236 Mr. Christie on magnetic 
more than counterbalanced by the time during which the 
glass and copper needles vibrated being greater than that for 
the magnetised needle. If the effects be reduced propor- 
tionally to the times, the terminal excess in the same time 
would be for the magnetised needle 13°.75, f° r the copper 
needle 5°.24, and for the glass needle 4°.7i. 
The diminution in the time of vibration of the magnetised 
needle when exposed to the sun, may at first sight appear 
singular, since there can be no doubt that, the temperature 
being higher, the intensity of the needle was less than when 
it was screened. This diminution in the time of vibration I 
have no doubt arose principally from the more rapid diminu- 
tion of the arcs of vibration in the former case than in the 
latter, and likewise in some measure from the needle not re- 
covering the same intensity of magnetism after being exposed 
to the high temperature of the sun’s rays which it previously 
possessed. That these causes operated to a great extent is 
clearly shown by the following observations which I after- 
wards made. The same needle, but having its weight 
increased to 252 grains, was first vibrated in the shade from 
90° as before, and three different observations gave me 
5 m 58 s .8, 5 ra 58 s . 6, 5 m 58 s . 6, as the time of performing 100 
vibrations, or 3 s . 587 as the mean time of 1 vibration, the 
terminal arc being 33 and thermometer 64.5. I then vibrated 
it, still in the shade, from 20°, and found that 50 vibrations 
were performed in 2 m 48\8, giving 3 s . 376 as the mean time 
of vibration, the terminal arc being 13 0 and thermometer 65°. 
Similar observations being immediately afterwards made 
with the needle exposed to the sun, gave 5“ 59 8 - 4 , 5 m 59 s . 4 , 
and 6 m 00 s , as the time of performing 100 vibrations, or 
