QUARTZ FIBERS. 333 



The mirror is so small and light that the moment of inertia is a two- 

 hundredth part of that wliich ]>eople ordinarily call the minute and 

 delicate needle of the Thomson mirror-galvanometer. With a fiber 

 only a few inches long, there is no difficulty in getting a period of oscil- 

 lation of 10 or 11 seconds. When the light from the lamp is reflected 

 and falls upon the scale, as it will be in a minute, then a movement of 

 the light from one of those great divisions to the next — that is, a move- 

 ment of 3 inches — will correspond to a twisting force such as would 

 be produced by pulling the end of a lever an inch long with a force of a 

 thousand-millionth part of the weight of a grain. It would be easy to 

 observe a movement ten or a hundred times less. My difficulty now is 

 that it is impossible to speak and at the same time to keep that spot 

 at rest, because the instrument is arranged to respond to a certain 

 note. This is not the predominating note of my voice, but since the 

 voice, like all other noises as distinguished from pure musical sounds, 

 consists of a great number of notes, every now and then the note to 

 which the instrument is tuned is sure to be sounded, and then it will 

 respond. Therefore, while I am speaking it is impossible to keep the 

 spot of light at rest. However, in order to show that the instrument 

 does respond to certain notes, even if feeble, with a degree of energy 

 and suddenness which I believe would never be expected, I shall with 

 these small organ pipes sound three notes. But I must explain before- 

 hand what I am going to do, as tlie sound of my voice will spoil the 

 experiment. I shall, standing as far away as I can get from the instru- 

 ment, first sound a note that is too high; I shall then sound a note 

 that is too low; and then I shall sound the note to which the instru- 

 ment is tuned. I must ask everyone during this experiment to be as 

 quiet as possible, as the faintest sound of the riglit sort will interfere 

 with the success of the experiment. [The first (wo notes sounded 

 loudly produced no result, while the moment the right note was heard 

 the light went violently off the scale and travelled round the room.] 

 W^hen this little organ pipe was blown at the farthest end of the room 

 this afternoon, it drove the light off" tlie scale almost as violently as it 

 did just now. 



[The Cavendish experiment of observing the attraction due to grav- 

 itation between masses of lead was then explained, and the actual 

 experiment, performed with apparatus no larger than a galvanometer, 

 in which the attracting masses were two pounds and fifteen grains, re- 

 spectively, in which the beam was only about five-eighths of an inch long, 

 and in which the total force was less than one ten millionth of the weight 

 of a grain, was then shown. The actual deflection on the scale was 

 rather more than ten feet, and eighty seconds were required for the 

 single oscillation. With this ai)[)aratus forces two thousand times as 

 small could be observed, though tiie fiber is, in comparison with others 

 that were made use of, exceedingly coarse. Forces equivalent to one 



