1889.] on Quartz Fibres. 555 



over and over again in a strong oxyhydrogen jet. Then, if a stand 

 of any sort has been placed a few feet in front of the jet, it will be 

 found covered with a maze of thread, of which the photograj^h on the 

 screen represents a sample. This is hardly distinguishable from the 

 web spun by this magnificent spider in corners of greenhouses and 

 such places. By regulating the jet and the manipulation, anything 

 from one of these stranded cables to a single ultro-microscope line 

 may be developed. 



And now that I have explained that these fibres have such valu- 

 able properties, it will no doubt be expected that I should perform 

 some feat with their aid which, up to the present 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 I 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 six-foot beam in one direction, and then if the 

 balls are reversed in position, the deflection will be in the other 

 dii'ection. Now, when it is considered how enormously greater the 

 earth is than these balls, it will be evident that the attraction due to 

 them must in comj^arison be excessively small. To make this evident, 

 the enormous ajDparatus you see had- to be constructed, and then, 

 using a fine torsion wire, a perfectly certain but small effect was 

 produced. The experiment, however, could only be successfully 

 carried out in cellars or specially protected j)laces, because changes 

 of temperature produced 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 

 Cavendish apparatus, weighing several hundredweight each, ai-e re- 

 placed by balls weighing If lb. only. The smaller balls of If lb. 

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

 is replaced 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 aj)paratus of Cavendish, 

 and, what is more important, the accuracy of observation is 

 enormously increased. 



The light from a lamp passes 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 were plane, the whole move- 

 ment of the light would be only about eight inches, but the mii-ror is 



* Dr. Lodge has been able, by an elaborate arrangement of screens, to make 

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



Vol. XII. (No. 83.) 2 p 



