3^ PRESIDENTIAL ADDRESS SECTION A. 



Cambridge, took up Galileo's work and first developed the laws 

 of motion, which he enunciated (in substance) as: — 



1. (The law of inertia). A body moves in a straight line 



with constant velocity unless interfered with. 



2. The "change of quantity of motion " (mass X velocity) 



varies as the " force " applied. [This law has been 

 changed by later thinkers to '" the rate of change 



3. Action and reaction are equal and o])posite. 



In these simple laws was embodied much original thought: 

 we have been educated on such lines that they seem to us straight- 

 forward and to a great extent obvious. But (i), the word 

 " mass " connotes a new idea, which the Iniman mind had not 

 grasi)ed before — this idea is entirely Newtonian. It implies that, 

 apart from ivcight ( wlu'ch is an old idea — iM-ehistoric). a bod}' 

 has a certain iinariablc property implied bv the words " (quantity 

 of stuff " or " matter." which has primarily nothing to do with 

 weight; the idea in Newton's mind seems to have been almost 

 as crudely objective as this: that matter consists of units of 

 stuff — "molecules," "atoms." if vou will — which are indivisible: 

 and bodies only differ from one another for motion purposes by 

 consisting of dift'erent numl)ers of these units, so that "ob- 

 viously" (dangerous word) each body has a "quantity of motion" 

 — say, 1,000 units moving each with 5 units of velocity implying 

 5 oco times as much motion as one unit moving with unit velocity. 

 Just as 1. 000 subscribers of £5 each may be regarded as 5.000 

 unit subscribers, or 1,000 men putting in 5 hours' work ^ 5,000 

 hours' work. This idea has 'become a commonplace to our civili- 

 zation ; but it is not fundamentally above criticism — it is one 

 I^K)int on which revolt is possible in the new era — as. for instance, 

 when the development of thought on the all-pervading" aether " 

 has led to the statement, " The mass of an electron must be re- 

 garded as wholly electric." 



The second word and idea that attracts attention is " force." 

 The second law can be regarded as a definition of the word 

 " force " — i.e.. force is that asoect of the cause of motion which 

 reveals itself in the amount of the deviation from uniform velo- 

 city in a straight line. We can quite well build up a complete 

 system on this definition, resolvine to ignore all the ideas attach- 

 ins". before Newton's time, to the word force, ignoring our muscu- 

 lar sensations. In acquirin(? or in teaching^ the orthodox Science 

 of Mechanics, and. I imagine also, in other Sciences, the process 

 seems to consist largely of sifting out a ijreat mass of pre- 

 conceived ideas and choosin.e a mere thread of them as forming 

 the true conceotion. the rest being- expected to conform with this 

 scheme or being- rejected as illusions. Indeed, is not this the 

 essence of the idea of " Science "? and is not the aim of mathe- 

 matical treatment to get away from " common-sense " and the 

 senses senerallv on to a more trnstworthv olane of abstract, 

 abrost superhunian, reasoning? But in his third h.w and in the 



