MATHEMATICAL AND PHYSICAL SCIENCE. 



[Diss. VI. 



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- 



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 YOUNG Strength of Materials, and Art of Construction (continued). TELFORD 

 Introduction of Iron into permanent Structures. Suspension Bridges. Tredgold; Mr 

 Hodgkinson ; M. Navier. Mr ROBERT STEPHENSON Tubular Bridges. 



(342.) It is a circumstance not uninstructive as to the 



Difficulty progress and achievements of science, that the 



and impor- g rea ^ es f; mo dern 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 



mechanical h ave laboured with minute and practical care, and 



of solTds. 68 w ith 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. 



(343.) The eighteenth century was in this, as in so many 

 Progress 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. 

 (344.) Robison, indeed, with the peculiar tact and skill 

 Thomas which I have already ascribed to him, wrote several 

 Young. papers (contributed to an early edition of the Ency- 

 clopaedia Eritannica, 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 THOMAS YouNG, 1 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- 



century, 



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 Encyclopaedia 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 (345.) 

 of substances usually called solid may be thus clas- A PP llc , a " 

 sified: (1.) Extending forces, or such as produce f orce to 

 elongation in a body when applied in a direct man- solids. 

 ner. (2.) Compressive forces. (3.) Force produ- Extension 

 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 c 

 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 increase 

 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 x^Vtftf P ar * 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 i^ or T ^ of the breaking weight. Thus, in 

 a recent experiment by Mr Edwin Clark, a bar of 



1 1 reserve to the chapter on Optics a fuller account of Young and his writings. 



