NA TURE 



\_Dec. 27, 168^ 



stance ; and it is a familiar fact liovv ven' important a part is played 

 in the steam-engine by the flywheel. Why should that be? Why 

 should this llyuheel be ,^0 important that it is only quite recently 

 that mechanicians have learned to do without it ? For this reason : 

 if a mass of matter such as a flywheel is once made to revolve, it 

 will retain that motion for a long; time, resisting any tendency to 

 an increase or decrease of its velocity. It. is in consequence of 

 this property which the revolving flywheel possesses that an 

 engineer is able to get over the dead points in his engine, whilst 



Fig. 27. — Rapidly rotating wheel gjprortcd at 



it also acts in preventing the engine making too sudden a start. 

 In addition to this, when we have a ma.'S of matter in the con- 

 dition of the revolving flywheel it has feme very peculiar 

 qualities, only observed when luchan^ass of matter is in motion. 

 If, then, we have a wheel .'o arranged that a very rapid rota- 

 tion is being; inij arted to it, it does not behave as it would when 

 at rest. These properties possessed ly a rofat ng tody can be 

 well shown by an instiiiment known as the gyroscope, of which 

 we shall speak more fully later on. It consists essentially ol a 



^^'NiiD 



Fig. 28. — Rotating disk of gyroscope, cc, knife edges ; A A, ED, adiusting 

 weights. 



disk to which a very rapid lotation can be imparted by a train 

 of wheels or by other means. If the disk be set rotating', it is 

 found to possess those curious qualities of which I have spoken. 

 If whilst rotating at a high velocity it be placed in the position 



.shown in Fig. 27, it will not fall, but will take on a movement 

 of revolution round the stand. 



F'rom considerations suggested by this and other similar ex- 

 periments, Foucault poinied out that it might be demonstrated 

 whether the eatth moved or whether she remained at rc-t. It 

 struck him that the problem should be attacked somewhat in 

 this manner : — 



Su.ipose the earth to be at rest, and that either at the north 

 or south pole a pendulum, suspended so that its point of support 

 had as little connection with the earth as possible — so that it 

 should, in fact, like the rotating flywheel, be independent of ex- 

 ternal influences, were set vibrating^ Then an observer at the 

 north or south pole would note that the swinoing pendulum {the 

 earth being considered as at rest) always had the same relation 

 to the objects on his horizon. But, said Foucault, suppose that 

 the earth does move. Then the swing of such a pendulunj 

 would not always be the same with regard to the places on the 

 observer's horizon. Let the earth be represented by a globe. 

 Suppose it to rotate from west to east. Place it with the north 

 pole uppermost, and set the pendulum, whose point of support is 

 disconnected from the rotating earth, vibrating. Then the 

 pendulum will appear to travel from left to right as the earth 

 rotates from right to left beneath it. Now suppose the pendulum 

 to be suspended in the same way at the sruth pole, right and 

 left now being chani^ed. The earth of course rotates in the same 

 direction as before, but the pendulum now appears to change the 



plane of its swing from right to left. At the equator the earth 

 simply rotates straight up and straight down beneath the 

 swinging pendulum. 



From these considerations it became evident to Foucault that, 

 if there were any possibility of demonstrating the movement of 

 the earth by means of the pendulum, the demonstration would 

 take this form. Provided it were possible to swing a pen- 

 dulum so that it should be as free as pes ibie from any influence 

 due to the rotation of the earth, and take that pendulum to the 

 north pole, it would appear to make a complete swing round the 

 earth in exactly the same time that it really takes the earth to 

 make a complete rotation beneath it. At the south pole ex- 

 actly the same thing would happen except that the surface of the 

 earth would appear to move in the opposite direction to what it 

 did at the north pole. Now it will be perfectly clear that if we 

 thus get a pendulum appearing to swing one way on account of 

 the true motion of the earth at the north pole and in the opposite 

 direction on account of the true motion of the earth at the south 

 pole ; at the equator, as we found in dealing with our model earth 

 and model pendulum, it will not change the plane of swing either 

 way, that is to say, the time taken by a pendulum to make a 

 complete swing will be the smallest possible at the poles, whilst 

 at the equator it will be infinite. 



At all places, therefore, between either pole and the equator 



