CHAPTER III 



WEIGHTS AND MEASURES 



IT has been well said by one of the leading physicists 

 of the 19th century that "when you can measure what 

 you are speaking about and express it in numbers you 

 know something about it, but when you can not, your 

 knowledge is of a meager and unsatisfactory kind ; 

 it may be the beginning of knowledge, but you have 

 scarcely in your thoughts advanced to the stage of 

 science. " In later chapters we shall see how science 

 began, and learn something of the uses of mathe- 

 matics. 



To tell "how much," we must have some means for 

 measuring, and a unit. For example, to measure a 

 length we take some length as a unit, that is, call some 

 definite length "one." We may then count how many 

 times this unit goes into the unknown length. The unit 

 we use for this purpose may be a length of any sub- 

 stance, as of string between two knots, for the material 

 of the standard is unimportant provided it is not such 

 as to change its length while we are measuring with 

 it. If a boy wished a measure of how fast he was 

 growing he might record his height each year on some 

 upright. He would not, however, choose a poplar 

 sapling. 



This illustration makes the use of a variable standard 



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