332 QUARTZ FIBERS. 



inence, when you attemipt to push the delicacy of your apparatus to the 

 extent that I have reached in the hoine-nuide apparatus which I liave 

 here this evening. I do not propose to give more than one ilhistration, 

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

 pose the source of heat that I used was a common candle, placed suflfi- 

 cieiltly far off to produce a convenient deflection. I began by placing 

 the candle about 100 yards away, but I was obliged to place the caudle 

 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 flyiug 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 experiment of the sim- 

 plest 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. 



Supposing you take a little mirror suspended at an angle of 45 de- 

 grees 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 fiber on which it is sus- 

 pended is about the fifteen-thousandth part of an inch in diameter. 



