1846.] 



THE CIVIL ENGINEER AND ARCHITECPS JOURNAL. 



11 



deration all the more praiseworthy hecause it forms a nearly unique con- 

 trast to the extravagant encomiums which inventors usually bestow on 

 their own productions. As, however, on consideration, we think we can 

 discern advantages in the present invention which are not quite apparent 

 at first sight, [and which are^not commented on in the description which 

 we have received, we have ventured to give our own version. The inventor 

 ■will have no cause to complain of the determination. 



It is not, however, to be understood that on a] subject so complicated as 

 the theory of suspension bridges, any judgment, however carefully con- 

 sidered, can have'xtlio same value as one pronounced ■ in simpler cases. 

 The general problem of the equilibrium of suspended chains is so dldicult 

 ■when applied to practice, that it is impossible to predict with certainty 

 the exact practical efl'ect of any untried arrangement. However, as in 

 the present instance, the inventors do not apply themselves to the compli- 

 cated questions respecting the relation of the various tensions to the 

 strength of material, but confine their attention to the means of preventing 

 oscillationt and undulations of the platform, the subject can be satisfac- 

 torily examined without the use of mathematics. 



The leading idea of the invention is so simple, that a very few words 

 will suffice to explain it. Each chain of the ordinary suspension bridge 

 is replaced by two lighter chains crossing each other. Each of these cross- 

 ing chains is suspended by its highest and lowest points, aid the two may 

 also be fastened at their points of intersection. The suspending rods are 

 fastened to each chain alternately, ;and the edge of the platform will be 

 therefore supported, throughout its length by a series of alternately long 

 and short rods. 



Every practical 'engineer 'knows 'the importance of strengthening a 

 structure by supports or ties arranged cross-wise. The roofs of railway 

 stations may be referred toas familiarjnstauces of the value of cross lies. 

 Wooden bridges, in which the beams are arranged diagonally, and cast 

 iron girders, with] cross ribs, are examples of the same principle. In factt 

 it may be laid down as a general principle that all structures where the 

 points of support are at considerable distances from each other, derive 

 stability and rigidity from a system of oblique rods or beams. 



Let us now see how this principle applies to the case before us. The 

 disturbances to which a suspension bridge is liable, from external forces, 

 are of two kinds — first, oscillation, or swinging of the chain from side to 

 side — secondly, undulation, or the vertical rising and falling of the several 

 parts of the platform. Now, with respect to this first kind of disturbances 

 it is obvious that a chain suspended from the tops of two piers must be 

 more pendulous than one fastened by its highest and lowest point. That a 

 catenary suspended by its two highest points is readily set in motion, and 

 can be maintained in motion by a slight force, may be readily seen from 

 the familiar instance of a child's swing. But it may also be seen from 

 the same instance, that if the lowest point of the curve be fastened, the 

 liability to oscillation is almost entirely removed. In the invention before 

 lis, this method of preventing oscillation is adopted, and the advantage is 

 further increased by the union of the two cross chains at their points of 

 intersection. 



The second kindof disturbances of suspension bridges — theundulatory — 

 is by far the most important. The height of the undulations of the Menai 

 bridge in a storm has been known to be some 8 or 10 feet, while the trans- 

 Terse oscillations were scarcely observable. By far the most important 

 part, therefore, of the invention of intersecting suspension chains is the 

 means they afford of preventing the rising and falling of the platform. 

 This effect is, however, not immediately obvious,' and we shall perhaps 

 have some little difficulty in rendering the explanation of it intelligible. 



What we wish to show is this — that theeflTect of two chains, both united 

 to the platform, is such that, where the deflection of the one chain would 

 tend to raise the platform, the other chain would tend to depress it — so 

 that the two chains would, during the passing of a load, counteract the 

 tendency of each to elevate or depress the several parts of the roadway. 



To make the point clear we will refer to the figure of the single-span 

 bridge in the accompanying plate. We will suppose that a heavy carriage 

 has got some distance across the bridge, but has not yet reached the centre 

 of it. Now let us see what is the effect of this load on each chain sepe- 

 ratelij — first, for instance, oa the chain which at this place is the highest. 

