874 
tivated precisely the same branches of science which 
gave distinction to the career of Coulomb. When we 
compare the two philosophers, we find that the former 
was the more discursive reasoner and experimenter, 
and was diverted, perhaps by the copiousness of his 
erudition, and the attention with which he studied 
the works of others, from doing full justice to his 
own original powers. The latter excelled as a ma- 
MATHEMATICAL AND PHYSICAL SCIENCE. 
[Diss. VI. 
thematician; he concentrated his efforts more me- 
thodically, and displayed the results to the world (so 
far as they were published) in a more consecutive 
and lucid form. In uprightness of character and 
high morality, the two philosophers bore a marked 
resemblance. They were both sufferers from bad 
health, and they died within about a year of each 
other, at nearly the same age. 
§ 3. THomas Youne—Strength of Materials, and Art of Construction (continued).—TELFORD 
—Introduction of Iron into permanent Structures. Suspension Bridges—Tredgold; Mr 
Hodgkinson ; M. Navier—Mr Rosert StEPHENSON—Tubular Bridges. 
(342.) It is a circumstance not uninstructive as to the 
Difficulty progress and achievements of science, that the 
pete a" greatest modern philosopher who preceded Newton 
the enquiry —Galileo—and one of the most eminent, if not 
into the — the most eminent, of his successors— Young—should 
NON have laboured with minute and practical care, and 
ve eiids, With corresponding success, on a subject apparently 
so humble and mechanical as the Strength of Mate- 
rials, and the Resistance of Beams to fracture. 
Newton himself condescended to swing pendulums, 
and to observe the collisions of elastic worsted balls. 
It is sufficient here to advert to the exceeding inte- 
rest of enquiries which throw so much light upon 
the internal constitution of bodies, and in some 
instances intimately connect them with the laws of 
vibration of elastic media, to which so much of 
Modern Physics is intimately allied, 
The eighteenth century was in this, as in so many 
other departments of science, sluggish and mechani- 
cal, or else abstract and ultra-geometrical. The 
learned labours of Euler and the Bernouillis on 
elastic curves, and the strength of pillars, were for 
the most part elegant mathematical amusements, 
and with the exception of the experiments of Mus- 
schenbroek in the earlier half of the century, and the 
skilful but more limited researches of Coulomb at 
its close, little valuable in the way of precise theory or 
of accurate data derived from practice had been added 
to this important branch of mechanical engineering. 
Robison, indeed, with the peculiar tact and skill 
which I have already ascribed to him, wrote several 
papers (contributed to an early edition of the Ency- 
clopedia Britannica, and printed in his collected 
works) full of acute observation and reasoning, 
adapted to the imperfect experiments of his time, 
and connected by sound scientific deductions, which 
are still well worthy of careful perusal ; but it was 
to the penetration of Dr Tuomas Youne,' who par- 
took strongly of Robison’s mechanical tastes, whilst 
he surpassed him in facility of mathematical resource, 
that we owe a great revision of the doctrine of the 
strength of materials. In the “Syllabus of Lec- 
(343.) 
Progress 
during the 
eighteenth 
century. 
(344.) 
Thomas 
Young. 
tures” (1802), into which he condensed, in a manner 
peculiar to himself, an incredible amount of positive 
knowledge ; in the Lectures themselves (1806), with 
the admirable ‘Catalogue of References ;” and in 
the articles on “ Bridges,” and the supplementary 
propositions on “ Carpentry,” which he contributed 
to this Encyclopedia—we find (stated, as usual, 
not without some obscurity) a multitude of theorems 
and problems embracing the whole principles of 
construction, and based upon mechanical laws and 
the most probable interpretation of experiments. 
The forces tending to alter the figure or dimensions 
of substances usually called solid may be thus clas- AP ae 
sified: (1.) Extending forces, or such as produce force to 
elongation in a body when applied in a direct man- solids.— 
ner. (2.) Compressive forces, (3.) Force produ- !xtension. 
cing detrusion, or the slipping of one portion of the 
substance over another. (4.) Force producing flexure. 
(5.) Torsion or twisting force. The resistance of bo- 
dies to extension was examined by Hooke and Grave- 
sande, and is held to be directly as the area of section 
of the body, and to increase directly as the amount of 
elongation produced, at least within certain limits. 
The measure of this resistance Young termed (not 
very happily) Modulus of Elasticity, expressing the Modulus of 
force required to produce unit of elongation (or to Elasticity. 
double the length) of a prism of the substance un- 
der experiment. This quantity may be measured 
either by the length of a depending prism of the 
substance which would produce the requisite strain, 
or more simply by the strain expressed in pounds 
or tons, which, supposing the elongations to merease 
without limit as the extending forces, would double 
the length of the prism under experiment, Thus, 
in round numbers, a bar of wrought iron an inch 
square will be extended ;5}5> part by a pressure 
of one ton—hence the modulus of elasticity is about 
10,000 tons. The elasticity of wrought iron remains 
perfect to about half the breaking weight, after which 
the elongations appear to double for each addition 
of about 44, or 5 of the breaking weight. Thus, in 
a recent experiment by Mr Edwin Clark, a bar of 
1 I reserve to the chapter on Optics a fuller account of Young and his writings. 
