344 The Mechanical Action of Light. iJuly, 
while it follows a piece of ice as a needle follows a magnet. 
With a large bulb, very well exhausted and containing a 
suspended bar of pith, a somewhat striking effet is pro- 
duced when a lighted candle is placed about 2 inches from 
the globe. The pith bar commences to oscillate to and fro, 
the swing gradually increasing in amplitude until the dead 
centre is passed over, when several complete revolutions are 
made. The torsion of the suspending fibre now offers re- 
sistance to the revolutions, and the bar commences to turn 
in the opposite direction. This movement is kept up with 
sreat energy and regularity as long as the candle burns. 
For more accurate experiments I prefer making the appa- 
ratus differently. Fig. 4 represents the best form. abisa 
glass tube, to which is fused at right angles another nar- 
rower tube, cd; the vertical tube is slightly contracted at e, 
so as to prevent the solid stopper d—which just fits the bore 
of the tube—from falling down. The lower end of the 
Fie. 5. 
stopper, de, is drawn out to a point; and to this is cemented 
a fine glass thread about o‘oor inch diameter, or less, ac- 
cording to the torsion required.* 
At the lower end of the glass thread an aluminium stirrup 
and a concave glass mirror are cemented, the stirrup being 
so arranged that it will hold a beam, fg, having masses of 
any desired material at the extremities. At ¢ in the hori- 
zontal tube is a plate-glass window cemented on to the 
tube. At 0 is also a piece of plate glass cemented on. 
* Some of the glass fibres used in these torsion balances are so fine that 
when one end is held between the fingers the other portion floats about like 
a spider’s thread, and frequently rises until it takes a vertical position. 
