246 
MR. W. CROOKES ON REPULSION RESULTING FROM RADIATION. 
The weights and dimensions of the various parts of the apparatus are as follows 
Weight of straw beam, mirror, magnetic needle, aluminium 
stirrup, and flat glass fibre, &c 
Average weight of six plain mica disks 
Average weight of six plain pith disks 
Length of straw beam, from centre of counterpoising pan 
to centre of disks 
Length of arm, from centre of suspension to centre of pan . 
Length of arm, from centre of suspension to centre of disks 
Glass torsion fibre (this was the same as I employed in the 
experiments described in my last paper, par. 199) — 
Length 
Thickness 
Torsion, with the glass weight hanging from it 
(186, 199) 
7' 25 grains. 
2-40 „ 
0-59 „ 
17'0 centimetres. 
7-6 
9-4 
23-0 
O’OOIS inch. 
- oscillation in 15'75 seconds. 
Fig. 2 shows the apparatus fitted up for experimentation. The disks are shown 
in position at a ; a brick wall, b c, has holes pierced through it in two places, as shown, 
one hole, cl, being opposite the centre mirror, and the other, e, opposite the disks. 
The aperture d is fined with card, lampblacked inside, and the interstices between it 
Fig. 2. 
and the bricks are well plugged with cotton wool. A water cell at d prevents radiant 
heat from the lamp getting to the apparatus. Through the hole e pass six card tubes, 
lampblacked internally, 20 millims. diameter and 23 centims. long. The tubes are 
firmly cemented to the wall, so that each shall be exactly central with its corresponding 
disk, and the outer end of each is closed with a cork. The space between the tubes 
and wall is well stuffed with cotton wool. The apparatus, being once fixed in position, 
is surrounded on all sides, as well as above and below, with cotton wool. Outside this 
is a row of glass bottles filled with water, and in front of all is a wooden screen. 
When protected in this manner, the inside of the apparatus is found to be free from 
