464 



♦ KNOWLEDGE ♦ 



[Dec. 5, 1884. 



THE EARTH'S SHAPE AND MOTIONS. 



Br EicHARD A. Proctor. 



CHAPTER VI.— THE EARTH'S ROTATION. 

 (Contimked from page 421.) 



"WJ E note then that if a ring of particles A B C D 

 VV (Figs 1 and 2) is rotating in direction A B C D, and 

 we wish it to rotate in direction « B e D, we must apply 

 forces tending to carry B in a direction contrary to that in 

 which we wish A to be carried, and in the same direction 

 as we wish C to be carried. On the contrary D must be 

 thrust in the same direction as A ia to be carried and in the 

 direction contrary to that in which we wish to be shifted. 

 The rule may be thus stated : — Having a rotating disc 

 A B C D, whose plane of rotation we wish to change, so 

 that two opposite points as A and 0, in Figs. 3 and 4, may 

 be shifted in opposite directions, at right angles to the 

 disc's plane, the intermediate points B and D not shifting, 

 we must move the disc as if trying to make the points B 

 and D move in the direction in which C and A, respectively 

 a quadrant in advance of them (according to the direction 

 of rotation), are required to move. 



The converse rule may be thus stated : — If a rotating 

 disc A B C D be so urged by impressed forces that were it 

 at rest the points B and D would be carried in opposite 

 directions at right angles to the disc's plane, the points A 

 and C not shifting, then will the jilane of rotation be 

 actually shifted as though the points A and C, a quadrant 

 behind B and D respectively, had been carried in the direc- 

 tions in which B and D are respectively urged. 



In all such cases the bodily translation of the rotating 

 disc must be considered separately. 



I have been careful to explain thus far the principles on 

 which the ordiuary experiments with the gyroscope depend. 

 They serve to show how and why the median plane of a 

 heavy rotating disc, spheroid, or sphere, resists any attempt 

 to change the position of its axis of rotation, and why such 

 change as does take place is at right angles to the direction 

 in which the external forces would move the axis of the 

 disc were it not rotating. ^loreover we can understand 

 why even considerable external forces acting on a heavy 

 disc rotating very rapidly, change very slightly the position 

 of its axis of rotation. 



But in Foucault's experiment for exhibiting the 

 earth's rotation by means of the gyroscope, everything 

 was done to prevent the rotiting disc from being 

 externally influenced. In the first place, the gyro- 

 scope is made very heavy, and set in very rapid rotation. 

 This makes its power of resisting a pressure tending 

 to change the position of its axis very great. Then, 



secondly, all possible care is taken to diminish the- 

 effect of the tendency which really acts upon it to change 

 the position of its axis. This force is the earth's rotation 

 acting upon the gyroscope, througli its supports, whatever 

 they may be. If we make the gyroscope as free as pos- 

 siVjle this force will be diminished as much as possible. 

 Now this is precisely what Foucault sought to do. In his 

 gyroscope for exhibiting the earth's rotation, there is first a 

 vertical ring suspended by a thread, and supported on a 

 pointed pivot which rests on an agate plane. So carefully 

 is this suspension managed that when the vertical ring is 

 loaded with the heavy disc (at rest), the merett breatb 

 causes it to shift its place. Next within the vertical ring 

 there is one jjlaced horizontally, supported on knife edges 

 which rest ou two suitable shoulders at the extremities of 



Fig. 3. 



Fig. 4. 



the horizontal diameter of the vertical ring. Thu.s this- 

 ring can oscillate freely on a horizontal axis. Lastly within 

 the horizontal ring, and on an axis at right angles to the 

 la.st-named one and also horizontal, is the massive disc. 



When the disc is set by suitable machinery in rapid 

 rotation, it tends, according to the principles already enun- 

 ciated, to preserve its axis of rotation in a fixed position. 

 If the earth were not rotating, it is clear that the axis of 

 the disc's rotation would preserve its posiiion unchanged 

 without any appreciable effect resulting. But if the earth 

 rotates, the axis of the disc can only maintain itself in an 

 unchanged position in space by an apparent motion withis 

 its supports. 



Suppose, for example, that there is a gyroscope at a point 

 A in the rotating earth PAP' (Fig 3), with the median 

 plane of its heavy rotating disc north-andsouth, as shown 

 in Fig. 6 at N S. Let this gyroscope be carried by the 

 earth's rotation around her axis P O P', to the position B. 



