IS-] LINEAR MOTION. 7 



platinum bar preserved in the Palais des Archives in Paris, a 

 legalized copy of which has been deposited at Washington, 

 D.C. The metre can be defined as the distance between two 

 marks on the standard metre when at a temperature of o C. 



In the F. P. S. system, the unit of length is the foot, i.e. -J- of 

 the standard yard. The original British standard yard is a 

 bronze bar preserved in London. For the United States the 

 yard is defined as the distance between the twenty-seventh and 

 sixty-third divisions of the brass standard yard kept in the 

 Bureau of Weights and Measures at Washington, when the bar 

 is at a temperature of i6| C. or 62 F. 



The relation between these two fundamental units of length 

 is, according to the United States Coast and Geodetic Survey 

 Bulletin No. 9, 1889, 



I cm. = 0.032 808 2 ft. 

 For practical use we have the following approximate relations : 



i m. = 3.2809 ft., i ft. = 30.48 cm., 



i cm. = 0.3937 in., i in. = 2.54 cm. 



15. The unit of angle is either the degree, i.e. -^ of one 

 revolution, or the radian, i.e. the angle measured by an arc 

 whose length is equal to the radius. 



If be any angle expressed in radians, and , a', a" the same 

 angle expressed respectively in degrees, minutes, seconds, we 

 have the relations 



7T o T i 7T it 



a= cc = a' = or, 



I 80 I08OO 648000 



or a = o.Oi/ 45 301 = o.OOO 29 1 <*' = 0.000004 8 5 a". 



