486 
THE ASTRONOMER ROYAL’S EXPERIMENTS 
is circular, and when positions had been selected for the centre of the compass (as will 
be mentioned), a circle somewhat larger than the compass-box was described in pencil 
with each of those posit'ions for centre ; and the compass could then be planted with its 
centre very accurately placed above the intended point. 
The compass-positions were thus prepared: — Upon a sheet of strong paper the plan 
of the magnet, 14 inches by 1‘4 inch, was laid down. On each side were drawn two 
parallel lines of the same length as the magnet, at distances respectively U5 inch and 
3’0 inches from the near edge of the magnet; these lines were divided each into ten 
equal parts, and thus in each line eleven points were obtained at intervals of U4 inch. 
From each of the four angles of the magnet as centre, two quadrants were swept — 
one with radius at 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 differ- 
ence only, that the longitudinal separation of the points taken for compass-centre was 
only T34 inch. 
A solid piece of wood was provided, in which was cut a concave 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 sufficiently 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 re-united 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. 
For 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 axis transversal 
to the direction which the compass-needle had taken before the constant magnet was, 
