ELECTRICITY. 333 



which, on the same axis, is an index K, which goes 

 round a small plate k, divided into 29^ equal parts, 

 for the days of the moon's age from change to 

 change. So that, in the time the great wheel A, 

 the earth D, and hour-index E, make 29i revolu- 

 tions, the moon I, and her index K, make only 

 one ; and in that time, by showing herself all round 

 to the observers, her different phases are exhibited 

 like those of the real planet. 



Having set the orrery near the prime conductor, 

 place a crooked wire from the conductor, so that 

 its point may be even with the great wheel B, and 

 tend to turn it in the direction a, b, c, d; turn the 

 glass globe of the electrical machine by the winch, 

 and a stream of fire will issue from the wire to the 

 wheel, and turn the whole of the moveable work ; 

 by which means, the earth D will be turned round 

 its axis, from west by south to east \ and, in such 

 turn of the earth, the index E will go round all the 

 24 hours on the dial-plate e. In the time the earth 

 and index turn 29i times round, the moon I will 

 turn once round her axis, showing all her various 

 phases ; and the index K will go over all the 29i 

 days of the moon's age on the plate K. 



Fig. 3. is another electrical orrery, showing the 

 motion of the sun, earth, and moon. The sun and 

 earth go round the common centre of gravity be- 

 tween them in a solar year; and the earth and 

 moon go round the common centre of gravity 

 between them in a lunar month. The ball S re- 

 presents the sun, E the earth, and M the moon, 

 connected by wires a c and b d: a is the centre of 

 gravity between the earth and moon. These three 

 balls, and their connecting wires are hung and 

 supported on the sharp point of a wire A, which is 



