252 
Mechanical Action of Light. [April, 
torsion, the force with which a perfectly elastic body like glass 
tends to untwist itself is direcftly proportional to the number of 
degrees through which it has been twisted; therefore, knowing 
how many degrees of torsion I must put on the fibre to lift up 
the i-iooth of a grain weight, I can tell how many degrees 
of torsion are required to lift up any other weight ; and con- 
versely, putting an unknown weight or pressure on the pith, 
I can find its equivalent in grains by seeing how much tor- 
sion it is equal to. Thus, if i-iooth of a grain requires 
10.000 degrees of torsion, i-5oth of a grain would require 
20.000 degrees ; and conversely, a weight which required 
5000 degrees torsion would weigh i-200th of a grain. Once 
knowing the torsion equivalent of i-iooth of a grain, the 
ratio of the known to the unknown weights is given by the 
degrees of torsion. 
Having thus explained the working of the torsion balance 
I will proceed to the adtual experiment. On the central 
mirror I throw a ray from the electric light, and the beam 
reflected on a particular spot of the ceiling will represent 
zero. The graduated circle j of the instrument also stands 
at zero, and the counter which I fasten on at the end l stands 
at o. The position of the spot of light reflected from the 
little concave mirror being noted, the torsion balance enables 
me to estimate the pressure or weight of a beam of light to a 
surprising degree of exactness. I lift up my little iron weight 
by means of a magnet (for working in a vacuum I am restricted 
in the means of manipulating), and drop it in the centre of 
the pith : it knocks the scale-pan down, as if I had placed 
a pound weight upon an ordinary balance, and the index ray 
of light has flown far from the zero-point on the ceiling. I 
now put torsion on the fibre to bring the beam again into 
equilibrium. The index-ray is moving slowly back again. 
At last it is at zero, and on looking at the circle and counter 
I see that I have had to make 27 complete revolutions and 
301 degrees, or 27 x 36o° + 3oT= 10,021°, before the force of 
torsion would balance the i-iooth of a grain. 
I now remove the weight from the pith-pan of my balance, 
and liberate the glass thread from torsion by twisting it back 
again. Now the spot of light on the ceiling is at zero, and 
the counter and index are again at o. 
Having thus obtained the value of the i-iooth of a grain in 
torsion degrees, I will get the same for the radiation from a 
candle. I place a lighted candle exadtly 6 inches from the 
blackened surface, and on removing the screen the pith 
scale-pan falls down, and the index-ray again flies across the 
