80 THE FOUNDATIONS OF SCIENCE 



and we have expressed acceleration in terms of our 

 fundamental concepts of length and time. 



In order to co-ordinate these purely conceptional 

 relations with experimental quantities and to apply 

 them to practical measurement, we need units. The 

 units of length arose originally from the average 

 dimensions of the human body, the yard from a 

 convenient arm-span, the foot from the length of a 

 man's foot. But, as the need of accuracy increased, 

 mechanical models of the units in general use were 

 constructed, and the legal yard became the length 

 between two marks on a standard bar, and the foot 

 the third part of that length. The French units, 

 framed by the confident science of the revolutionary 

 period, took the dimensions of the earth as a standard. 

 But the legal metre is no longer the ten-millionth 

 part of one earth quadrant, but the length between 

 marks on a standard bar, which a re-measurement 

 of the earth has shown appreciably to differ from the 

 supposed relation. 



The idea of time depends on human consciousness, 

 but as this differs from man to man, and even in one 

 man at different stages of his life, the periodicity of 

 astronomical phenomena was early taken as a better 

 practical standard. The day or the year may be 

 taken as our fundamental unit, and the second, or 

 practical scientific unit, defined as a fraction of either 

 of them. With the units of length and time, and the 

 derived units of velocity and acceleration, any system 



