the Directive Power of Magnets, kc. 223 



the same length as the magnet, at distances respectively 1*5 

 inch and 3*0 inches from the near edge of the magnet; these 

 lines were divided each into ten equal parts j and thus in each 

 line eleven points were obtained at intervals of 1*4 inch. From 

 each of the four angles of the magnet as centre, two quadrants 

 were swept: — one with radius 1*5 inch, at whose extremity 

 and bisection points were taken for the compass-centre; and 

 one with radius 3*0 inches, which was twice bisected, and of 

 which the extreme point and the three bisection- points were 

 taken for the compass-centre. These points were used for the 

 magnet both with its edge and with its flat side towards the 

 compass. A similar process was adopted in using the galvanic 

 coil, with this difference only, that the longitudinal separation 

 of the points taken for compass-centre was only 1*34 inch. 



A solid piece of wood was provided, in which was cut a con- 

 cave channel, less than half a cylinder, such that when the 

 galvanic coil, or the large magnet with its flat side towards the 

 compass, was laid in the channel, its axis was sensibly at the 

 same height as the needle of the small compass. With the 

 magnet's edge towards the compass, that condition was suffici- 

 ently secured by merely laying its flat side upon the board. 

 The paper with station-points, being laid in proper position 

 upon the board and secured by nails, was cut along the middle 

 of the channel and crosswise at its ends, so that it could be 

 bent down into the channel to permit the magnet or coil 

 to take its proper position. When observations were finished, 

 the paper was detached from the board, and the edges which 

 had been cut were reunited by cementing a piece of paper 

 behind. 



The observation (as will be seen) consisted, in every case, of 

 observation of the direction taken by the small needle. And 

 this observation was made solely by the eye. The observer, 

 looking endways of the small needle, made two pencil-dots 

 upon the paper, corresponding to the line of the needle-axis 

 produced as it appeared to his eye. If, from erroneous position 

 of the eye, a parallactic error is produced in the position of the 

 two pencil-dots, this error is detected as soon as the compass 

 has been removed and an attempt has been made to draw a 

 line of direction through the station-point of the compass ; 

 and, to correct it, all that is required is, to draw through the 

 station-point a line parallel to the line joining the two dots. 

 The whole of this operation is extremely accurate. 



Eor measuring the intensity of the magnetic force exerted on 

 the compass-needle, I determined, after consideration, to adopt 

 the statical method — that is, to place a constant magnet in a 

 definite position above the compass-needle, with its magnetic 



