13/8 AN INTRODUCTION TO CYBERNETICS 



because it removes this particle of dust but because, in a lifetime, 

 it removes particles again and again— as many times as are necessary. 

 Most of the physiological regulators act again and again, as often 

 as is necessary. And the coastal lifeboat saves lives not once but 

 again and again. If, in the last few chapters, we have spoken of 

 "the regulatory response" in the singular, this is only because the 

 single action is typical of the set, not because the set necessarily has 

 only one element. 



So many of the well-known regulations are repetitive that it is 

 difficult to find a regulation that acts once only. A possible example 

 is given by an observatory making plans so as to have everything 

 ready in case a supernova should occur, an event not likely to occur 

 twice in the director's lifetime. Various possibilities would have 

 to be considered — in which part of the sky it might appear, whether 

 during day or night, the spectral and other peculiarities which would 

 determine what particular type of plate and filter should be used in 

 photogi-aphing it, and so on. In making his plans, the director 

 would, in fact, draw up a table like that of S.11/4, showing the 

 uncertainties (D) to be feared, the resources {R) available, and the 

 outcomes {E). Inspection of the table, as in Ex. 11/4/4, would then 

 enable him to decide whether, in all cases, he would get what he 

 wanted. 



There are, therefore, cases in which the regulation has to be 

 exerted against a non-repetitive disturbance, but they are uncommon. 



From here on we shall consider the case in which the disturbance, 

 and the regulatory response, occur more than once; for such cases 

 show constraint, of which advantage can be taken. 



13/8. The constraint occurs in the following way. 



The basic formulation of the regulatory process referred to a set 

 of disturbances but assumed only that the separate elements in the 

 set were distinct, nothing more. Like any other quantity, a disturb- 

 ance may be simple or a vector. In the latter case, at least two main 

 types are distinguishable. 



The first type was discussed in S. 11/17: the several components of 

 the disturbance act simultaneously; as an air-conditioner might, at 

 each moment, regulate both temperature and humidity. 



The second type is well shown by the thermostatically-controlled 

 water bath; it can be regarded as a regulator, over either short or 

 long intervals of time. Over the short interval, "the disturbance" 

 means such an event as "the immersion of this flask", and "its 

 response" means "what happens over the next minute". Its 

 behaviour can be judged good or bad according to what happened 



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