Mr. Seaward oti Suspension Chain Bridges. 427 



Notwithstanding the great change that has been made from 

 the primitive plan of the suspension bridge as above described, 

 still the original form of the susjjending chaiji has been in- 

 variably preserved : why it has been so preserved is not easy 

 to determine, because it is quite certain that a suspension 

 bridge can be built equally well without having the suspend- 

 ing chains in that paiticular form. 



Fig. 2 is the elevation of a suspension bridge, wherein the 

 catenary curve is not employed ; the platform of the roadway 

 being supported by straight diagonal rods attached to the tops 

 of the two towers. A suspension bridge may be built on this 

 plan, with the same quantity of materials, that shall possess 

 double the strength of one formed on the common plan, as 

 will be fully demonstrated in the course of the following ob- 

 servations. 



It is well known that the strain or stress on the chain at 

 the two points A and B (fig. 1.), supposing them to be at the 

 same altitude, is as the co-secant of the angle CBH, formed 

 by the direction of the chain, at the point of suspension, and 

 a horizontal line BH : radius being as half the weight of the 

 whole bridge, including the chains and all adventitious load- 

 ing. Now as it is of great importance to reduce as much as 

 possible the height of the towers, it generally happens that 

 the aforesaid angle CBH becomes very small ; and conse- 

 quently its co-secant very considerable, compared with ra- 

 dius; which is the reason that in many cases the strain at 

 the points of suspension is increased to three or four times the 

 absolute weight of the whole bridge. 



But v/hat is particularly deserving of notice, is, that it is not 

 the middle part only of the roadway and loading that has to 

 be supported under this small disadvantageous angle CBH; 

 but the whole roadway and loading fi'om end to end (E to F) 

 have to be thus supported. But if, instead of employing the 

 catenary curve, the roadway were to be suspended by a num- 

 ber of straight diagonal rods oB, 6B, cB (fig. 2.), and sup- 

 posing each rod to support an equal portion of the load, it is 

 plain that the stress upon the different rods will vary consi- 

 derably ; for although the strain upon «B, 6B, &c. (where the 

 angle aBH, iBH, &c. is small) would still be very great; yet 

 when we come to the rods AB, zB &c., we shall find that the 

 strain upon them would be reduced to about one quarter of 

 what it is on wB, 6B, &c., which is a most important consider- 

 ation ! 



And again : supposing (what is not true) that the strain 



upon the whole of tlie diagonal rods at B (fig. 2.), taken to- 



,'J 1 1 2 gether, 



