20 PHYSICAL SCIENCE 



One of the earliest advances in exact science 

 was the power of counting and the resultant 

 method of expressing quantities as numbers. In 

 spite of its essential nature, the capacity for so 

 doing is by no means innate ; nor is it even yet 

 properly developed among all the races inhabit- 

 ing this globe. In order to measure quantities, 

 it is necessary to choose or invent some unit, 

 and then to count the number of times that 

 unit is comprised in the quantity to be measured. 

 In a civilised country the unit of length is taken 

 as the length between two marks on a certain 

 standard metallic bar. In England there is a 

 standard yard, and in France a standard metre. 

 In fact, both these units are arbitrarily selected 

 for their convenience, though the original idea 

 of the metre was derived from a connection with 

 the supposed dimensions of the earth. 



Like the unit of length, the unit of time is 

 arbitrary, and ultimately rests on a measure of 

 our sequence of consciousness. Again we have 

 to choose some arbitrary unit, which, in this 

 case, should always contain, under similar con- 

 ditions, a similar amount of human consciousness. 

 For purposes of the convenience of daily life the 

 obvious unit to select is the day, while the 

 sequence of the seasons suggests another equally 

 arbitrary unit — the year. The exact relation 

 between these two units can only be determined 

 by careful astronomical observation. Wrong 

 determination and consequent re-determination 

 have led from time to time to necessary changes 

 of calendar ; while the partial adoption of these 

 changes has resulted in the inconvenient differ- 

 ences of date in vogue among the various nations. 



