 

 opinion between us. We still, however, are unable to assent to the expe- 

 diency of considering the resistance and the moment of it separately. Mr. 

 Neville says, " there are many circumstances in practice which may alter 

 the position of the centre of pressure, whilst the pressure itself remains 

 unchanged :" he could not have made an admission more in our favour, 

 for this shows that the determination of the pressure by itself adds abso- 

 lutely nothing to our stock of knowledge. 



In the case taken in the present letter — and, indeed, in all other cases, 

 where the sum of the pressures varies as h'^ or the square of the distance 

 from the top of the vvall — it is easy enough to find the centre of pressure, 

 because the pressures themselves follow a known hydrostatic law. But in 

 some of the most important cases the pressure does not vary as /i^; for 

 instance, the equation 8, page 4 of Mr. Neville's memoir, which refers to 

 one of the very commonest cases, that of a wall supporting a bank sloping 

 upwards behind it, gives an altogether difTerent law. We do not at present 

 see how it is possible to ascertain the centre of pressure in this instance. 

 (The result, by the way, is identical, mu(o(« miidini/is, with equation 453 

 in Moseley's " Principles of Engineering," and Mr. Neville's investiga- 

 tion, though more general, has the advantage of much greater simplicity.) 



Is there not an error in equation 28, page 1 1 of (he memoir ? Putting 

 5=0 fthat is, supposing the upper surface of the bank inclined at the angle 

 of repose), the right hand side of the equation vanishes, and, consequently, 

 on the left-hand side cz^<p ; which leads to the strange conclusion that, 

 when the upper surface is inclined at its natural slope, the plane of frac- 

 ture is parallel to the plane of repose. At the top of the next page, Mr. 

 Neville gives a definite value to the horizontal pressure when the wedge 

 of earth is resting ou a plane of repose. But surely, by the principles of 

 the inclined plane, when a body rests on a plane inclined at angle, of which 

 the tangent is the co eflicieut of friction, the body will have no tendency to 

 slip forward, and cannot, therefore, require an additional horizontal pres- 

 sure to support it. 



We scarcely know what is meant by the allusion to Telford in the 

 present letter, unless there be an accidental mistake of his name for that of 

 Tredgold;— we said, last month, that Mr. Neville had condemned the 

 latter from having misunderstood his meaning. On referring to Tredgold's 

 original paper, we find his conclusion to be in effect this, — that if the 

 sloping bank be carried up so high that the plane of fracture meets the 

 natural slope of the upper surface, all the pressure on the wall is caused 

 by the wedge, included by the plane of fracture, the wall, and the upper 

 surface : in other words, however high the bank of earth may be, the 

 additional height, beyond the place where the plane of fracture meets the 

 surface of the scarp, adds nothing to the pressure on the wall. Mr. Ne- 

 ville says this conclusion is erroneous! But a moment's reflection will 

 show that all the earth behind and above the plane of fracture is in a slate 

 of rest by itself, and cannot possibly add to the pressure on the wall. 

 Tredgold further says that if we were to adopt the now exploded doctrines 

 of Belidor and Rondelet, we should get a different and an absurd conclu- 

 sion. This is all perfectly correct. Tredgold is not infallible, but he ought 

 not to be blamed for errors committed by others— he has plenty of his own 

 to answer for.] 



Subsidence or the Preston Viaduct.— This viaduct consists of 27 

 arches, and it crosiea the turnpike road from London to Brighton, near the Brighton 

 Urmium. It has been finished several weeks, and the principal portion of the wooden 

 centre, supporting the arches taken away.— April 11, the men engaged in baUasting the 

 line over the viaduct discoversd symptoms of the middle arch, which crosses the tum- 

 pike-road, having given way. Woikmen were immediately employed In placing timber 

 to support the arch. It was found to have sunk a foot or 18 inches, and will have to be 

 taken down and rebuilt. This arch was of wider span than the others, and was the only 

 elliptic arch in th« viaduct. Some persons foolishly ascribe IU subsidence to Us being 

 built in that form, but it Is more probably owing to the continued wet weather, as another 

 of the arches has since been found to be cracked. They will both have to be taken down 

 and rebuilt. This lin« was to have been opened In May, but In comequence of this unto- 

 ward event the opening muat be deferred. 



