34- Mr. W. Sturgeon on the Distribution of 



tides on the surface of the disc will be at right angles to the 

 several positions which the needle is thus found to assume 

 whilst the disc is in rotatory motion ; and it was from nume- 

 rous experiments and observations of this kind, whilst the 

 needle was placed over various parts of the surface, that the 

 necessary data were discovered, and the recurving forces care- 

 fully traced out. 



The process by which the distribution of polarity on the 

 surface of the disc has been determined being now understood, 

 no further explanation will be necessary to illustrate the sin- 

 gular recurving directions of the excited forces which are sup- 

 posed to actuate the needle on the upper surface of the disc, 

 under the two conditions of rotation, than merely to refer to 

 fig. 16.* and 17. The exterior arrows indicate the directions 

 of motion given to the disc; and the two systems of small 

 recurving arrows in each figure show the distribution and 

 direction of the forces which impel the needle and urge it to a 

 position at right angles to the aggregate of any portion of those 

 forces over which it may be placed during the revolving mo- 

 tion of the disc. 



It will be observed, by comparing fig. 16. and 17, that the 

 direction in which the aggregate of the forces recurves is 

 nearly if not completely reverse by simply changing die re- 

 volving motion of the disc. The arrows which indicate the 

 direction of those forces are seen to issue from the front of the 

 exciting magnetic pole in fig. 16, but are re-entering at that 

 point in fig. 1 7. In the former figure also, the arrows are 

 seen re-entering on both sides of the magnet, near to the edge 

 of the disc; but in the latter figure the arrows issue forth from 

 both sides of the magnet, along the same edge ; so that the 

 force in the edge of the disc is as decidedly reversed as it is 

 in any part of the area by simply reversing the revolving mo- 

 tion. The curious change in the direction of the force in the 

 edge of the disc is beautifully illustrated by the following ex- 

 periment. 



Experiment 20. Let the axis of the disc be placed horizon- 

 tally east and west, and consequently the plane of it will be 

 coincident with the plane of the meridian. Let the horse-shoe 

 magnet be so arranged as to embrace the south edge of the 

 disc between its poles, its plane horizontal, and coincident 

 with that in which the axis of rotation is situated. Let also 



* Since my former communication went to press I have had an oppor- 

 tunity of repeating; my experiments on the surface of the disc ; from the 

 results of which I have been induced to offer fig. 16. as a more faithful 

 representation of the distribution of the force in the central parts than 

 that which Is shown by fig. 11. (Plate III.) vol. xi. 



