May 3, 1888] 



NA TURE 



bridges, described and illustrated in the second section of 

 the work. But while in the animate kingdom the mammoth 

 animals have become extinct from insufficient mobility 

 and relative strength to carry their own weight, the con- 

 verse operation is observable in engineering construction. 

 Bone and muscle are of the same strength as formerly, 

 but the improved manufacture of steel has placed in the 

 hands of the engineer a material with which he can safely 

 attempt his mammoth creations ; and should metal- 

 lurgical science provide commercially for the engineer a 

 new metal, as strong as, or stronger than, steel, but of less 

 weight — say, aluminium — then we may expect to see still 

 more marvellous developments in bridge building. 



The bridge, on a large scale, resembles the mammoth 

 or giant in requiring its whole strength to keep itself up- 

 right ; and one of the most interesting theoretical ques- 

 tions discussed in the present treatise is the consideration 

 of the maximum span possible with the material in hand — 

 say, steel. When the span is large, the greatest economy 

 in details must be practised, as the chief stress is due to 

 the dead weight of the bridge, and not to the relatively 

 insignificant weight of the moving load. Thus in the Forth 

 Bridge a weight of 20,000 tons of steel is required in a 

 single span to provide it with the necessary strength to hold 

 itself up, so that the stresses due to a train of 200 tons 

 running across may be left out of account. 



The weight of metal worked into a bridge is at once 

 a measure of the stresses in the material, and also of the 

 quantity, and consequently the cost, of the material used. 

 The author employs the customary units of engineers, 

 the pound or ton as a measure of force and of weigh, 

 and measures stresses in pounds or tons per square inch. 

 He does not find it necessary to express his stresses in 

 poundals per square foot, nor does he measure quantity 

 of material in units of mass, which are g pounds or tons, 

 as we are taught in theoretical text-books. 



The mathematical student, to whom the book is 

 partially addressed, will find it, while valuable as a hand- 

 book for a practical engineer, at the same time stimu- 

 lating to his imagination in the realms of pure Abstract 

 Mechanics, which at present run the risk of wandering 

 away from reality, because the writers of modern text- 

 books of mathematics do not look to the wonderful 

 creations of modern engineering science for illustrations 

 of theory. Thus the methods of Graphic Statics, largely 

 employed in this treatise, arose out of the requirements 

 of an engineer's office : a draughtsman was found using 

 the method, and Prof. Maxwell seized upon it and elevated 

 it to the rank of a new method in Mechanics. 



Scientific treatises on Practical Mechanics are more 

 common in America, where the requirements of opening 

 up a vast continent have given great employment to the 

 engineer and the bridge-builder ; and it must be owned 

 that these treatises are far superior to our own. But we 

 hope the present treatise will do something to take away 

 this reproach. 



We may flatter ourselves that the Forth Bridge 

 now in progress is the greatest thing of the kind in 

 the world, but a rival in the Poughkeepsie Bridge 

 is projected. These two bridges will exemplify the 

 difference of practice of the Old World and the New. 

 In our practice the whole bridge is riveted up into a rigid 

 structure as much as possible ; while in America the 



articulated system of triangular cells, with pin joints per- 

 mitting rotation, is adopted, the stress in individual 

 members being thus a simple pull or thrust. So far the 

 American system has scored one in securing the contract 

 for the Hawkesbury Bridge in Australia. This system 

 affords the best theoretical illustrations of elementary 

 Statics— the subject of Part I. of the present treatise —until 

 the question of the bending moment (it is gratifying to 

 find the term "tendency to break" of the abstract 

 treatises discarded) comes into consideration, when the 

 Old World bridge affords the requisite illustrations. 



In Part 1 1 1., on the " Strength of Materials," the author 

 begins with the resistance of columns and struts to flexure, 

 and here theory and practice have long worked together 

 almost in harmony. The expression " breaking load " of 

 a column — to mean the load which just starts flexure of 

 the column — is apparently usual, but like the expression 

 "tendency to break" should now be discarded for some- 

 thing more suitable. The theoretical strength of a column, 

 according to Euler, which requires the assumption that the 

 column is \m\\a.\\y ftcrfect/y straight, and the actual strength 

 against flexure, are represented in a diagram (p. 160) ; and 

 the author has shown very ingeniously how the actual state 

 of things encountered in practice can be imitated theo- 

 retically by a strut composed of two flanges of unequal 

 elasticity (p. 163). Such a strut will begin to curve imme- 

 diately as the load is gradually applied, and will thus repre- 

 sent very closely the actual behaviour of a continuous 

 column, as great variations are found experimentally in the 

 elasticity of iron or steel in specimens cut from one piece 

 of metal (p. 167). When crushing or tearing takes place 

 from continually applied pressure or tension, only em- 

 pirical formulae are suitable ; but, as in actual structures 

 the stress is kept by Board of Trade rules much below the 

 elastic limit, the theoretical equations depending essen- 

 tially on Hooke's law, that Tension and Extension are 

 in the ratio of the Elasticity of the material, may be 

 employed. Even with the low stresses permissible 

 by law, Wohler's researches on the fatigue of metals 

 show that permanent deformation may keep on accu- 

 mulating, and, in consequence, modern engineering 

 practice is in some respects not so daring as formerly. 

 Gordon's empirical rules (§ 124) (originally due to 

 Tredgold) have been shown by Prof. J. H. Cotterill to 

 rest on a theoretical basis, if the compression of the 

 material due to the thrust previous to flexure is taken 

 into account. 



For very long spans, the only two rival methods of con- 

 struction are the cantilever and the suspension principles, 

 of which the Forth Bridge and the Brooklyn Bridge are 

 the great respective examples. In the Cantilever method 

 we build out equally on each side of a pier, so as always to 

 preserve stable equilibrium, while in the suspension method 

 the roadway is suspended from the chains or steel ropes. 

 The chief drawbacks of the suspension principle, its 

 defect of stiffness and great sensibility to changes of 

 temperature, are shown by the author to be avoidable by 

 the system of bracing in his "rigid suspension bridge'' 

 (Fig. 22). 



The disastrous fall of the Tay Bridge Viaduct in a 

 hurricane has forcibly redirected the attention of en- 

 gineers to the importance of the theory of wind-pressure 

 and wind-bracing (Chapter XXIV.), and now we may 



