Transactions

MASON, PHILBRICK—MATHEMATICS OF SURGE VESSELS AND AUTOMATIC AVERAGING CONTROL 601the momentary correspondence11. . . . in order to restore . . . thesensed variable precisely to its set value.” This definition,carefully framed so as to apply in general, is worthy of note at atime when terminology is still in a fluid state. If anything therehas been too little tendency to generalize—apparent in the literatureof automatic control (or automatic regulation). Untilplaced on an independent footing and freed of all specializingconcepts, this subject will never attain recognition as a branch ofknowledge in its own right as we know it deserves to be.The discussion of E. W. Yetter and J. C. Peters is devotedprincipally to electrical analogs of the hydraulic system and controllingequipment treated in the paper. Such analogy may leadto a quicker perception, by many electrical engineers, of the dynamicphenomena described, but it may be remarked that theprocess of analogy is traditionally the other way around; hydraulicanalogs serving to make more tangible the functionalperformance of electric-circuit elements and circuits. A greatmany engineers we feel sure, mechanical engineers for example,11 The authors would add the words “or proportionality.”would not agree to the relatively higher development of mathematicsin electrical as compared with other technical fields. Thentoo, the rudimentary sort of wave mechanics which is representedin the standard alternating-current theory is not limited in applicabilityto electric circuits. It is significant that the operationaltreatment, although actually no more involved than any other,is versatile to the extent that it yields the wave-mechanics, or frequency“spectrum,” solution if interpreted one way and the completetransient solution if interpreted another, both solutionscoming down from the same operational form.An entire series of mechanical, thermal, pneumatic, and/electricalanalogs may be placed in correspondence with, and will adequatelyrepresent, the hydraulic system assumed in the paper.Beyond those introduced in the discussion, a number of otherelectrical analogs are possible and could suffice as models for thehydraulic prototype. A mechanical interpretation, in which theflows become displacements and the capacitance of the vessel isreplaced by a massive body, can be traced out in complete detailand is easy to visualize. Under this latter analogy the problemof automatic control is seen as a true problem in shock-absorbing.