WEIGHTS AND MEASURES 25 



seem very foolish, l but such a possibility must always 

 be guarded against, especially when precise measure- 

 ment is desired. For example, a steel tape line varies 

 in its length, increasing on hot days. In surveying 

 over large areas, as in the case of the U. S. Coast and 

 Geodetic Surveys, extreme precautions are taken in 

 measuring a base line to compensate for such linear 

 expansion. If a tape is used care is also taken that it 

 shall always be under the same tension. 



For the measurement of length it was natural that 

 the earliest units should be connected with the human 

 body. Thus lengths were measured in spans, in digits, 

 in feet, or in the length of a step or pace. Longer 

 distances were measured in terms of a day's march or 

 journey. These units all differ from man to man, 

 so that if two persons measure the same distance they 

 will express it differently, because they will count 

 different numbers of units. 



Thus it came about when men wished to have a 

 common understanding that they had to agree as to 

 whose foot or pace should be used as the standard. 

 Many times these lengths were referred to the body 

 of a king or of a priest. In order that the chosen units 

 should be commonly available for comparison, they 

 were sometimes marked on the wall of the city or of 

 a temple. This method of recording standards per- 

 sisted until comparatively recently, for there are 

 European cathedrals on the outer walls of which 

 standards are marked. 



1 The use in measurements of a standard which changes, without 

 making proper allowance for such changes, we recognize as a form 

 of mistake which is frequently made in the popular discussions of 

 social and economic relations. 



