QUARTZ FIBERS. 323 



by this maguificeut spider in corners of greenhouses and such jilaces. 

 By regulating the jet and the manipulation, anything from one of these 

 stranded cables to a single ultra-microscope line may be developed. 



And now that I have explained that these libers have such valuable 

 properties, it will no doubt be expected that I should i)erform some 

 feat with their aid which, up to the i^resent time, has been considered 

 impossible, and this I intend to do. 



Of all experiments the one which has most excited my admiration is 

 the famous experiment of Cavendish, of which 1 have a full size model 

 before you. The object of this experiment is to weigh the earth by 

 comparing directly the force with which it attracts things with that 

 due to large masses of lead. As is shown by the model, any attraction 

 which these large balls exert on the small ones will tend to deflect this 

 6-foot beam in one direction, and then if the balls are reversed in posi- 

 tion the deflection will be in the other direction. Now, when it is con- 

 sidered how enormously greater the earth is than thesB balls, it will be 

 evident that the attraction due to them must be in comparison excess- 

 ively small. To make this evident the enormous apparatus you see 

 had to be constructed, and then, using a fine torsion wire, a perfectly 

 certain but small effect was jiroduced. The experiment however could 

 only be successfully carried out in cellars and underground places, 

 because changes of temperature iiroduced effects greater than those 

 due to gravity.* 



Now I have — in a hole in the wall — an instrument no bigger than a 

 galvanometer, of which a model is on the table. The balls of the Cav- 

 endish apparatus, weighing several hundredweight each, are replaced 

 by balls weighing 1;^ pounds only. The smaller balls of 1| pounds are 

 replaced by little weights of 15 grains each. The 6-foot beam is re- 

 placed by one that will swing round freely in a tube three-quarters of 

 an inch in diameter. The beam is, of course, suspended by a quartz 

 fibre. With this microscopic apparatus, not only is the very feeble 

 attraction observable, but I can actually obtain an effect eighteen times 

 as great as that given by the apparatus of Cavendish, and, what is 

 more important, the accuracy of observation is enormously increased. 



The light from a lamp i)asses through a telescope lens and falls on 

 the mirror of the instrument. It is reflected back to the table, and 

 thence by a fixed mirror to the scale on the wall, where it comes to a 

 focus. If the mirror on the table wore plane, the whole movement of 

 the light would be only about 8 inches, but the mirror is convex, and 

 this magnifies the motion nearly eight times. At the present moment 

 the attracting weights are in one extreme position, and the line of light 

 is quiet. I will now move them to the other position, and you will see 

 the result — the light slowly begins to move, and slowly increases in 



* Dr. Lodge has been able, by aa elaborate arraugemeut of screens, to make this 

 attraction just evident to an audience. — C. V. B. 



