THE MECHANICAL ACTION OF LIGHT. 275 



but gives way when additional weight is added. You see, then, the 

 great strength of a fibre of glass, so fine as to be invisible to all who 

 are not close to it, to resist a tensile strain. 



Now I will illustrate another equally important property of a glass 

 thread, viz., its power to resist torsion. Here is a a still finer glass 

 thread, stretched horizontally between two supports ; and in order to 

 show its position I have put little jockeys of paper on it. One end is 

 cemented firmly to a wooden block, and the other end is attached to 

 a little instrument called a counter a little machine for registering 

 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 gravitation. 



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 

 two square inches of pith, C, at one end, is balanced on a very fine 

 fibre of glass, DD r , 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 inclosed 

 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 uniform state of tension, i^is a piece of cocoon silk. 

 J 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 lubri- 

 cation 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 E 

 will throw the beam out of adjustment. If, on the other hand, I 

 place a weight on the piece of pith 0, that end of the beam will fall 

 down, and I shall have to turn the handle, E, round and round a cer- 



