CHAMBERS'S INFORMATION FOR THE PEOPLE. 



which is to be taken out of the store accumulated 

 in the train ; for how many feet, then, will that 



store last ? Evidently for 8>7 ^ 63 = 2863 feet. 



Of the moving-power applied to a machine, 

 only part goes to do useful work ; another part is 

 expended uselessly in overcoming the friction of 

 the machine, and other resistances to its trans- 

 mission. For every 100 units of work applied to 

 the handle of a crane, possibly only 70 are done 

 upon the load. In this case, the fraction *?&, or -7, 

 expresses the efficiency of the machine, and is 

 sometimes called its modulus. The working 

 values of different machines may thus be exactly 

 compared. Of machines, for instance, for raising 

 water, the modulus of the bucket-wheel is -6, and 

 that of the Archimedean screw, -7. 



SOURCES OF POWER. 



The principal moving forces are, the muscular 

 power of men and animals ; the weight of bodies, 

 or gravity ; wind, and the flow of streams ; the 

 expansive force of steam ; electricity ; and mag- 

 netism. Water, steam, and electricity, as sources 

 of moving power, are treated of in other parts of 

 the work. The labouring-power of animals varies 

 greatly with the way or position in which they exert 

 their muscular strength. In turning a handle or 

 winch, the pressure exerted at different parts of the 

 circuit is very different ; but it has been found 

 that a man can exert for a considerable time a 

 mean pressure of 30 Ibs. while moving the handle 

 through 1 20 feet per minute. This would give 

 3600 units of work a minute, or about -J of a horse- 

 power. Another estimate of labour of this kind, 

 when continued for eight hours a day, makes the 

 work per minute only 2600, or -fa of a horse-power. 

 Rowing is one of the most advantageous ways of 

 exerting muscular strength, the effective work 

 being 4000. In working at the tread-mill the man 

 exerts his strength to raise his own body, and the 

 weight of the body in descending turns the wheel. 

 The work got in this manner is 3900. 



ENERGY. 



A moving body, as we have seen, has in it a 

 store of force called Energy, and the exact amount 

 of this store can be stated in the number of foot- 

 pounds of work it can do before being brought to 

 a stand. But this is not the only form in which 

 energy exists. A ram suspended ten feet above 

 the head of a pile has in it, while yet at rest, a 

 store of working power in virtue of its position ; 

 and it requires only to be let go in order to bring 

 that power into action. While resting on the 

 head of the pile after the impact, it has no work- 

 ing power, if we except the pressure of its own 

 weight. It must be again raised so as to have a 

 free space to fall, and then it is invested anew 

 with energy equal in amount to what has been 

 spent in raising it. A store of force thus quiescent, 

 but capable of being called into action, is called 

 potential energy ; while that which exists in a body 

 in motion is called actual or kinetic (from a Greek 

 word signifying to move) energy. A ram de- 

 scending upon a pile, a cannon-ball flying towards 

 its mark, a running stream driving an undershot 

 wheel, the wind pressing a sail, all are examples 

 of actual or kinetic energy. As examples of 



2.'4 



potential energy that can be directly used for 

 mechanical purposes, we may cite a coiled watch- 

 spring, a drawn bow, the charged receiver of an 

 air-gun, a store of water with a fall or head. In a 

 common clock we see the potential energy of the 

 wound-up weights gradually converted into actual 

 energy in the motions of the parts. The very 

 sound of the bell is a vibratory motion derived 

 from the energy of the driving weight. 



Correlation of Physical Forces. In treating of 

 HEAT (see No. 13), it was shewn that mechanical 

 motion could be converted into heat, which is 

 itself a motion among the molecules of bodies, 

 and therefore a form of energy. The latent heat 

 of steam is a store of potential energy, which, in 

 the steam-engine, is converted into mechanical 

 power. If that power is made to drive a shaft, 

 and a brake is applied, heat to any amount may 

 be reproduced ; or if it is made to turn a common 

 electric machine, or a magneto-electric one, elec- 

 tric currents will result, attended with attractions 

 causing sensible motions, and with heat and light. 

 Light, again, causes chemical combinations and 

 decompositions, as in photography ; and chemical 

 action in the voltaic battery gives rise in its turn 

 to electricity, heat, light, and magnetism. It is 

 facts like these, shewing the convertibility of one 

 force into another or into others, that constitute 

 what is called the Correlation of Physical Forces. 



Conservation of Energy. But there is some- 

 thing more than mere convertibility in the different 

 forms of energy ; there is constant equivalence. 

 If, in rubbing two pieces of wood against each 

 other, energy has been expended equal to that 

 of lifting a pound-weight 722 feet high, the heat 

 produced is sufficient to raise a pound of water 

 one degree of F. This relation is constant, and 

 722 foot-pounds is thus the mechanical equivalent 

 of one unit of heat. The equivalents of the 

 other physical forces have not been determined,, 

 although an approximation has been made in 

 the case of light. But everything tends to- 

 prove that whatever transformations energy may 

 undergo, none of it is ever lost. In every 

 machine it is only a portion, greater or less, of 

 the motive power that is effective for the work 

 intended. But the residue, though said to be lost,, 

 so far as useful work is concerned, is not annihi- 

 lated ; it is mostly expended in overcoming the 

 friction of the machinery, and in doing so it is 

 transformed into heat. A part of it is spent in 

 noise, but this also, in the end, results in heat,, 

 which is still energy. Energy, in the shape of 

 heat, has a constant tendency to uniform diffu- 

 sion ; but although it thus becomes dissipated, it 

 does not cease to exist. This truth has become 

 the cardinal principle of natural philosophy in 

 recent times under the name of the conservation 

 of energy ; and nearly all the ordinary physical 

 laws can be shewn to be involved in it. Thus a 

 planet has a certain amount of potential energy 

 in virtue of its distance from the sun, just as a 

 stone has when suspended above the surface of 

 the earth. That energy must therefore become 

 less as the planet moves from its aphelion towards 

 its perihelion; what then becomes of it? The 

 difference appears in the increased velocity of 

 the planet, being thus transformed from potential 

 energy into actual. Numerous illustrations of this 

 important doctrine will occur in the treatises on 

 ELECTRICITY and the STEAM-ENGINE. 



