CHAP. IV., 3.] 



MECHANICS. YOUNG TELFORD. 



wrought iron, one inch square and ten feet long, ex- 

 tended TO 1^7 of its length for every ton of weight 

 up to 12 tons, from which point the extensions nearly 

 doubled successively for every two tons of load, and 

 the bar was finally torn asunder by 23 tons. 

 (346.) The compression of bodies proceeds (like the ex- 

 empress- tension) at first uniformly with the load. Some bo- 

 dies resist compression more than extension (as cast 

 iron); some the reverse (as wrought iron). Sub- 

 stances give way under compression after different 

 fashions. Hard bodies divide into prisms parallel to 

 the compressing force ; slender elastic bodies bend 

 laterally ; soft bodies bulge horizontally ; bodies of 

 a medium hardness divide into wedges, and the sur- 

 faces slide along the plane of spontaneous fissure, 

 etrusion. Detrusion marks more particularly the mode of giv- 

 ing way by the sliding of surfaces in the interior of 

 solids. Though seldom due to force directly applied, 

 it is an important element in most cases of the rup- 

 ture of semiductile solids. 



(347.) The force of flexure is that by which the resist- 

 lexure of ance of the greater number of solids is most easily 

 overcome, but which it is of most importance to re- 

 sist ; as when a beam is fastened by one end into a 

 wall, and loaded at the other, or when it spans a 

 horizontal space. It had not escaped the notice of 

 James Bernouilli, Duhamel, and other writers of the 

 earlier part of the 17th century, that the fibres on 

 the concave side of a loaded beam are in a state of 

 compression and not of extension, and that there is 

 therefore a point, or rather a line, in every beam, in 

 which the fibres are neither extended nor compressed. 

 But the clear modification of the theory prevalent 

 in the time of Leibnitz and Marriotte which this 

 consideration introduced, was probably first deve- 

 loped by Coulomb, Robison, and Young, who in their 

 respective publications insisted upon it with great 

 judgment ; and it is difficult to overrate its import- 

 ance in mechanical engineering, although the first 

 great canon of Galileo remains still true, that the 

 ultimate strength of a solid rectangular beam varies 

 as the breadth and as the square of the depth. 

 The writings of Young and Robison did not im- 

 mediately attract the attention of practical men, 

 and Coulomb, who was by far the ablest French 

 experimenter on subjects of mixed mechanics, 

 seems to have done less on the theory of strains 

 producing flexure than in the case of torsion, which 

 he studied with so much success, and applied to 

 such excellent purpose. Nevertheless in his memoir 

 on the Resistance of Masonry, in the 7th vol. of the 

 " Memoires Presentts" (1776), he had already laid 

 down very clearly the effect of compression on abeam. 1 



It is, however, to Young that we owe the application 

 of these principles in unfolding their legitimate conse- 

 quences. In a series of remarkable propositions con- 

 tained in the writings I have quoted (344), he assigns 

 numerical relations between the flexure of a beam 

 under almost every supposable circumstance, and 

 the resistance of the material to direct strains. These 

 results have been extensively used by all subsequent 

 writers. They are not equally verified in all classes of 

 substances. This, however, is not wonderful ; flexure 

 is not due to direct compressive and extending strains 

 alone ; deformation may take place in a solid without 

 appreciable change of density, thus giving rise to 

 some of the nicest questions in molecular physics. 



The laws of Torsion, as laid down by Coulomb, (348.) 

 have been mentioned in the last section (340). Torsion. 



The mathematical investigations of Young on (349.) 

 mechanical problems were conducted with bold di- Young's 

 rectness and in defiance of the generalizing methods me f h . anical 

 and symmetrical notation of foreign writers on such 

 subjects. But his pre-eminent sagacity in laying hold 

 on the salient points of the questions he discussed, 

 and in conducting his argument to a practical con- 

 clusion, was unequalled, and deserves imitation. 2 



A great revival in the study of the properties of (350.) 

 elastic matter, as regards strength, took place about General 

 the year 1820, probably in consequence of the in- useof 

 troduction of wrought iron into the construction of i ron> 

 suspension bridges, which has been attended with 

 important results. 



THOMAS TELFORD, though neither the contriver of (351.) 

 suspension bridges, nor the introducer of them into Telford ; 

 Britain, 3 deserves notice from the superior boldness 

 and solidity of the noblest work of the kind which 

 has yet been executed the Menai Bridge. Telford 

 (and the same may be said of his contemporary 

 Rennie) was more distinguished as a man of judg- 

 ment, integrity, and experience, than as eminently 

 original or philosophical. In this respect both yield 

 to Smeaton, who, with Watt, was the founder (each 

 in his own department) of modern engineering. But 

 the beautiful and truly workmanlike structure of the 

 Menai Bridge inaugurated the era of the extensive 

 introduction of that admirable material, WROUGHT 

 IRON, into great permanent structures exposed to 

 heavy strains. Cast iron had been used much ear- 

 lier, as in the bridge erected at Colebrookdale in 

 1777 by Mr Derby, and in the very beautiful arch 

 at Sunderland, which dates from 1796. The span 

 of the Menai Bridge is 580 feet, the whole quantity 

 of iron used was 2186 tons, the transverse section 

 of the suspending chains or bars was 260 square 

 inches, supporting a strain of 1094 tons. This 



1 Reprinted in the Theorie des Machines Simples, Paris, 1821. 



8 A copious selection from Young's mechanical writings may be found in his Miscell. Works edited by Dr Peacock, vol. ii. 



3 Captain Samuel Brown erected the first considerable chain bridge in this country across the Tweed in 1819. 



