2/10 DYNAMIC SYSTEMS 



tures of some places are held constant, the variations of the 

 other temperatures are observed after the initial moment. 



In physiology, the variables might be the rate of a rabbit's 

 heart-beat, the intensity of faradisation applied to the vagus 

 nerve, and the concentration of adrenaline in the circulating 

 bloocj. The intensity of faradisation will be continuously under 

 the experimenter's control. Not improbably it will be kept first 

 at zero and then increased. From a given instant the changes 

 in the variables will be recorded. 



In experimental psychology, the variables might be ' the number 

 of mistakes made by a rat on a trial in a maze ' and 4 the amount 

 of cerebral cortex which has been removed surgically '. The 

 second variable is permanently under the experimenter's control. 

 The experimenter starts the experiment and observes how the 

 first variable changes with time while the second variable is held 

 constant, or caused to change in some prescribed manner. 



2/9. The detailed statement just given about what the experi- 

 menter can do and observe is necessary because we must (as later 

 chapters will show) be quite clear about the sources of the experi- 

 menter's knowledge. 



Ordinarily, when* an experimenter examines a machine he makes 

 full use of knowledge ' borrowed ' from past experience. If he 

 sees two cogs enmeshed he knows that their two rotations will not 

 be independent, even though he does not see them actually rotate. 

 This knowledge comes from previous experiences in which the 

 mutual relations of similar pairs have been tested and observed 

 directly. Such borrowed knowledge is, of course, extremely use- 

 ful, and every skilled experimenter brings a great store of it to 

 every experiment. Nevertheless it must be excluded from any 

 fundamental method, if only because it is not wholly reliable: the 

 unexpected sometimes happens; and the only way to be certain 

 of the relation between parts in a new machine is to test the 

 relation directly. 



2/10. While a single primary operation may seem to yield little 

 information, the power of the method lies in the fact that the 

 experimenter can repeat it with variations, and can relate the 

 different responses to the different variations. Thus, after one 

 primary operation the next may be varied in any of three ways : 



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