CHAPTER XI 



RATES 



WHEN one speaks of the speed of a passing automobile 

 as so many miles, say thirty, per hour, he is expressing 

 his estimate of an instantaneous rate, for he does not 

 mean that the machine will travel thirty miles in the 

 hour but that at the given instant its motion is such 

 that a continuance at this rate would result in this 

 displacement. The estimate is the result of previous 

 experiences, conscious or otherwise, in estimating 

 distances and times. Although the observer does 

 not make actual measurements of these two magni- 

 tudes and then perform an arithmetical operation 

 upon them, the method is fundamentally that which 

 was applied in the " speed traps " of the earlier days 

 of automobiles. In such traps two observation stations 

 were established along the roadway at measured dis- 

 tances apart and telephone connections were provided 

 between the observers. The time consumed by any 

 machine in traversing the known distance could then 

 be obtained and hence the speed in miles per hour, by 

 taking the ratio of the distance in miles to the time in 

 hours. 



The method admitted of a determination only of the 

 average speed through the trapped distance. Stories 

 were told, therefore, of drivers who astonished the 



128 



