October i6, 1890] 



NA TURE 



607 



h its dark edge foremost. The clockwork will now bring the 

 moon back again, and convert the nineteen-day moon into a 

 nine-day moon, one in which the bright edge goes forward. 

 What I want you to notice, and it will be perfectly evident, is this, 

 that the spot of light will now go up the scale suddenly, will then 

 rise to a maximum position, and will then fall slowly until the 

 terminator is reached, which proves that in the former case the 

 slow rise and sudden fall, or the present sudden rise and slow 

 fall was not a peculiarity of the instrument, but was due to the 

 fact that the different points of the moon radiated in the manner 

 which I have stated. There is one point which, as the moon 

 has now left the instrument, I should like to show ; that is, that 

 it is a real moon and not a mere slide. That is shown by gradually 

 moving the sun round. Now it is at right angles to the line of 

 view, and we have got the half moon. As it goes round, the 

 moon continues waning, appearing more like a new moon, and 

 at last we have an eclipse of the sun, which may be annular 

 if the proportions of the apparatus are properly arranged. 



I wish now to make a few statements as to the delicacy of 

 apparatus that can be made with the help of quartz fibres. I 

 would wish you most distinctly to understand that it is not 

 sufficient to go into a shop and buy apparatus as it is now 

 made, replace the silk by quartz, and to suppose you can get a 

 degree of delicacy such as I have shown you. That is not 

 sufficient. If you take out the silk and put in a quartz fibre the 

 apparatus will be much improved, and you can then increase 

 its delicacy. You will then escape the troubles due to silk ; 

 but one after the other a new series of disturbances will appear, 

 and anything like ultimate, extreme and minute accuracy 

 will still seem out of the question. Now it has been my 

 business to eliminate one by one these disturbing influences. I 

 will not weary you with a description of them all, and the 

 methods by which they may be certainly provided against. 

 These disturbing causes, which at the present time with instru- 

 ments carrying a silk fibre are not even known to exist, or 

 if known to exist, are practically of no consequence what- 

 ever, come one by one into prominence, when you attempt 

 to push the delicacy of your apparatus to the extent that I have 

 reached in the home-made apparatus which I have here this 

 evening. I do not propose to give more than one illustration, 

 and as this is one which I found out by accident, and which at 

 the time very much annoyed me, I imagine that it may be of 

 interest to explain the circumstances under which this was 

 observed. 



In the experiments I made on the heat of the moon and the 

 stars it was necessary to determine to what degree of delicacy 

 the apparatus could be brought — that is to say, to determine 

 what deflection would be produced by a known and familiar 

 source of radiation. For this purpose the source of heat that I 

 used was a common candle, placed sufficiently far off to produce 

 a convenient deflection. I began by placing the candle about 

 100 yards away, but I was obliged to place the candle at a 

 distance of 250 yards. At that distance I could not conveniently 

 at night turn the shutter on and off with a string. Therefore I 

 adopted the more simple and practical plan of asking my niece to 

 stand at the top of the hill and to pull the string when I gave the 

 signal. The signal was nothing more nor less than my saying the 

 word " on " or " off," so that without moving I could observe the 

 deflection due to the heat of the candle at that distance. Those 

 were the circumstances, but when I shouted "on," before the 

 sound could have reached my niece at the top of the hill, the 

 spot of light had been driven violently off the scale. This seemed 

 as if, as I suspected at the time, one of my little eight-legged 

 friends had got inside the apparatus, and feeling the trembling 

 due to the sound, struck forward, as the diadema spider is known 

 to do, and tried to catch the thing that was flying by. But 

 further experiments showed that this was not the case. It 

 happened that the sound of my voice was just that to which the 

 telescope tube would respond. It echoed to that note, the 

 instrument felt the vibration of the air, and that was the result. 



In order to show that an instrument will feel the motion in 

 the air under the influence of sound, I have arranged an experi- 

 ment of the simplest possible character. I should say that the 

 first instrument of this kind was made many years ago by Lord 

 Rayleigh ; but I feel sure that even he would not be prepared 

 for the delicacy to which apparatus on this principle can be 

 brought. It simply depends upon this familiar and well-known 

 fact. A card or a leaf allowed to drop through the air does not 

 fall the way of the least resistance — that is, edgeways — but it turns 

 into the position of greatest resistance, and falls broadside on, or 

 it overshoots the mark, and so gets up a spin. 

 NO. 1094. VOL. 42] 



Supposing you take a little mirror suspended at an angle of 

 45° to the direction of the waves of sound, the instant sound- 

 waves proceed to travel, that mirror turns so as to get into such 

 a position as to obstruct them. The mirror that I have for this 

 purpose weighs about the twentieth part of a grain, and the fibre 

 on which it is suspended is about the fifteen-thousandth part of 

 an inch in diameter. The mirror is so small and light that the 

 moment of inertia is a two-hundredth part of that which people 

 ordinarily call the minute and delicate needle of the Thomson 

 mirror-galvanometer. With a fibre only a few inches long, there 

 is no difficulty in getting a period of oscillation of ten or eleven 

 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 three 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 im- 

 possible 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 beforehand what I am 

 going to do, as the sound of my voice will spoil the experiment. 

 1 shall, standing as far away as I can get from the instrument, 

 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 in- 

 strument is tuned. I must ask everyone during this experiment 

 to be as quiet as possible, as the faintest sound of the right sort 

 will interfere with the success of the experiment. The first two 

 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. When this little organ pipe was blown 

 at the farthest end of the room this afternoon, it drove the light 

 off the scale, almost as violently as it did just now. 



[The Cavendish experiment of observing the attraction due to 



gravitation 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 respectively, 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 



i single oscillation. W^ith this apparatus forces two thousand 



i times as small could be observed, though the fibre is, in com- 



i parison with others that were made use of, exceedingly coarse. 



Forces equivalent to one million-millionth of the weight of a 



! grain were stated to be within the reach of a manageable quartz 



! fibre.] 



! Now that I have shown all that my limited time has permitted 

 me, I wish finally to answer a question which is frequently put 

 ! to me, and which possibly some in the room may have asked 

 themselves. The question may be put broadly in this form : 

 "These fibres no doubt are very fine, and very wonderful, 

 but are they of any practical use ? " This is a question which 

 I find it difficult to answer, because I do not clearly know what 

 is meant by " practical use." If by " a thing of practical use" 

 you mean something which is good to eat or to drink, or if you 

 mean something which we may employ to protect ourselves 

 from the extremes of heat or cold or moisture, or if you mean 

 — and this is a point which those who have studied biology will 

 perhaps appreciate more than others — something which may be 

 made use of for the purpose of personal adornment ; if that is 

 what you mean by "practical use," then with the exception of 

 the possibility of being able to weave garments of an extra- 

 ordinary degree of fineness, softness, and transparency, quartz 

 fibres are of no " practical use." But if you mean something 

 which will enable a large and distinguished body of men to 

 do that which is most important to them more perfectly than has 

 been possible hitherto — I allude of course to the experimental 

 philosopher and his experimental work, which after all has laid 

 the foundations upon which so much that is called practical 



