APPENDIX 



Units. To measure any physical quantity a certain 

 definite amount of the same kind of quantity is used as the 

 unit. For example, to measure the length of a body, some 

 arbitrary length, as a foot, is chosen as the unit of length; 

 the length of a body is the number of times thfa unit is con- 

 tained in the longest dimension of the body. The unit is 

 always expressed in giving the magnitude of any physical 

 quantity ; the other part of the expression is the numerical 

 value. For example, 60 feet, 500 pounds, 45 seconds. 



In like manner, to measure a surface, the unit, or stand- 

 ard surface, must be given, such as a square foot; and to 

 measure a volume, the unit must be a given volume, such, 

 for example, as a cubic inch, a quart, or a gallon. 



Systems of Measurement. Commercial transactions in 

 most civilized countries are carried on by a decimal system 

 of money, in which all the multiples are ten. It has the 

 advantage of great convenience, for all numerical operations 

 in it are the same as those for abstract numbers in the dec- 

 imal system. The system of weights and measures in use 

 in the British Isles and in the United States is not a dec- 

 imal system, and is neither rational nor convenient. On 

 the other hand most of the other civilized nations of the 

 world within the last fifty years have adopted the metric 

 system, in which the relations are all expressed by some 

 power of ten. The metric system is in well-nigh universal 

 use for scientific purposes. It furnishes a common numer- 

 ical language and greatly reduces the labor of computation. 



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