CHAMBERS'S INFORMATION FOR THE PEOPLE. 



Here the weight is doubled as before, but the 

 extent of rubbing surface remains unaltered ; it 

 would be natural, therefore, to expect that this 

 would make a difference, and that, though the 

 friction would, of course, be increased, the increase 

 would be less than in the former case. Experi- 

 ment, however, shews that there is no difference, 

 and that the friction is just double in both cases. 

 In short, the unexpected and important fact is 

 established, that, within certain limits, the friction 

 of any two surfaces increases in proportion to the 

 force with which they are pressed together, and is 

 wholly independent of the extent of the surfaces in 

 contact. 



The amount of friction between two bodies is 

 thus a constant fraction or proportion of the force 

 with which they are pressed against each other. 

 This fraction differs for the different kinds of 

 surfaces. Thus between oak and cast-iron, it is, 

 as already stated, about |, or more exactly, -38 ; 

 for wrought-iron on wrought-iron (we speak at 

 present of dry surfaces, without grease or unguent 

 of any kind), it is '44 ; for brass upon cast-iron, 

 22. This constant fraction (expressing the pro- 

 portion between the pressure of two surfaces and 

 their friction) is called the coefficient of friction for 

 these two surfaces. 



Friction is very much diminished by the use of 

 grease or unguents. The coefficient of wrought- 

 iron upon oak, which, in the dry state, is "49. is 

 reduced by the application of water to '26, and by 

 dry soap to -21. The result of experiments on this 

 subject is stated to be, 'that with the unguents, 

 hog's-lard and olive-oil, interposed in a continuous 

 stratum between them, surfaces of wood on metal, 

 wood on wood, metal on wood, and metal on 

 metal (when in motion), have all of them very 

 nearly the same coefficient of friction, the value of 

 that coefficient being in all cases included be- 

 tween '07 and '08.' Tallow gives the same co- 

 efficient as the other unguents, except in the case 

 of metals upon metals, in which the coefficient 

 rises to 'io. 



The most important fact, perhaps, and one that 

 could hardly have been anticipated before experi- 

 ment, is, that the friction of motion is wholly in- 

 dependent of the velocity of the motion. 



The resistance to the motion of a wheeled 

 carriage proceeds from two sources : the friction 

 of the axle, and the inequalities of the road. The 

 resistance of friction to the turning of a shaft in 

 its bearings, or of an axle in its box, has evidently 

 the greater leverage, the thicker the journal or the 

 axle is ; the axles of wheels are accordingly made 

 as small as is consistent with the required strength. 

 The resistance that occurs between the circum- 

 ference of the wheel and the road, constitutes 

 what is called rolling friction. There are on all 

 roads, to a greater or less extent, visible rigid 

 prominences, such as small stones, in passing over 

 which the wheel and the load resting on it have 

 to be lifted up against gravity. But even were 

 these wanting, the hardest road yields, and allows 

 the wheel to sink to a certain depth below its 

 surface ; so that in front of the wheel there is 

 always an eminence or obstacle, which it is at 

 every instant surmounting and crushing down. 

 This is the case even on iron rails, though of 

 course to a much less extent than on any other 

 road. Now, for overcoming this resistance, it can 

 be shewn on the principle of the lever that a large 

 222 



wheel has the advantage over a small one ; and 

 by numerous experiments, the fact has been fully 

 established, that on horizontal roads of uniform 

 quality and material, the traction varies directly 

 as the load, and inversely as the radius of the 

 wheel. 



The best direction of traction in a two-wheeled 

 carriage, is not parallel to the road, but at a slight 

 inclination upward, in proportion to the depth to 

 which the wheel sinks in the road. 



On a perfectly good and level macadamised 

 road, the traction of a cart is found to be ^V of the 

 load; that is, to draw a ton, the horse requires 

 to pull with a force equal to 75 Ibs. On a railway, 

 the traction is reduced to TOT of the load, or 

 to 8 Ibs. per ton. For the addition to traction 

 occasioned by an incline on the road, see page 215. 



The force of friction is often directly employed 

 in mechanics. It is used, for instance, to com- 

 municate motion by means of belts, chains, &c. 

 It is the force that holds a knot. It is specially 

 useful when a machine, with great momentum, 

 has to be checked or arrested in its motion. The 

 best example of this is the brake used on railways. 

 By means of a system of levers, blocks of wood 

 are made to press against the circumferences of a 

 number of the carriage-wheels ; and thus the 

 momentum of a train weighing hundreds of tons, 

 and moving with a velocity of perhaps 50 miles an 

 hour, is gradually destroyed in a wonderfully short 

 space of time. 



WORK. 



Work, in the mechanical sense, implies two 

 things ; it implies that something is moved, and 

 that force or pressure is required to move it In 

 other words, all mechanical work involves the 

 exertion of a continuous pressure over more or 

 less space. To bear or resist a pressure without 

 motion is not work. A man standing still with a 

 burden on his back is not working, any more than 

 a post that sustains the end of a beam. The 

 hand, fig. I, does no work while it merely holds 

 the weight suspended, although it requires force 

 to do so ; for a fixed catch or pin above the lever 

 would do the same. But when the upward pressure 

 of the lever is overcome by the hand through the 

 space PP', a certain amount of work is done. 

 Nor does motion, where there is no resistance, 

 constitute work. While a cannon-ball is flying 

 through the air, it is doing no work (if we neglect 

 the resistance of the air that is overcome) ; it only 

 works when it forces its way through an obstacle, 

 as the planks of a ship's side. 



The Unit of Work. When one man lifts six 

 gallons of water from a well twenty feet deep, 

 while another lifts eighteen gallons from the same 

 well ; it is easy to see that the one has done three 

 times as much work as the other. But in order 

 that we may compare exactly the quantities of 

 work in operations of different kinds, we must 

 have some common standard to refer to. Just as, 

 in comparing, the height of a mountain with a 

 distance on a road, we take a standard or unit of 

 length, say a foot, and, applying it to both, find 

 that the height of the mountain contains 800 of the 

 unit, and the length of the road 900, and thus get 

 a precise knowledge of their relative magnitudes ; 

 so it is necessary to have a unit of work, and thus 

 be able to say of different operations, that they 

 contain each so many of such units. 



