BiCKERTON. — On a simple Method of illustrating Motions of the Earth. 165 



night, seasons, the solstices, and equinoxes and the precession of equinoxes, 

 eccentricity of orbits, the hnes of the globe, etc. It has also the advantage 

 over the ordinary model of possessing nearly all the dynamical peculiarities 

 of the heavenly bodies themselves. It is, as it were, a double pendulum, 

 and so it may easily be made to illustrate the laws of motion, resultant 

 motion, the properties of the pendulum, Foucault's pendulum, and a large 

 number of facts of both the kinetics and kinematics of dynamics. The 

 accompanying diagram (Plate IIb.) represents it on a large scale, and shows 

 it in use. The angle is shown exaggerated. 



The model consists of a ball of wood or other material with a thick 

 knitting-needle through it : this represents the earth and its polar axis ; 

 the ends of the needle are sprung into two centre punch dents in a light 

 brass ring, the ball thus rotates on the needle as on an axis. This brass 

 ring is hung in a vertical plane in such a manner that the needle makes an 

 angle of 23° to the vertical. There are also other points of suspension for 

 exaggerating the inclination, to render the phenomena more evident. The 

 two cords are attached to the ceiling so as to hang parallel. 



On swinging the apparatus as a conical pendulum, the direction of the 

 axis remains all the time parallel to itself. If a lamp be placed in the 

 centre of this cone, and the ball be made to spin, the phenomena of day and 

 night and summer and winter are at once illustrated. The solstices and 

 equinoxes are of course shown with the greatest readiness ; the equator, 

 tropics, and polar circle also show themselves, and the peculiarities of polar 

 seasons can, of course, readily be shown. By making the swing of the pen- 

 dulum an ellipse instead of a circle, and placing the lamp at a focus, the 

 long winter and short summer of great eccentricity are explained. This 

 illustratioir of the rate of motion during eccentric orbits is, of course, 

 not mathematically accurate. With the apparatus moving in an ellipse it 

 becomes easy to explain the reason why, in the northern hemisphere, the 

 sun is nearer in winter than in summer. By merely spinning the whole 

 model on its two cords, and so twisting them up, the precession of the equi- 

 noxes is readily understood. 



By these two experiments it is easy to render Croll's theory of glaciation 

 intelligible, by taking a card to represent the moon's orbit to the plane of 

 the ecliptic the causes of the lunar and solar eclipses and their cycles are 

 rendered intelligible. The whole of the motions being due to inertia, and 

 the centrifugal j)oint being the centre of the circle, we have a true central 

 force acting on the body. Thus planetary dynamics is almost exactly repre- 

 sented. 



By taking off the ring and hanging two similar balls the exact isocran- 

 ism of equal length pendulums may be shown, and this may be amphfied 



