324 QUARTZ FIBERS. 



movement. In K) seconds it will have acciuired its highest velocity, 

 and in 40 more it will have stopped at 5 feet 8i inches from the start- 

 ing point, after which it will slowly move back again, oscillating about 

 its new position of rest. 



It is not possible at this hour to enter into any calculations ; I will 

 only say that the motion you have seen is the effect of a force of less than 

 one ten-millionth of the weight of a grain, and that with this apparatus 

 1 can detect a force two thousand times smaller still. There would be 

 no difficulty even in showing the attraction between two No. 5 shot. 



And now in conclusion, I would only say that if there is anything 

 that is good in the experiments to which 1 have this evening directed 

 your attention, experiments conducted largely with sticks and string 

 and straw and sealing-wax, I may perhaps be pardoned if I express my 

 conviction that in these days we are too apt to depart from the simple 

 ways of our fathers, and instead of following them, to fall down and 

 worship the brazen image which the instrument-maker hath set up. 



11. * 



Before I enter upon the subject upon which I have to address you, I 

 wish to point out that, quite apart from any deficiency on my part which 

 will be only too apparent in the course of the evening, it is my inten- 

 tion to commit two faults which may well be considered unpardonable. 

 In the first place, I shall speak entirely about my own experiments, even 

 though I know that the iteration of the first personal pronoun for the 

 space of one hour is apt to be as monotonous to an audience as it is 

 wanting in taste on the part of a lecturer. In the second place, I am 

 going almost to depend upon the motions of a spot of light to illustrate 

 the actions which I shall have to describe, in spite of the fact that it is 

 impossible for an audience to get up any enthusiasm when watching 

 the wandering motion of a spot of light the result of the mauiiiulation 

 of a mystery box, of which it is impossible to see the inside. These 

 however are faults which are the immediate consequence of the nature 

 of my subject. 



Physicists deal very largely with the measurement of extremely mi- 

 nute forces, which it is of the utmost importance that they should be able 

 to measure accurately. Now, forces may be considered under two 

 aspects. It may be that the force which is developed and which has 

 to be measured is a twist, in which case the twisting force may be ap- 

 plied to the end of a wire directly, when the amount through which 

 that wire is twisted is a measure of the twisting force. Or the force 

 may be a direct pull or a push, which may also be measured by the 

 twist of a wire if it is applied to the end of a lever or arm carried by the 

 wire. 



" Lecture delivered on September 8, 1890, at the Leeds meeting of the British Asso- 

 ciation. 



