220 Mr. Christie on magnetic 
In order to ascertain the effect which changes of tempera- 
ture have on the times of vibration of a needle, it is neces- 
sary to know the temperature of the needle itself during the 
observations, and I saw no better means of ascertaining this, 
even approximately, than to vibrate it in the shade and then 
exposed to the rays of the sun, and to consider the tempera- 
ture of the needle to be that indicated by a thermometer 
near to it. On my first doing this, I found, that although I 
could easily mark the 50th vibration when the needle was 
shaded, I could not distinguish beyond the 40th when it was 
exposed. I at the same time found that the time of vibra- 
tion was slightly diminished at the higher temperature, 
instead of being increased, as I had reason to expect. As 
however the needle was not vibrated in the same spot in the 
two cases, the diminution in the time of vibration and of the 
arc when it was exposed, might be independent of the change 
of temperature and of any influence in the solar rays. To 
avoid any uncertainty arising from difference of disturbing 
causes in two situations, I placed the compass out of doors, 
with a screen composed intirely of wood, supported at the 
height of four feet above it, and by removing which the rays 
of the sun struck directly on the needle. A thermometer 
having the bulb near to the compass-box indicated nearly 
the temperature of the needle. When the shutter was up, 
so that the needle vibrated in the shade, I could very dis- 
tinctly note the 100th vibration; but when it was removed 
and the needle vibrated exposed to the sun's rays, I could 
not so distinctly mark the 75th. I made use of a needle six 
inches long, weighing 42.75 grains, and contained in a brass 
compass-box with a glass cover : the needle was suspended 
by a fine hair, and commenced vibrating 30° from zero. 
