THE CIVIL ENGINEER AND ARCHITECT'S JOURNAL. 



[Ja.v 



would dive fi73 ft. 10 in. for tlie length of tlip front. The east and 

 west fronts, by the s:iine reckoninjr, would he (ilS ft. 10 in. ; and, 

 as the lenjjth of Uuckinirhuni Palace is stated in the ./'(«)•»«/ at 

 .S50 feet, the proportion is exartly two-tifths. The height of 

 Caserta is 120 feet ; of Buckiiiifham Palace, 77 feet. 



I visited Caserta thirty years ajjo, in company with Mr. Woods 

 and M. Soiss, the eminent Belgian architect, and the notes of that 

 day are now before me. 



"The central passage or vestibule leading from the entrance is 

 2i feet wide, and 2t feet high to the springing of the semicircular 

 vault. The steps of the middle Hight of tlie stairs are 2'2 feet 

 long, and the two return flights each 14 feet long (100 steps in all), 

 and are of white and reddish variegated marble : there are statues, 

 trophies, &c. The vestibule above is handsome, hut (in my eyes 

 at that time) in bad style. The chapel (which is opposite to the 

 staircase) has coupled columns, of Sicilian marble. The lower 

 part of the chapel is lined with slabs cut from the Giallo Antico 

 columns, removed from Purruoli," &c. &c. 



Let Mr. Blore give to the Palace a staircase at all resembling 

 this, and the world will forgive him the faults of his front ; many 

 f>[ which have, no doubt, been forced upon him, as ujjon his great 

 predecessor, by the necessity of providing accommodation for so 

 many people. 



I have read Mr. Elmes's Epitome of the Lives of English Archi- 

 tects with great satisfaction, and beg to expi-ess my hope that he 

 will carry out his " present intention" without delay. I would take 

 the further liberty of suggesting to him that the illustrations he 

 promises ought to consist mainly of unedited specimens, or at least 

 of those of which the engravings are least accessible, or the works 

 containing them least known. A list, referring to the books in 

 which the most meritorious of each architect's productions are to 

 be found, would l>e very valuable. 



It should be remembered that size is not the criterion of merit. 

 How many of us country architects are forced to take the counsel 

 of our excellent frienil, Percier, and in despair of executing large 

 works, to bestow greater care upon lesser ones .'' 

 I am, &c., 



Voric, Dec. 13, 18t7. Robert Sharp, Architect. 



COWPER'S INVERTED ARCH BRIDGE. 



Sin — In reflecting on the construction of Mr. Cowper's bridge, I 

 think it is interesting to notice that the boiler plates are rivetted 

 together, so that the pressure on the piers and abutments must now 

 be vertical instead of oblique, as in the common suspension bridge. 



Hence, then, in principle it may be said to coincide with the 

 (u-dinary girder bridge, but its alteration in form suggests an im- 

 ]iortant advantage, which it possesses ; for, in the girder bridge 

 (and especially when dealing with long bearings), there must 

 always be this objection, viz., that by reason of the gravity of the 

 material itself, independently of any additional influence of a load 

 placed upon it, it is constantly tending to assume a curved form, 

 and such curved form not being the natural position of its particles, 

 it is constantly tending to rupture ; but in the " inverted arch 

 bridge" the material is thrown into that form (or nearly so) which 

 it would take if perfectly fte.rihlc, and then is made perfectly rigid. 

 So that, as regards its own gravity, there is no further alteration 

 of form, of any practical importance, to be apprehended. 



With regard to the alteration of form which might result from a 

 load being put upon it, Mr. Cowper has already pointed out the 

 j)reventivc measure, viz., the giving to the vertical dimension of 

 the plate such a magnitude as to bring the line of strain within 

 the plates. 



I think the name might have been more aptly chosen : "Inverted 

 arch" is suggestive only of form, and not of principle, and might 

 (it seems to me) with eijual projjriety he applied to the ordinary 

 suspension bridge. I am, &c., 



G. W. Richard. 



*,,* It is not cl^-ar that it would be practicable to make the 

 '■ inverted arch bri<lge" so rigid as to exert only vertical pressures 

 on the abutments, and to act as a curved girder. 



Suppose its span 200 feet and dejjth I feet, and that a weight of 

 :iO tons (engine and tender) rests at its centre. Then, by the 

 ordinary ))rinciples of statics which apply to girders, we may easily 

 find the horizontal strains of tension and com]iression which this 

 weight alone produces at the centre. Considering the half-structure 

 as a separate statical system, the forces acting upon it have equal 

 moments about the jioint of sn])port in the abutment : or half 

 the weight X the half-span = the moment of the couple of tension 



and compression created at the centre of the bridge. The length of 

 the arm of couple is indeterminate, but (since the total depth is 

 ■1 feet) it is a favoural)le supposition to take it at 3 feet. Hence, 

 calling the horizontal strain P, we have 



P X 3 = 15 X 100, or P = 500 tons ! 



