308 THE LANGUAGE AND CONCEPTS OF CONTROL 



ical phenomena, analogies which can be used to enable a continuously 

 varying measurement to be recorded, amplified, analyzed or operated upon, 

 and the results used as an immediate control on the process. Analogies can 

 be very simple. A small-scale drawing can be used in the solution of a geo- 

 metrical problem of finding the height of a tree from the length of its shadow. 

 The sliderule is an analogue of logarithm tables. The addition of two con- 

 tinuously varying numbers can be done by superposing two electrical cur- 

 rents, each in a separate circuit and proportional to one of the numbers, and 

 measuring the total current through a common part of the circuit. 



This principle has been built into analog computers. Much of the analog 

 computer is electrical, but mechanical wheels, gears, cams, and levers, and 

 magnetic and electromagnetic devices are used wherever they can provide a 

 closer analogue to the real process being represented. Such computers are 

 ideal instruments for solving simultaneous and differential equations, as will 

 be shown in an example in a later section. 



Many continuously varying systems are suited to analogies of this sort. 

 Generally speaking there are continuous processes in the living thing, the 

 most easily recognizable ones being at the molecular level, continuous ex- 

 pression of which was detailed in Chapter 8. The general control of the sys- 

 tem is a result of control of each process at the molecular level. Thus the 

 speeds of the parts control the general health of the whole, and the general 

 health of the whole in turn adds the fine adjustment to the speeds of the 

 parts. 



However, on a larger scale analog control is not so easy to recognize, 

 partly because the physiological basis for digital control by the pulsating 

 nervous system is easier to study experimentally than the continuous varia- 

 tion which are superimposed on the pulses; and partly because this language 

 of control has not yet been successfully used to describe chemical regulatory 

 systems such as the endocrine glands. * One can find many examples of 

 analogies used disparts of a controlling system in the living thing, but one .is 

 hard put to it to describe clearly at this time a full analog computer which is 

 in complete control of part of the living system. Many neurophysiologists now 

 feel that the digital computation may be only a small part of the complete 

 story of control, even in the central nervous system. 



New Dimensions 



In summary, then, the human being, and indeed every living organism, 

 has control operations which might be described in the same terms used to 

 describe digital and analog computers. How fruitful this description will 



+ See Schueler's recent book for examples of pharmacological control. 



