30 GENERAL SCIENCE 



matter and space to agree on certain standards by which 

 they shall be measured. There have been such standards 

 used by all the civilized nations, but they have been of 

 various values. For example, the unit of length corre- 

 sponding to the English foot has had different lengths 



FIG. 22. Standard Yard Bar. 



in different countries. This unit was usually derived 

 from the supposed average length of the human foot. 

 In England, the yard was finally established as the 

 standard unit of length and was supposed to represent 

 the length of the arm of Henry the First, but later the 

 standard was more definitely fixed and is now the length 

 of a metal bar kept in London (Figure 22). The United 

 States uses the same standard, a .copy of the standard 



Courtesy of Bureau of Standards, Washington, D.C. 

 FIG. 23. United States National Prototype Meter Bar. 



yard being kept at our Capitol. The foot was arbi- 

 trarily taken as one third of the yard, and the other units 

 were derived in the same way. There are 12 inches in 

 a foot, 3 feet in a yard, 5J yards or 16| feet in a rod, and 

 320 rods in a mile. 



