1889.] Mr. a V. Boys on Quartz Fibres. 547 



WEEKLY EVENING MEETING, 



Friday, June 14, 1889. 



William Huggins, Esq. D.C.L. LL.D. F.E.S. Vice-President, 

 in tlie Chair. 



C. V. Boys, Esq. F.E.S. 



Quartz Fibres. 



In almost all investigations wMch the physicist carries out in the 

 laboratory, he has to deal with, and to measure with accuracy, those 

 subtle, and, to our senses, inappreciable forces to which the so-called 

 laws of nature give rise. Whether he is observing by an electrometer 

 the behaviour of electricity at rest, or by a galvanometer the action 

 of electricity in motion ; w^hether in the tube of Crookes he is investi- 

 gating the jDower of radiant matter, or by the famous experiment of 

 Cavendish he is finding the mass of the earth — in these and in a host 

 of other cases he is bound to measure, with certainty and accuracy, 

 forces so small that in no ordinary way could their existence be 

 detected ; while disturbing causes, which might seem to be of no 

 particular consequence, must be eliminated if his experiments are to 

 have any value. It is not too much to say that the very existence of 

 the physicist depends upon the power which he possesses of producing 

 at will, and by artificial means, forces against which he balances 

 those that he wishes to measure. 



I had better, perhaps, at once indicate in a general way the 

 magnitude of the forces with which we have to deal. 



The weight of a single grain is not to our senses appreciable, 

 while the weight of a ton is sufficient to crush the life out of any one 

 in a moment. A ton is about 15,000,000 grains. It is quite possible 

 to measure, with unfailing accuracy, forces which bear the same 

 relation to the weight of a grain that a grain bears to a ton. 



To show how the torsion of wires or threads is made use of in 

 measuring forces, I have arranged what I can hardly dignify by the 

 name of an experiment. It is simply a straw hung horizontally by a 

 piece of wire. Eesting on the straw is a fragment of sheet-iron 

 weighing ten grains. A magnet, so weak that it cannot lift the iron, 

 yet is able to pull the straw round through an angle so great that the 

 existence of the feeble attraction is evident to every one in the room. 



Now it is clear that if, instead of a straw moving over the table 

 simply, we had here an arm in a glass case and a mirror to read the 

 motion of the arm, it would be easy to observe a movement a hundred 

 or a thousand times less than that just produced, and, therefore, to 



