426 A Resultant System for the [Oct. 



32. In paragraphs 24, 25, the principle that is to guide the con- 

 struction of the longitudinal beams has been given, viz. as the third 

 force acting by tension horizontally to preserve the equilibrium with the 

 oblique force and that of gravity ; and in paragraph 9, full explanation 

 of the reason of the above arrangement has been entered into, and it 

 has also been shown that provision can be made to meet the several 

 amounts of tension acting on the beam in the horizontal line. If this 

 were all that the longitudinal beam had to perform, a construction 

 similar to Fig. 10, would answer the purpose, and the section of the 

 different portions might diminish from the centre, towards the standards 

 in proportion to the variation of the strains produced by the auxiliaries, 

 but as these beams are intended to bear the vertical weight of the 

 platform together with the heavy traffic load, and other contingencies, 

 a compact or uniform section should be retained in bridges of small 

 span equal to that demanded at the centre, which will be the most 

 advantageous to the system, and facilitate the actual construction, 

 though in larger spans a considerable reduction of section may be 

 effected between the centre and standards. 



33. The "Resultant" system as above elucidated, cannot surely 

 fail to present many valuable points for recommendation, professing, as 

 it does, practically to coincide with the theoretical and analytical con- 

 clusions of the author of the " Memoir" under notice, and moreover, 

 whilst it is divested of the positive defects of both the systems which 

 have been simultaneously reviewed, a powerful resultant is obtained 

 from the composition of the advantages or forces of each of them. 

 This system has been somewhat hastily " damned with faint praise," 

 by some, because they would not take the trouble to ascertain its prin- 

 ciples of construction ; it has been passed over by others, from absolute 

 inability to understand them, simple as they are, but from what has 

 been shown above it will be clear that, with the condemnation of the 

 "Resultant" system, the uniform must be included, the latter being 

 nothing more than an extreme case of the general system in which the 

 strain on the chain is a maximum, and the horizontal tension is 0, 

 whilst the system of Mr. Dredge in a way aims at, (but does not 

 attain,) the opposite extreme, where the tension on the chain is a mini- 

 mum, and that on the horizontal line a maximum. 



34. It now remains to show another advantage of the "Resultant" 

 system with a diminishing chain. The annexed Fig. 17, is the con- 



