233 
influence in the solar rays. 
produced by heating the box was to increase considerably 
that arc ; yet I considered that it must remove all doubt on 
the subject, if the box in which the needle vibrated were of 
wood, or other non-metallic substance. I resolved therefore 
at the earliest opportunity to repeat my experiments, and to 
make additional ones with an apparatus, in the construction 
of which I had scrupulously excluded metal, the suspension 
of the needle alone excepted. The compass-box is of maho- 
gany, the sides being of thick pasteboard : the graduations 
are on paper. Through a perforation in the middle of the 
glass cover a glass tube is fixed, and down this the suspend- 
ing wire, which is fixed above, passes into the compass- 
box. The needle can be released 90° from zero by means 
of a wooden or glass pin, turning in the side of the box. It 
would have been very desirable to have repeated the experi- 
ments in a vacuum, as the effect which I had observed to be 
produced by vibrating the needle exposed to the sun, might 
possibly be attributed to changes taking place in the medium 
in which the needle moved ; but as there would be extreme 
difficulty in doing this without having metal in the vicinity of 
the needle, I proposed to vibrate needles of copper and of 
glass, by the force of torsion, both in the shade and exposed 
to the sun, to observe the terminal arcs under those circum- 
stances, and to compare the effects with those obtained with 
a magnetised needle. The needles employed are all of the 
same form ; their length 6 inches, breadth in the middle 1.5 
inches, their sides being nearly circular arcs. The magnet- 
ised needle was suspended by a very fine hard brass wire. 
No. 35, between -j— and ~~ inch in diameter, and 10 inches 
in length ; the glass needle was suspended by the same 
