ME. W. CEOOKES ON EEPULSION EESTJLTING FEOM EADIATION. 
371 
Eig. 17. 
h 
Yertical section of cross tube at the centre. Yertical section of cross tube at the end 
containing the pith. 
fg are two narrower tubes sealed into a b, as shown, the part d e being 17 inches long, 
and fg 38 inches long, d e is \ an inch in diameter, and is closed at the end ; f g is 
a \ of an inch diameter, h i is a thin glass beam, having a glass weight at the end Ji , 
and a flat surface of lampblacked pith (j) at the end i. The pith (j) exposes exactly 
2 square inches of surface. It is prevented from curling up in the vacuum by cutting 
it partly through at intervals, as shown, k is a silvered mirror, cemented on to the 
centre of the glass beam ; 1 1 are two knife-edges of glass, finely ground and polished ; 
m n is a very fine fibre of glass cemented to the beam (hi) beneath the mirror (k). 
The end (n) of the glass torsion-fibre is attached to the solid stopper (r), which is 
carefully ground and polished into the contracted part of the tube (s). The other end 
of the torsion-fibre (to) is cemented to a filament of silk (to o ) ; this silk is attached to 
one end of a steel spring (p), which is held firmly at the other end by the solid rod (q) 
cemented to the glass tube (d e ). The torsion-fibre (to n) passes over the knife-edges 
(l l), which thus support the beam. The tension of the spring (p) is so adjusted that the 
glass fibre shall remain stretched under a constant pull of about 100 grains, tt is a 
circular card cemented to the end of the tube (g) and divided into 360 degrees. On 
the stopper ( r ) is fixed a pointer (u), which revolves with it, and marks the degrees 
through which it has turned. The complete revolutions are recorded on a “ counter ” ( w ). 
Motion is given to r by a torsion-handle ( v ). The apparatus is connected to the pump 
by the spiral (x), which works in a stopper tap (y) most ingeniously devised by my friend 
and assistant Mr. C. H. Gimiugham, so as to enable the instrument to be discon- 
nected from the pump and attached to it again without interfering with the ex- 
haustion*. 
The stopper (r) is very accurately ground and polished in the tube at s, as long a 
surface as possible being in contact. It should be lubricated by setting fire to a piece 
of plain india-rubber, and allowing the melted drops to fall on the contact surfaces. 
* I have asked Mr. Gtimingham to send a description of this tap, and some useful improvements he has 
introduced in the mercury-pump, to the Eoyal Society. 
