1848.] Construction of Iron Tension Bridges. 423 



the centre, which has the pull of 3+2 + 1 acting on it ; the connecting 

 link between 2 and 3, being strained with the tension of 2+1, and that 

 between the parts 1 and 2, with the strain due to the part 1 only. Now 

 the outer longitudinal beams of the system stand in the place of the 

 connecting links of the above Fig. 10, and are exposed to the varying 

 tensile forces as described along the whole length, the amount of each 

 of which admits of easy calculation, and whilst the precise spot of the 

 greatest effect can be exhibited, the exact amount in every portion of 

 the system can be accurately ascertained, and consequently provided for. 



25. The following Figs. 11 and 12, will show the relative tensions 

 in the oblique and horizontal directions, in both Mr. Dredge's and the 

 present "Resultant" systems. Fig. 11, showing the strains where the 

 oblique rod angles vary, as practised by Mr. Dredge from 10° to 60°, 

 and Fig. 12, the strains where the variation of the angles is only from 

 25° to 45°. (See Figs. 11 and 12). 



The force of gravity being represented by unity in both cases the 

 extreme difference in the amount of tension in the oblique rods of Mr. 

 Dredge's combination is as 5 to 1, and in the horizontal beam as 10 to 

 1, (Fig. 11.) whilst in the "Resultant" system under adoption, as 

 shown in (Fig. 12.) the variation of tensions in either direction between 

 the centre and standard is as 1.4 to 2.2 greatly to the advantage of the 

 latter. 



26. Now to apply the same principle of the composition of forces 

 to the chain, so that the system may be in equilibrio. The span, width 

 of roadway, its construction, the spaces between the oblique rods, and 

 angle of the central one being determined, the weight to be assigned to 

 each set of auxiliaries may be safely assumed at 120 ibs. per square 

 foot of platform, including the weight of the structure. 



27. The tension on the centre, or horizontal link may be arbitrarily 

 assumed, i. e., it may be made any proportion of the link at the point of 

 suspension, thus tapering the chain ^d, ^th or nth, part of the sectional 

 area of the upper link, for it is evident that by the arrangement of the 

 angles formed by the first link from the centre and first set of oblique 

 rods, the strain on the centre link may be =0, or =1000 tons, as is 

 shown in annexed Figs. 13 and 14, where it is clear (Fig. 13.) that the 

 tension on the centre link c. b. is increased or diminished as the line 

 c. e. (the prolongation of a. c.) approaches nearer to c. b. or c. d.; 



