STABILITY 4/12 



4/11. This definition of stability conforms to the requirement 

 of S. 2/8 ; for the observed behaviour of the system determines 

 the field, and the field determines the stability. 



Feedback 



4/12. The description given in S. 4/1 of the working of the 

 Watt's governor showed that it is arranged in a functional circuit : 

 the chain of cause and effect is re-entrant. Thus if we represent 

 4 A has a direct effect on B ' or ' A directly disturbs B ' by the 

 symbol A — > B, then the construction of the Watt's governor may 

 be represented by the diagram : 



Speed of 

 engine 



Distance 

 between 

 weights 



Velocity 



of flow 



of steam 



(The number of variables named here is partly optional.) 



Lest the diagram should seem based on some metaphysical 

 knowledge of causes and effects, its derivation from the actual 

 machine, using only primary operations, will be described. 



Suppose the relation between ' speed of engine ' and 4 distance 

 between weights ' is first investigated. The experimenter would 

 fix the variable ' velocity of flow of steam '. Then he would try 

 various speeds of the engine, and would observe how these changes 

 affected the behaviour of ' distance between the weights '. He 

 would find that changes in the speed of the engine were regularly 

 followed by changes in the distance between the weights. He 

 need know nothing of the nature of the ultimate physical linkages, 

 but he would observe the fact. Then, still keeping i velocity of 

 flow of steam ' constant, he would try various distances between 

 the weights, and would observe the effect of such changes on the 

 speed of the engine ; he would find them to be without effect. 



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