The metal must he tolerably thick to resist five hundred tons 

 pressure on the upper, and tension on the lower, side of the bridge 

 at its centre! This dithculty is formidable enough when the struc- 

 ture is considered as all one piece, liut becomes insuperable when 

 the efl'ect of joints is taken into account. It is not to be overcome 

 by any system of rivetting and dove-tailing, however intricate. 



Though it be easy to calculate the amount of the horizontal 

 strains at the centre of the curved beam, it is not easy to estimate the 

 sectional or transverse dimensions necessary to resist those strains; 

 for our knowledge of the transverse strength of wrought-iron is 

 much less than of cast-iron. Some idea may, however, be (dttained 

 from analogy. The "inverted arch," if it sustains itself by its 

 rigidity as a girder, may, for all purposes of calculating the strength 

 at its centre, be considered as a horizontal beam 200 feet long and 

 4 feet deep, with an effective width of 1 foot to 1 foot 6 inches at 

 the utmost. Now, the proposed Menai tubular bridge will be 450 

 feet long, but its de])th will be thirtg feet, and width fifteen feet : 

 also its upper and lower sides will be composed of several thick- 

 nesses of metal, as the former will consist of two, and the latter of 

 one, series of cellular compartments. The analogy between the 

 Menai bridge and the inverted arch is complete in several respects : 

 both are tubular, both are to be composed of rivetted wrought-iron 

 plates, and both are designed for railway traffic. The sectional 

 dimensions of the Menai bridge are suggested by actual experi- 

 ment, and are never considered too great. Is it not, therefore, 

 abundantly evident that a beam of |ths the span of the Alenai tube, 

 hut with only an eighth or twelfth its depth and width, ivould not 

 be rigid enough to sustain itself as a beam .' 



If suspended from, instead of resting upon, the abutments, it 

 might perhaps be prevented from actually falling, but it would 

 certainly bend. If the point of suspension he supposed to be at 

 the upper edge of the end of tlie beam, the transverse strains of 

 deflection will be somewhat reduced by the curvature of the beam; 

 but it would be difficult to show that this advantage would not be 

 far more than compensated for by the increase of length, and there- 

 fore of material, wliich the curvature renders necessai-y. 



It is important to remark, that if the only requisite.i'or security 

 were that the depth of the chain should "include any alteration in 

 the curve of the strain," that depth should not be uniformly i feet. 

 It should be nothing at the centre of the span and the point of sus- 

 pension, and gradually increase up to some intermediate part. The 

 highest and lowest points of a catenary may be always chosen 

 arbitrai'ily. 



The argument that the chain would not be distorted because it 

 is "of such depth as to include any alteration in the curve," is 

 vague and inconclusive. It certainly cannot stand ground against 

 deductions from the fundamental equations of statical equilibrium. 

 The reasoning given above is a simple, ordinary application of the 

 elementary principles of mechanics ; these are not to be opposed 

 by a mere hypothesis, which is too subtle to be made the suliject of 

 rigid investigation. All that can he said of this hypothesis is, that 

 it is not neces.mrily true. A number of independent chains might 

 be hung from the abutments, and to each might be given that form 

 which it would, if perfectly free, assume of itself when the load 

 at some particular stage of its transit hung from that chain. alone. 

 Then it is clear that, while no connection existed among the se- 

 veral chains, the load acting on each in succession would not tend 

 to distort any of them, i. e. would not produce transverse strains. 

 But it does not follow that this would be the case when all the 

 chains were bound up together in one connected mass. 



The " inverted arch bridge" does not seem to be an advantageous 

 compromise between the principle of the girder and the suspen- 

 sion chain. An intermediate condition misses the advantages of 

 both those structures : for if the inverted arcli be only partially 

 rigid it is subject to needless and prejudicial transverse'strains; — 

 if it be as rigid as a girder, why unnecessarily increase its length 

 by curving it.? The idea of our correspondent that the curvature 

 obviates its tendency to deflect by its own weight, seems to us un- 

 founded ; for however much the structure was bent when lirst 

 put' up, it would tend to bend still more when its ends merely 

 rested upon the abutments. We cannot positively undertake to 

 assert that the suspension of a curved beam has no peculiar advan- 

 tages ; hut they have not yet been pointed out, and we are unable 

 to discern them. — Editois. 



