1846.] 



THE CIVIL ENGINEER AND ARCHITECT'S JOURNAL. 



83 



near low water only — a period at which the delirery upon the shores 

 most offensive to the public ; »ud although this part of the evil might iu 

 some measure he remedieil by the acloptioa of the plans proposed by Mr. 

 AValker and Mr. I'a;.',e, for extending liie existing sewers into low water, 

 still the contamination of the ri»er would remain the same. At all other 

 states of the tide, the sewage is pent up and held back, these large sewers 

 becoming reservoirs, to contain the quantity collected during tliose periods 

 iu which the height of Ike tide prevents its discharge into the river. 



Au'ilher evil now arises, not more offensive than injurious : the foul air 

 Dot beingcarried off with the current into the river, necessarily and naturally 

 rises through any openings it can find, and a stream of noxious effluvia is 

 evolved. 



A further disadvantage resulting from this penning up of the sewage, is, 

 that the current through the sewers being checked, the water becomes qui- 

 escent, and the heavy particles previously held in mechanical suspension 

 are deposited, and accumulate. When the current in the sewers re-com- 

 mences, it is slow, depending upon the rate at which the tide falls into the 

 river. Until it has fallen below the level of the pent-up sewage, there is 

 no fall in the sewer. Then as the tide falls inch by inch, so docs the fall 

 in the sewer increase, but not in the same ratio, because the water being 

 always running, the relative difiVrence between the two levels is always 

 diminishing. The case would be ditlerent were the sewer water held back 

 by mechanical means, until there was a sufficient fall of tide in the riter ; 

 but it is not so— the process is gradual, and no fall is obtained sufficient to 

 scour away the accumulated deposits. Hence the necessity of manual la- 

 bour to clear away these offensive deposits, which must be brought to the 

 surface, or for machinery for carrying it off by Bushing, and the demand for 

 larger supplies of water, which vs'ould be useless unless there was an unin- 

 terrupted current in the sewers. 



The following is an outline of Mr. Wicksteed's own plan of making the 

 sewage manure available. He proposes to collect it in one large reservoir 

 where it is to be dried and packed like gunao for sale. As regards tlie 

 northern side of the Thames, it is proposed to construct a Circular Sewer 

 of eight feet diameter ; to extend from the end of Gros»enor-ro:Kl, to pass 

 through, in an easterly direction, TothiU-street, Westminster Abbey-yard, 

 King-street, Whitehall. Strand, to the end of Fleet-street; from tlience a 

 sewer of twelve feet diameter in continuation across through Ludgate-hill, 

 the south side of St. Haul's Church-yard, Watling-stieel, Cannon-street, 

 Tower-hill, l?atcliffehigh%vay, Commercial-road, under the river Lea, above 

 the Iron Bridge, and from thence in a straight line through the West Mam 

 5Iarshes, to the proposed works, iu an angle formed by the western banks 

 of Barking Creek and northern banks of the Thames. 



The necessary communication between the present sewers and the inter- 

 cepting sewer will be effected by means of shafts from the top of the pro- 

 posed sewer to the underside of the existing sewers, so that whatever so 

 flows through the present sewers must flow into the intercepting sewer ; 

 unless in case of long continued rains or storms, when if much more than 

 double the usual quantity of water should pass down, then 

 as soon as the intercepting sewer is fully charged, the 

 surplus water would run oil through the old channels into 

 the Thames. The sewers will, however, at these times, be relieved by 

 an additional outlet of large capacity, the proposed sewer forming a com- 

 munication with the Thames at Barking. All the flaps at the mouths of 

 the present sewers will have to be made water-tight, to prevent the wa- 

 ter of the Thames flowing at high water into the intercepting sewer, unless 

 required, although this, of course, will not prevent their being available tor 

 allowing the surplus waste to flow into the Thames when necessary. 



The new sewer has a fall of 12 inches per mile, which wU\ give a velo- 

 city of 120J feet per minute in the 12 feet sewer, and 982-10 feel per 

 minute in the S feet sewer. The main sewer will terminate in a receiving 

 reservoir, in the Barking Marshes. The engines will be equal to an aggre- 

 gate power of 1,060 horses, and will be capable of raising, wlien worl^ed at 

 their full power, 56 feet high, 18,112,320 cubic feet in 2-1 hours, equal to 

 more than 2^ times the present ordinai'y quantity of sewer water. Tiie 

 sewer water will be raised into reservoirs sufficiently elevated (o allow of 

 its solid contents being deposited at a level above the Trinity high water 

 mark, so that it can easily be shipped, or loaded into railway trucks, and 

 that the refuse liquid may be discharged at all states of the tide. And 

 lastly, the deposit iu tlie reservoirs will be removed periodically, and dried 

 by artificial means, and then compressed and packed up, ready for irans- 

 missiuu by land or water. 



TUBULAR BRIDGE OVEK THE MENAI. 



The following is an abstract of the report on this gigantic undertaking, 

 made by Mr. Robert Stephenson to the Chester and Holyhead ltail«aj 

 Company, and read at their last meeting. 



" I have throughout the experiments carefully studied the results as tl'cy 

 developed themsehes, and I am satisUed that the views i ventured to ex- 

 press twelve months ago were in the main correct, and tliat the adoption 

 of a wrought- iron tube is the most efficient, as well as the most economical 

 description of structure that can be devised for a railway bridge across the 

 Menai Straits. 



