On the Nature and Laws of Friction. 19 



care to have all the parts sufficiently strong, and so well con- 

 nected together, as to be able to sustain the weight he has cal- 

 culated it to bear ; but I have not made any calculations of the 

 actual strength of the bridge which Mr. Telford proposes, though 

 I am satisfied that a bridge of that construction may be made 

 sufficiently strong for the purpose. 



Can there be any difficulty in making the piers capable of 

 bearing the bridge ? — None in the world. 



Does not the whole depend merely upon a matter of calcula- 

 tion ? — Certainly, and the judgement of the person who puts it 

 together. 



You have made no calculation as to the weight and bearing of 

 the bridge ? — I never have, because I never saw the plan until it 

 was just now produced before me. 



[To Mr. Telford.] — What greater power of resistance than 

 the actual weight of the bridge itself have you provided for in 

 your calculation ? — Rather more thati four times the power. 

 The whole of the bridge is 489 tons, and the power of suspen- 

 sion I calculate equal to 2016 tons. 



What is the sectional area of your cables ? — I have given the 

 relative power to the weight; it is more than four times. 



State the sections of your cables ? — The section of the cable 

 (taking it as a single cable) is 192 square inches. 



[To Mr. Rennie.] — Will a bridge, constructed upon these 

 data, be sufficient to bear a power of resistance four times greater 

 than its own weight ? — I should think it would ; but if it was 

 my own case, I should make it a little more, that is to say, I 

 should make it four times beyond the strength requisite to carry 

 its own weight. 



[To Mr. Telford.] — Would there be any difficulty in giving 

 this bridge that additional strength ? — None at all ; only the 

 additional expense of that quantity of iron ; you may have any 

 quantity of iron which will give the proportional power. 



III. On the Nature and Laws of Friction : being a Continna- 

 tion of the Paper on the same Sulject, given in our last Vo- 

 lume*. By Mr. Thomas Tredgold. 



On the Friction of Rolling Bodies. 



vJne of the most simple, and at the same time one of the most 

 important applications of the rolling motion, is that of whcel- 



• See vol. liii. p. H. 



B 2 carriages : 



