the Laws of Magnetic Forces. 279 



the frame which sustains the graduated arc ; they are mounted 

 on very delicate pivots, terminating in fine points *. 



6. The five wheels just described, with the graduated arc, are 

 sustained by a projecting frame of brass ABD ; and the whole is 

 supported by a vertical column of wood or brass DE, about four- 

 teen inches high. The frame of brass ABD projects six inches 

 from the column, and is united to it at D by means of a small 

 nut and screw. The column DE is screwed, at its lower extre- 

 mity, into a circular base B', of 10 inches diameter, supported on 

 three adjusting screws, g, h, k. There are two lines of silk, each 

 three inches in length, 6cm, ban, Figs. 1. & 2., which pass from 

 the point b in opposite directions, over the circumference of the 

 wheel abed, and terminate in two small hooks m, n : these lines 

 are secured close to the point b on each side of it, by means of 

 a small knot, and by passing them through holes drilled in the 

 circumference, as in Fig. 2. The circumference is slightly grooved 

 to receive these lines, and prevent them from slipping over the 

 edge of the wheel. 



The line bcm sustains a small cylindrical piece of soft iron, or 

 otherwise a small cylindrical magnet #, Fig. 1 ., which being first 

 attached to a loop of silk f , is suspended on the hook at m. From 

 the opposite hook n there is suspended in a similar way a cylin- 

 drical counterpoise of wood W, the lower half of which is im- 

 mersed in distilled water. The water is contained in a cylindri- 

 cal vessel of glass, whose interior diameter is so great that any 



* The opposite extremities of the pivot-holes are faced with small portions of 

 fine watch-spring, as at ef, Fig. 1 . which mark the centres of the two front wheels. 

 Thus all friction which might possibly arise from the occasional contact of the shoul- 

 der of the pivot is prevented. 



t The loop is formed by a doubled piece of fine silk, inserted in a small hole 

 drilled vertically into the centre of the upper part of the cylinder x, and secured 

 there by a small peg of wood passed down into the hole between its two extremities. 