" In the course of the experiments, it is true, some unexpected and ano 

 malous results presented themselves; but none of them tended, in my mind 

 to show that the tubular form was not the very btst fur obtaining a 



rigid roadway for a railway over a span of 450 feet, which is the absolute 

 requirement for a bridge over the iMenai Straits. 



" The first series of experiments was made with plain circular tubes, the 

 second with elliptical, and the third with rectangular. In the whole of 

 these, this remarkable and unexpected fact was brought to light, viz., that 

 in such tubes the power of wrought-iron to resist compression was much 

 less than its power to resist tension, being exactly the reverse of that which 

 holds with cast-iron ; for example, in cast-iron beams for sustaining weight, 

 the proper form is to dispose of the greater portion of the material at the 

 bottom side of the beam, whereas, with wrought-iron, these experiments 

 demonstrate beyond any doubt that the greater portion of the material 

 should be distributed on the upper side of the beam. We have arrived 

 therefore at a fact having a most important bearing upon the construction 

 of the tube, viz., that rigidity and strength are best obtained by throwin" 

 the greatest thickness of material into the upper side. 



" Another instructive lesson which the experiments have disclosed is 

 that the rectangular tube is by far the strongest; that the circular and ellip- 

 tical should be discarded altogether. 



" This result is extremely fortunate, as it greatly facilitates the mechani- 

 cal arrangements for not merely the construction, but the permanent main- 

 tenance of the bridge. 



" We may now, therefore, consider that two essential points have been 

 finally determined — the form of the tube and the distribution of the ma- 

 terial. 



" The only important question now remaining to be solved is, the abso- 

 lute ultimate strength of a tube of any given dimensions. This is, of cour.-e, 

 approximately solved by the experiments already completed ; but Mr . 

 Hudgkinson very properly states, tliat others, with tubes of more varied 

 dimensions, should be continued, iu order to clear up some anomalies which 

 still exiit. 



" The formula, as at present brought out by Mr. Hodgkinson, gives the 

 strength of a rectangular tube of the dimensions I proposed — viz , 450 feet 

 long, 15 feet wide, by 30 feet high (assuming the plates to be one inch 

 tiiick) equal to 1,100 tons applied in the centre, including the weight of the 

 tube Itself ; but, deducting the latter, equal to 747 tons in the centre, or 

 double this, supposing the weight to. be uniformly distributed over the 

 whole 450 feet. 



"This amount of strength, although sufficient to carry any weight that 

 can in practice be placed upon the bridge, is not sufficiently in excess for 

 practical purposes. It is on this ground, therefore, I have requested Mr. 

 Hodgkinson to devise a few more experiments in the shape best calculated 

 to free the formula from all ambiguity. In the meantime, however, as I 

 consider the maiu question settled, I am proceeding with the designs and 

 working plans for the whole of the masonry, which I expect to have the 

 pleasure of submitting to you in a fortnight from this time. 



" You will observe in Mr. I'airbairn's remarks, that he contemplates the 

 feasibility of stripping the tube entirely of all the chains that may be re- 

 quired in the erection of the bridge; whereas, on the other hand, Mr. 

 Hodgkinson thinks the chains will be an essential, or at all events a useful 

 auxiliary, to give the tube the requisite strength and rigidity. This, how- 

 ever, will be determined by the proposed additional experiments, and does 

 not interfere witii the construction of the masonry, which is designed so as 

 to admit of the tube with or without the chains. 



" The application of chains as an auxiliary, has occupied much of my 

 attention, and I am satisfied that the ordinary mode of applying them to 

 suspension bridges is wholly inadmissible in the present instance ; if, 

 therefore, it be found hereafter necessary or desirable to employ them iu 

 conjunction with the tube, another mode of applying them must be devised, 

 as it is absolutely essential to atfach them in such a manner as to preclude 

 the possibility of the smallest oscillation. 



" In the accomplishment of this I see no difficulty whatever; and the 

 designs have been arranged accordingly, in order to avoid any further 

 delay. 



"The injurious consequences attending the ordinary mode of employing 

 chains in suspension bridges were brouglit under my observation in a very 

 striking manner on the Stockton and Darlington Railway, where I was 

 called upon to erect a new bridge 'or carrying the railway across the river 

 Tees, in lieu of an ordinary suspension bridge, which had proved an entire 

 failure. 



" Immediately on opening the suspension bridge for railway trafHc, the 

 undulations into which the roadway was thrown, by the ineviiatile uu- 

 equal distribution t»f the weight of the train upon it, were such as to 

 tlireaten the instant downfall of the whole structure. 



" These dangerous undulations were most materially aggravaletl by the 

 chain itself, for this obvious reason, — that the platform or roadway which 

 was constructed with ordinary trussing for the purpose of rendering it 

 comparatively rigid, was suspended to the chain, which was perfectly 

 flexible, all the parts of the latter being in equilibrium. The structure 

 was, therefore, composed of two parts, the stability of the one being totally 

 incompatible with that of the other ; for example, the moment an unequal 

 distribution of weight upon the roadway took place, by the passage of a 

 train, the curve of the chain altered, one portion descending at the point 

 immediately above the greatest weight, and consequently causing some 

 other portion to ascend in a corresponding degree, which necessarily raised 

 ihe platform with it, and augmented the undulation. 



" So seriously was this defect foual to operate, that immediate steps 

 were lakeu to support the platform underneath by ordinary trussing ; ill 



11* 



