278 Mr HARRIS'S Experimental Inquiries concerning 



application of a very simple principle, aided by an easy and deli- 

 cate mechanism, I have sought a means of observing the action 

 of one magnet on another, or that of magnetised upon unmagne- 

 tised iron or steel, so as to estimate either the final result of the 

 compound action, or the separate forces of which such action is 

 compounded. 



4. Plate X. Fig. 1. represents an instrument which may be 

 considered as a species of balance with equal arms. There is a 

 light wheel of brass abed, Figs. 1. & 2. about two inches diame- 

 ter, whose centre i is placed in that of an arc MIN. This arc is 

 the quarter part of a circle, having a radius of between six and 

 seven inches : it is divided into 180 equal parts; 90 in the di- 

 rection IN, and 90 in the direction IM ; the point I being the 

 bisection of the arc, and marked zero. There is a short steel pin 

 which projects at b for about half an inch from one of the arms 

 of the wheel, through the circumference : this pin sustains an 

 index b I, Fig. 1 . formed of a light straw, which being tubular, is 

 easily placed on it, so as to fit sufficiently tight ; the distant ex- 

 tremity of this index is cut in the manner of a common writing 

 pen, and is carefully tapered to a fine point. From the opposite 

 arm at d, a similar pin projects, on which is screwed a very 

 small brass ball, which being adjusted either nearer to or farther 

 from the centre, is made so nicely to counterbalance the index, 

 that the wheel, when resting on its axis, is almost indifferent as 

 to position, the index remaining on any part of the arc, or nearly 

 so. 



5. The axis of this wheel abed is formed for a short distance 

 at each extremity into fine cylindrical pivots, which rest upon 

 the angles formed by four lesser or friction-wheels : these are al- 

 so about two inches in diameter, are constructed in the lightest 

 way possible, and are placed two of them before, and two behind 



