250 Mechanical Action of Light. [April, 
the number of revolutions. I now turn this handle till the 
fibre breaks, and the counter will tell me how many twists 
I have given this fibre of glass. You see it breaks at twenty 
revolutions. This is rather a thicker fibre than usual. I 
have had them bear more than 200 turns without breaking, 
and some that I have worked with are so fine that if I hold 
one of them by the end it curls itself up and floats about 
the room like a piece of spider’s thread. 
Having now illustrated these properties of glass fibres, 
I will try to show a very delicate experiment. I want 
to ascertain the amount of pressure which radiation 
exerts on a blackened surface. I will put a ray of light 
on the pan of a balance, and give you its weight in grains, 
for I think in this Institution and before this audience I may 
be allowed a scientific use of the imagination, and may 
speak of weighing that which is not affeCted by gravi- 
tation. 
The principle of the instrument is that of W. Ritchie’s 
torsion balance, described by him in the “ Philosophical 
Transactions ” for 1830. The construction is somewhat 
complicated, but it can be made out on reference to the 
diagram (Fig. 11). A light beam, a b, having 2 square inches 
of pith, c, at one end, is balanced on a very fine fibre of 
glass, D d', stretched horizontally in a tube ; one end of the 
fibre being connected with a torsion handle, E, passing 
through the tube, and indicating angular movements on a 
graduated circle. The beam is cemented to the torsion fibre, 
and the whole is enclosed in glass and connected with the 
mercury pump by a spiral tube, f, and exhausted as perfectly 
as possible. G is a spiral spring, to keep the fibre in a uni- 
form state of tension. H is a piece of cocoon silk. 1 is a 
glass stopper, which is ground into the tube as perfectly as 
possible, and then highly polished and lubricated with melted 
india-rubber, which is the only substance I know that allows 
perfect lubrication and will still hold a vacuum. The pith, c, 
represents the scale-pan of the balance. The cross-beam, A B, 
which carries it, is cemented firmly to the thin glass fibre, D, 
and in the centre is a piece of mirror, k. Now the cross-beam 
A B and the fibre D being rigidly connected together, any twist 
which I give to the torsion handle Ewill throw the beam out 
of adjustment. If, on the other hand, I place a weight on the 
piece of pith c, that end of the beam will fall down, and I 
shall have to turn the handle, E, round and round a certain 
number of times, until I have put sufficient torsion on the 
fibre d to lift up the beam. Now, according to the law of 
