186 3Ir. G. Johnstone Stoney [Feb. 6, 



What Fouce is. 



Many persons suppose that force, mass, energy and so on have an 

 independent existence in nature. In reality they are functions of the 

 motions that occur in nature, and of relations between these motions. 

 These j^articular functions are so constantly presenting themselves in 

 our dynamical calculations, that it is most convenient to have definite 

 names and symbols for them, but it is a grave error to mistake them 

 for the real agents, and not to recognise that they really are merely 

 contrivances, most useful contrivances, for simplifying the description 

 of complex phenomena, so as to enable us to grasj) sufficiently those 

 parts of a problem that are essential for the purpose we may happen 

 to have in view. 



It is sometimes said — " Force is the cause, motion the effect." 

 This statement needs much amendment. Force is not any existing 

 thing. When we speak of so much force as acting in any particular 

 case, we only mean to indicate that some sufficient cause of accelera- 

 tion is present acting up to that particular degree which is capable of 

 producing a change of motions of a specified amount ; and the con- 

 venience of the term is that it successfully avoids the necessity of 

 paying attention to what the cause is. 



The reason why a stone falls is not, as is sometimes supposed, 

 that an agent which can be called the force of gravitation is acting 

 upon it. The real cause is that the great mass of molecular motions 

 called the earth is in sufficient proximity to that other mass of 

 motions called the stone. The cause why a bolt is projected from a 

 cross-bow is that by the action of the bow the molecular motions on 

 the front of the string have been brought into sufficiently close 

 relation to the molecular motions at the back of the bolt. The word 

 force expresses that one or other of these causes, or some other cause 

 capable of producing the effect, is present ; with the great convenience 

 that it relieves us from the necessity of determining which of the innu- 

 merable possible causes is the one acting in the case we are dealing 

 with — a problem unnecessary, and often too difficult for solution. 

 The term and its mathematical symbol are therefore most useful by 

 enabling us to get on with our work, when we want to find out lioio 

 much effect will arise, and do not care whether the real agent is a 

 cord pulling, a spring urging, a magnet attracting, or any other. 

 But in every case treated of in the science of dynamics (i. e. in every 

 case in which we are studying the drifting about of masses of mole- 

 cular motions), the real cause is that certain motions (whether that 

 vast accumulation of motions called the earth, the end motions of the 

 cord, the front motions of the spring, or wliatever they may be) have 

 come into the requisite position with relation to that other aggregation 

 of motions that is moved. 



What Mass is. 



Similarly the symbol used by mathematicians and called by them 

 the mass of an object, suppose of a stone, is the convenient con- 