 The load rests on the platform, and tends to sink it, and this tendency is 

 communicated by one of the long rods to the upper chain. If, however, 

 the action of the rod pull down the chain at this point, we know by the 

 nature of suspended chains, that the whole of the curve will be acted upon, 

 rising at some"points and sinking at others. Now let us see what the 

 effect of the passing load is on^that chain which at the place in question is 



lowest. Here the effect of the load is transmitted by one of the short rods, 

 which tends to depress this chain also ; and here again,5 if the chain be 

 actually depressed, the whole curve will be acted upon, [rising at some 

 points and sinking at others. But if the matter be attentively considered* 

 it will be found that the parts of the one chain which tend to rise are 

 directlij above those parts of the other chain which tend to sin/c, so that these 

 tendencies are opposite to each other, and are counteracted by the 

 attachment of both chains to the platform. 



Or, to express the same thing another way, a passing load tends to alter 

 the whole curvature of both chains — to draw the two chains, in some 

 places, closer together, and, in others, wider apart, than they are in their 

 positions of equilibrium. In the ordinary single-chain suspension bridges 

 there is nothing but the weight of the chain and platform to resist this 

 tendency to alter the forms of the curves; but in this double-chain bridge 

 the alteration is resisted by the tensions or thrusts of the connecting rods. 



If we have succeeded in making this point clear, the reader will see 

 that these counteractions occur, not at one or two points of the bridge, but 

 at every point of it. Everywhere therefore there will be a tendencyito 

 bring the two chains closer together, or move them wider apart, and these 

 effects are everywhere prevented by the connexion of the vertical rods, each 

 of the rods exerting either a tension or thrust to resist the effect of the 

 passing load. The two curves cannot either recede from or approach 

 each other on account of the rods, so that the case is altogether different 

 from thatjofithe ordinary suspension bridge; for there a passing load 

 acts only on the suspending rods immediately adjacent to it— but here 

 every vertical rod of the whole bridge is acted upon by the load, and resists 

 its tendency to depress the platform. 



The effect therefore seems to be to convert the whole structure into a 

 system almost perfectly rigid, to give it, without any additional weight of 

 iron, nearly the stiffness and stability of a girder. This is by far the most 

 important part of the invention, and deserves the careful consideration of 

 the engineer. 



It is a point worth noting, that the vertical rods would not always be in a 

 state of tension as in other suspension bridges, but occasionally would have 

 to resist compression also, (where the load tended to bring parts of the two 

 curves closer together), and therefore these rods would have to be made 

 with more than the usual rigidity. 



Of course on so complicated a subject as that of suspension bridges, no 

 opinion on practical points is perfectly trustworthy till verified by experience. 

 We cannot determine exactly to what extent rigidity might be attained in 

 a suspension bridge on this new principle, still we think enough has been 

 said to show that the rigidity would certainly be far greater than what can 

 be obtained in the common suspension bridges, without additional stiffening 

 by strong diagonal ties. These greatly increase the weight of the structure, 

 and are generally inapplicable from the difficulty of finding convenient points 

 for attaching them. 



BEVIBV^S. 



Companion to the Almanac, or Year Book of Information for the year of-^t 

 our Lord 1840. London : Charles Knight, Vimo.ipp. 260. 

 This is the nineteenth of a series of annual volumes published under the 

 superintendence of the Society for the Diffusion of Useful Knowledge. 

 The information in this volume is collected under two heads — first, "mathe- 

 matics, natural history and philosophy, chronology, geography, statis- 

 tics," &:c. — secondly, " the legislation, statistics, public improvements, 

 and chronicle of 1845." The first chapter is on the " earliest printed al- 

 manacks," and treats very fully of the history and successive improve- 

 ments of the calendar. This paper is written by Prof. De Morgan ; it 

 contains, however, no display of his mathematical genius, but exhibits 

 a vast amount of antiquarian erudition. Wc have chosen one or two short 

 extracts, as well for the curious nature of the information they afford, as 

 to give a general idea of the style of the paper. Speaking of the difficul- | 



ties which the Church experienced in determining the time of Easter, and 

 reviewing the various remedies proposed for these difficulties, it is ob- 

 served — 



" Regiomontanus is the next after Bacon who declared that the nsus ecclesim 

 was not in accordance with the decrela patrum ; he does not attempt the '■ 



usual historical falsehood of fixing the existing method upon Eusebius and j 



the other Nicene bishops, but refers it (and truly, as we showed in our last 



