(346.) 
Compress- 
ing force. 
Cua. IV., § 3.] 
wrought iron, one inch square and ten feet long, ex- 
tended y5$5y 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. 
The compression of bodies proceeds (like the ex- 
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, 
MECHANICS.— YOUNG—TELFORD. 
875 
It is, however, to Young that we owe the application 
of these principles in unfolding their legitimate conse- 
quences. Ina 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, 
Torsion. 
have been mentioned in the last section (340). 
The mathematical investigations of Young on 349.) 
mechanical problems were conducted with bold di- Young's 
rectness and in defiance of the generalizing methods sake p rm 
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,? 
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- weet os 
troduction of wrought iron into the construction of jron.” 
suspension bridges, which has been attended with 
important results, 
Tomas Tet¥orp, though neither the contriver of (351.) 
suspension bridges, nor the introducer of them into Telford; 
Britain,* deserves notice from the superior boldness S¢sPension 
Detrusion. 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 flewure is that by which the resist- 
Flexure of ance of the greater number of solids is most easily 
beams. —_ 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 bridges: 
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 Presentés” (1776), he had already laid 
down very clearly the effect of compression on a beam,* 
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, wroucutT 
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 10943 tons, This 
2 Reprinted in the Théorie des Machines Simples, Paris, 1821. 
2 A copious selection from Young's mechanical writings may be found in his Miscell. Works edited by Dr Peacock, vol. ii. 
* Captain Samuel Brown erected the first considerable chain bridge in this country across the Tweed in 1819. 
