MR. W. CROOKES ON REPULSION RESULTING- FROM RADIATION. 
245 
it is cemented at d to a well-ground stopper, so as to admit of adjustment. When in 
position, cement (83) is run round the stopper. At c, the point of junction between 
the torsion fibre and the straw beam, is a silvered glass mirror. At the end, e, of the 
beam is a small pan to hold the weights counterpoising the disks which are suspended 
to the other end. A flat stirrup of aluminium, at f fits stiffly on the straw beam, and 
Fig', L 
d 
carries a flat glass fibre, f g, cemented to it so as to allow of no play ; the straw beam, 
the aluminium hook, and the glass fibre being perfectly rigid. The experimental disks 
are fixed on the glass fibre by means of a touch of cement at the back. The vertical 
tube is arranged to hold six disks, the top one, h, being always the same standard 
lampblacked pith ; the others, i, j, Jc, l, and m, being changed each time. A small 
magnet, n, attached to the central mirror, and controlled by a bar magnet outside, 
gives the power of bringing the beam to zero, should it happen to get out of adjust- 
ment, without having to melt the cement and alter the angle of the torsion fibre by 
turning the stopper d. Plate glass caps at o and p, cemented to the ground edges 
of the tubes, give access to the interior ; o allows the counterpoises to be adjusted in 
the pan, and p allows the aluminium stirrup to be unhooked and the whole of the 
disks to be lifted out together. The apparatus is connected to the mercury pump 
by the arm and spiral q. 
