4 Wilde, Evolution of tJie Mental Faculties. 



vanishing at the equator previous to changing its direc- 

 tion in the southern hemisphere. Experiments have 

 shown that in the latitude of Bristol, 51^ 28' N., the rate 

 of rotation is about 11 "75 per hour = 307 hours for a 

 complete revolution.* At Colombo, Ceylon, Lat. 6° 56' 

 N. the rate of rotation was found to be i°-87 per hour = 

 192 hours per revolution.f 



A few years after the publication of Foucault's experi- 

 ments, I designed a gyroscope of larger dimensions than 

 those generally constructed, which not only exhibits the 

 earth's rotation, but also other principles of motion that 

 have not so far been demonstrated by that instrument. 



It consists of a heavy disc of gun-metal, six inches 

 diameter, and weighs 4-5 lbs. The spindle on which it 

 revolves is mounted in rings so as to turn freely in any 

 direction. The disc, with its rings, can be supported in 

 the usual manner from below, or be suspended on knife 

 edges from above by means of a fine wire, according to 

 the property of the rotating body it is desired to illustrate. 

 Motion is given to the disc by means of a friction wheel 

 driven by an electro-motor or by a cord unrolled from 

 its axis. 



When the disc is rapidly rotated, the vertical plane of 

 rotation appears to turn horizontally round the vertical line, 

 in the same direction as the stars, and in the same manner 

 as the plane of oscillation of a pendulum, from which the 

 diurnal rotation of the earth in the opposite direction is 

 inferred. I have, however, found that the axial rotation 

 may be made directly visible to an audience, by projecting a 

 vertical band of light from the slit of an electric lantern 

 upon a small mirror attached to the vertical axis of the 

 gyroscope and from thence upon a screen. A black band 



* Phil. Mao., (4) Vol. 4, pp. 272-275. 

 t //'/(/., Vol. 2, p. 410. 



