2/4 DYNAMIC SYSTEMS 



Thus, the position of a limb can be specified numerically by co- 

 ordinates of position, and movement of the limb can move a pointer 

 on a dial. Temperature at a point can be specified numerically 

 and can be recorded on a dial. Pressure, angle, electric potential, 

 volume, velocity, torque, power, mass, viscosity, humidity, sur- 

 face tension, osmotic pressure, specific gravity, and time itself, 

 to mention only a few, can all be specified numerically and 

 recorded on dials. Eddington's statement on the subject is 

 explicit: ' The whole subject matter of exact science consists of 

 pointer readings and similar indications.' ' Whatever quantity 

 we say we are " observing ", the actual procedure nearly always 

 ends in reading the position of some kind of indicator on a 

 graduated scale or its equivalent.' 



Whether the restriction to dial-readings is justifiable with living 

 subjects will be discussed in the next chapter. 



One minor point should be noticed as it will be needed later. 

 The absence of an entity can always be converted to a reading on 

 a scale simply by considering the entity to be present but in 

 zero degree. Thus, ' still air ' can be treated as a wind blowing at 

 m.p.h. ; 4 darkness ' can be treated as an illumination of foot- 

 candles ; and the giving of a drug can be represented by indicating 

 that its concentration in the tissues has risen from its usual value 

 of per cent. 



2/4. It will be appreciated that every real ' machine ' embodies 

 no less than an infinite number of variables, all but a few of which 

 must of necessity be ignored. Thus if we were studying the swing 

 of a pendulum in relation to its length we would be interested in 

 its angular deviation at various times, but we would often ignore 

 the chemical composition of the bob, the reflecting power of its 

 surface, the electric conductivity of the suspending string, the 

 specific gravity of the bob, its shape, the age of the alloy, its 

 degree of bacterial contamination, and so on. The list of what 

 might be ignored could be extended indefinitely. Faced with 

 this infinite number of variables, the experimenter must, and of 

 course does, select a definite number for examination — in other 

 words, he defines an abstracted system. Thus, an experimenter 

 once drew up Table 2/4/1. He thereby selected his variables, 

 of time and three others, ready for testing. This experiment 

 being finished, he later drew up other tables which included new 



15 



