March 30, 1883. 



kno\a;"ledge 



193 



Potential energy is the energy possessed by a mechanical 

 system as regards the relati\e positions of its different 

 parts with respect to one another. 



Kinetic energy is the energy possessed by a mechanical 

 system as regards the relative motions of its different 

 parts with respect to one another. 



Maxwell gives the following definition of the principle 

 of conservation of energy : — " The total energy of a system 

 is a quantity which can neither be increased nor diminished 

 by an}' actions between the parts of the system, though it 

 may be transformed into any of the forms of which energy 

 is susceptible." 



That is to say, the energy stored up in any 'system is a 

 constant, and is always the same in whatever form it may 

 appear, provided it always remains within the system ; if, 

 however, for instance, it appears stored up in the form of 

 heat, the heat might be radiated off from the system to 

 surrounding objects or into space, the energy stored up in 

 the system would then, of course, be diminished. 



Although energy cannot be destroyed, and must always 

 e.'tist, all the energy at present a\ailaV)le for useful work 

 may become in the course of time only available for work 

 useless to man ; and this transmutation of useful energj' 

 into useless is one of the theories of the end of the world, 

 which would then become dead, as the moon is now sup- 

 posed to be. 



Returning to the advertisement quoted in the commence- 

 ment of this paper, if the magneto machine were simply 

 required to rotate, the only work that need be expended 

 would be that necessary to overcome the friction of the 

 bearings and of the air (which latter, by the way, the 

 writer of the advertisement altogether ignores), and al- 

 though this may be reduced to a minimum, it can never be 

 altogether overcome, except by running the machines in 

 vacuo, which is, of course, utterly impracticable. But in 

 order to produce a cuirent of electricity at all, we must 

 expend work, and consequently absorb energy, which re- 

 appears in the heating or other effects produced by the 

 ■current. 



The work required to produce a current of electricity 

 varies as the square of the current multiplied by the resist- 

 ance opposing the passage of the current. The resistance 

 opposing the passage of the current may consist, either of 

 the resistance of the conductors conveying the current, 

 which is analogous to the resistance to a current of water 

 flowing in a pipe caused by the friction of the water 

 against the inside of the pipe ; or of an opposing electro- 

 motive force, tending to produce a current in the opposite 

 direction, which is analogous to back pressure in the case 

 of a current of water. 



This current of electricity produced by the rotation of a 

 dynamo machine may be employed to do mechanical work, 

 such as to turn an electromotor, and thus drive some 

 machine, or to produce heat, as in the case of the electric 

 light. In the case of a current produced by means of a 

 battery, the work that has to be expended to produce the 

 current is the result of the absorption of the energj- that 

 was stored up in the zinc when it was separated from 

 oxygen, for which it has a natural affinity. 



It is thus impossible to produce a current of electricity, 

 *ither by means of a battery or l>y means of a dynamo or 

 magneto machine, without the absorption or transformation 

 of energy. 



I also noticed, some months ago, a paragraph which 

 appeared, I think, in one of the daily papers. The gist of 

 the paragraph was this. A number of accumulators or 

 secondary batteries, that had been iharged by passing a 

 current of electricity through them, were to be employed 

 on board a vessel to drive an electromotor, which was to 



be attached to the screw, and thus propel the boat, the 

 forward motion of which was then to drive a dynamo, from 

 which the accumulators could be recharged. 



The wiseacre who wrote this article evidently did not 

 perceive that if this were possible we should oV)tain per- 

 petual motion without further troulilc, which, by the theory 

 of conservation of energy, is impossible. The energy stored 

 up in the accumulators is .absorbed in producing the work 

 necessary to propel the vessel, and in order to turn the 

 dynamo for the purpose of recharging the accumulators, or, 

 what amounts to the same thing, make the forward motion 

 of the boat do so, we must expend the work necessary to 

 produce the required current and to overcome the friction 

 of the bearings of the dynamo, itc, which will absorb 

 energy. So we should have to take out of the accumu- 

 lators the energy absorbed in performing the work of 

 propelling the boat plus the energy absorbed in performing 

 the work of driving the dynamo, while the quantity of 

 energy wc should store in these or other accumulators that 

 were charged 1 'V the dynamo would be the energy absorbed 

 in producing; the work requisite to turn the dynamo minus 

 the energy al)Sorbed in the work of overcoming its friction 

 and of heating its coils. We should thus be actually con- 

 verting useful energy into useless energy. By useful 

 energy, I mean energy that can be employed to perform 

 useful work : such work cannot be obtained from energy 

 stored up in the heated bearings and coils of a dynamo. 



HOW TO USE OUR EYES. 

 By John Browning, F.E..A.S. 



-lY. 



(Contintiedfrom p. 162.) 



A GREAT many persons have one eye more sensitive 

 to colour than the other, and this leads me to say 

 that many more persons than would be supposed have two 

 odd eyes. In some, the eyes differ in colour ; in others, in 

 focus ; and again in others, in their sensitiveness to light 

 In some cases, when spectacles are required, the difference 

 between the eyes may be corrected by using lenses of 

 diflerent power. 



And here I may, with advantage, give a few hints on 

 spectacles — a subject on which the public have less general 

 knowledge than any with which I am acquainted. As a 

 result of this, there is probably more quackery practised in 

 this direction than in any other. 



It is, above all, when we have passed middle age, and 

 are compelled to apply to the optician for artificial aid to 

 vision, that we require to know " how to use our eyes." 



We are all interested in spectacles, for those of us 

 who do not require them ourselves have relatives or 

 friends who do, and we may save them from ha\'ing their 

 pockets picked or their eyesight injured. 



There is no particular age at which spectacles are 

 certain to lie required, lien can seldom see well without 

 tliem after they are forty-live, or women after forty. Very 

 often indeed women require them at thirty, and do them- 

 selves irremediable injury by not using them. In many 

 cases they fear looking old, but more often they think 

 that the longer they can put off using them the better. 

 This is a great mistake. Once they cannot see clearly by 

 lamplight or gaslight without holding the object further 

 from them than usual, they require spectacles, and by 

 working or reading without them they may bring on dis- 

 tressing headaches, or do their eyes an injury which no 

 optician can afterwards remedy. 



A correspondent of mine has aptly named the stage at 

 which spectacles are first required as becoming " Brad- 

 shaw blind." 



