﻿illustrate the Laws of Motion. 333 



The apparatus which is described in works on physics as 

 available for the purpose of proving them, consists merely of a 

 very few instruments, among which Atwood's machine is that 

 most commonly referred to. This meagreness contrasts strangely 

 with the profusion of apparatus which has been devised for the 

 illustration of the elements of other branches of science (electri- 

 city, for example) ; yet surely it would be as desirable to prove 

 to a student the second law of motion by direct experiment, as 

 it is to demonstrate experimentally the laws of electrical induc- 

 tion. I have communicated this paper, thinking that any contri- 

 bution, however small, to the list of apparatus available for 

 this purpose might prove useful to others, as it has been to 

 myself. 



A certain principle should always be borne in mind by the 

 arranger of an experiment which is to be used for purposes of 

 elementary instruction. A law A is required to be proved ; and 

 an experiment is performed which demonstrates the fact B ; it 

 is then shown by logical inference that the truth of the fact B 

 necessitates the truth of the fact A. Thus A has been proved by 

 two distinct steps — (1) the experiment proving B, (2) the chain 

 of reasoning connecting A with B. For the instruction of a 

 class of beginners, it is essential that the experiment be so se- 

 lected that the second step be reduced to a minimum, or, if pos- 

 sible, totally dispensed with. To take an illustration from the 

 subject with which we are engaged. Suppose it be required to 

 prove that a body will fall sixteen feet in the first second. The 

 distance may, of course, be determined most accurately by find- 

 ing the time of vibration of a pendulum ; but the subsequent 

 chain of reasoning would be quite unintelligible. Atwood's 

 machine might be made to give some approximation to the value ; 

 but it likewise does not indicate the direct result, but rather 

 something from which the direct result is to be inferred by cal- 

 culation. This appears to me one of the defects in AtwoocPs 

 very beautiful machine : it possesses neither great accuracy nor 

 great simplicity in the interpretation of its results ; and one or 

 the other, if not both, of these features should characterize every 

 experiment. Besides, in its ordinary form Atwood's machine is 

 quite unadapted for use in the lecture-theatre. 



The arrangements now to be described have been designed 

 with a view of proving the points required as directly as possible 

 and with the minimum amount of subsequent reasoning. 



It should be mentioned that whatever has been necessary for 

 the purpose of supports and framework has been constructed out 

 of the very beautiful system devised by Professor Willis, and de- 

 scribed in his ' System of Apparatus for the use of Lecturers and 

 Experimenters in Mechanical Philosophy ' (Weale and Co.). 



