ON PASSIVE STRENGTH AND FRICTION. 117 



cumference, but in the middle between both ; and in Europe it is generally 

 thicker and firmer on the south-east side of the tree. Although iron is 

 much stronger than wood, yet it is more liable to accidental imperfections ; 

 and when it fails, it gives no warning of its approaching fracture. The 

 equable quality of steel may be ascertained by corrosion in an acid ; but 

 there is no easy mode of detecting internal flaws in a bar of iron, and we 

 can only rely on the honesty of the workman for its soundness. Wood, 

 when it is crippled, complains, or emits a sound, and after this, although 

 it is much weakened, it may still retain strength enough to be of service. 

 Stone sometimes throws off small splinters when it is beginning to give 

 way ; it is said to be capable of supporting by much the greatest weight 

 when it is placed in that position, with respect to the horizon, in which it 

 has been found in the quarry. 



It is obvious that when the bulk of the substance employed becomes very 

 considerable, its weight may bear so great a proportion to its strength as to 

 add materially to the load to be supported. In most cases the weight in- 

 creases more rapidly than the strength, and causes a practical limitation of 

 the magnitude of our machines and edifices. We see also a similar limit 

 in nature : a tree never grows to the height of 100 yards ; an animal is 

 never strong enough to overset a mountain. It has been observed that 

 whales are often larger than any land animals, because their weight is 

 more supported by the pressure of the medium in which they swim. 



The force of friction which resists the sliding of different bodies on each 

 other, seems to be intimately connected with that lateral adhesion or ri- 

 gidity which is opposed to the internal displacement of the parts of a 

 single body, by the effect which we have denominated detrusion ; and 

 when the friction is considered as resisting pressure rather than motion, it 

 approaches still more nearly to the same force. It is probably derived in 

 great measure from the strength of the protuberant particles, which must 

 be broken, bent, or compressed by the motion of the bodies on each other : 

 but it is not always that the existence of such particles can be asserted, 

 much less can they be made perceptible to the senses, and we can only ex- 

 amine the effects which they may be supposed to produce, by immediate 

 experiments on the forces required to counteract them. Such experiments 

 have been made on a very extensive scale by Musschenbroek* and 

 Coulomb,f and many of their results have been confirmed by Mr. Vince,J 

 in a simple and elegant manner. 



With a few exceptions, the friction of all solid bodies is either perfectly, 

 or very nearly, a uniformly retarding force, neither increasing nor di- 

 minishing when the relative velocity of the bodies concerned is changed. 

 The friction of some rough substances is a little increased with the velocity, 

 but, as they become more polished, this variation disappears. When, how- 

 ever, the motion is wholly extinct, and the bodies remain in contact with 

 each other, their adhesion is usually greater thkn the friction, and by a 

 continuation of the contact, it may become twice or even thrice as great, 



* Introductio ad Philosophiam Naturalem, 2 vols. 4to, Leyd. 1762, i. 145. 



f Mem. des Savans Etrangers, x. 161. 



On the Motion of Bodies affected by Friction, Ph. Tr. 1785, kxv. 165. 



