230 Mr. Christie on magnetic 
the diminution of the terminal arc. That this diminution 
could not arise from the increased temperature of the com- 
pass-box, the previous experiments have, I trust, satisfactorily 
proved : I think I may therefore safely conclude, that during 
the vibration of a magnetised needle exposed to the solar 
rays, these rays have an influence on the needle independent 
of the effect produced by heat, which tends to bring the 
needle to a state of rest. 
In the first observations which I have described it appears 
that, although we might expect that, when the needle 
vibrated in the sun, the increased temperature would dimi- 
nish the intensity, yet a slight acceleration took place in the 
time of vibration. Whether this is to be attributed to the 
same cause or not, it is perfectly analogous to what I have 
observed on vibrating a metallic disc under the influence of a 
magnet : the arc of vibration was much more rapidly dimi- 
nished than when the disc vibrated uninfluenced by the 
magnet, and the times of vibration were also diminished. A 
copper disc 7 inches in diameter having its axis of suspen- 
sion perpendicular to its plane, and at a small distance from 
its centre, made 286 vibrations before the arc of vibration 
was reduced to a certain extent ; but on vibrating it between 
the poles of a horse-shoe magnet it made only 23 vibrations 
before the arc was reduced to the same extent. The times 
in which the disc performed 24 vibrations at different trials 
were : 
Freely. 
Between the poles 
the magnet. 
24 vibrations in . 
35.15 seconds 
. . 34.6 seconds. 
24 
24 .... . 
2 4 
. 35.18 * . . 
. . 34.6 
Time of 1 vibration . 
. 1.4635 . . 
. . 1.4427 
