ME. W. CEOOKES ON REPULSION RESULTING FROM RADIATION. 
339 
impulse when the needle rests in the cup. The apparatus being arranged as shown, 
the cap is cemented on and the whole is exhausted. 
144. When the vacuum is within a few millimetres of being perfect, the arms of this 
instrument move when a candle is brought near ; as the pump continues working rota- 
tion commences, which gets more and more rapid, until, with the candle close to the 
glass, several revolutions are made per second. When well exhausted, the following 
experiments were tried : — 
A flask of boiling water, placed 1 inch from the outer glass, caused the arms to set at 
right angles to the line joining the flask and the pivot, showing that the heat from 
boiling water acts on black and white surfaces equally (128). 
Copper at 400° C. kept the arms revolving, at first quickly, then slowly, until, as the 
copper cooled, the rotation stopped, and the black and white surfaces ultimately set 
equidistant from the hot metal (128). 
A candle always set the arms revolving, when it was near enough for the force to 
overcome the friction. 
This showed that rotation was possible, and that it would be kept up as long as the 
radiation lasted ; and I accordingly devised a form of apparatus which would enable 
this action to be shown with greater facility. Owing to there being only two disks, the 
action of light was not uniform, as if it struck the arms at the end, instead of at the side, 
movement would not be commenced. Also the cement joint rendered it impossible to 
get the vacuum very good, whilst it took away from the permanent character of the 
instrument. After many trials of different arrangements, an instrument was made which 
had none of these defects, whilst it showed the movement of rotation in a very convenient 
manner. 
145. The apparatus is shown in fig. 6. It consists of four arms, of very fine glass, 
passing horizontally through pieces of pith (5), and 
afterwards bent twice at right angles, as shown in 
the figure. Through the centre of the pieces of 
pith (b) is passed vertically the point of a very fine 
sewing-needle («), which rests in a glass cup (c) 
blown on to the end of the glass tube e. At the 
end of each glass arm is fastened a thin disk of pith, 
white on one side and lampblacked on the other, 
the black surfaces of all the disks facing the same 
way. The whole is enclosed in a glass bulb blown 
on to the end of a wide tube, f is a piece of 
cement to keep the support ( e ) in its place, g is 
the tube containing cocoanut-shell charcoal ; the 
other end is sealed on to the mercury-pump. The 
exhaustion is effected as already described (131) ; 
and the apparatus is then sealed off, with the char- 
Fig. 6. 
