418 A Resultant System for the [Oct. 



therefore of the structure must depend on the capability of the longi- 

 tudinal beams to resist tension. 



Mr. Dredge has in fact carried the principle too far, and has conclud- 

 ed that, because the lowest point of a chain is that of least tension, 

 such an arrangement may be effected by which there shall be none at 

 all. He has also assumed perfect rigidity for his platform, which is 

 composed of a flexible combination, and which, if in the slightest degree 

 displaced, causes collapsion of the whole. 



15. The third defect in the extreme Taper chain system is the 

 great obliquity of the central auxiliaries, and the great difference in the 

 angles of obliquity ; varying from 10° at the centre to about 65° at the 

 standards ; the strains to which they are exposed by equal weights are 

 consequently very unequal. This conclusion hardly requires elucidation, 

 but the subjoined diagram (Fig. 3.) drawn to a scale, and on the princi- 

 ple that, when thrte forces are in equilibrio the strains in each direction 

 are proportional to the sides of a triangle in the direction of the forces, 

 shows the actual tension on the central oblique rod, and in that nearest 

 the standard, of a bridge constructed strictly on Mr. Dredge's system, 

 the angles of attachment being 59° 19 ; at the standards, and 9° 30 ; at 

 the centre. (See Fig. 3) or as in Fig. 4, the weight being in both 

 cases expressed by unity. (See Fig. 4). 



The tension on the first oblique rod from the pier will be 1.18 and 

 the horizontal tension 0.6, whilst that on the central oblique rod will be 

 6.14, and on the horizontal line 6.05, so that equal sections of iron 

 are strained in the proportion of 6 to 1 . 



16. The advantages of the above system are, first, that a considera- 

 ble portion of the platform is supported by rods direct from the stand- 

 ards, thus leaving a diminished tension due to the chain, and secondly, 

 by the oblique action of the auxiliary rods the system is retained under 

 the dominion of a certain amount of Tension, rendering the roadway 

 free from the injurious effects of undulation and vibration, and making 

 the transit more firm and pleasant. 



17. The defects of the Uniform chain system are, 



1st. The whole weight of the bridge is supported by the chains, 

 rendering them very heavy, massive and costly, as also more suscepti- 

 ble of receiving the impulse, which in storms is the primary cause of 

 the destructive motion given to the roadway. 



