2 UNITS OF MEASUREMENT. 



to form a layer one centimetre in thickness. About 

 10,000 leaves of the finest gold leaf piled one upon 

 another would have together a thickness of only one 

 millimetre ; yet each one of these leaves possesses a 

 definite although extremely small thickness. A thread 

 wound from the cocoon of a silkworm is so fine that 

 only its length is appreciable by the naked eye ; still it 

 has a certain breadth arid thickness (-j-^th of a milli- 

 metre), as may be seen with the help of a magnifying 

 glass. 



. In physical experiments the metre and its subdivisions 

 are frequently used for measuring dimensions. The 

 metre is very nearly the ^^oth P ar ^ ^ a mer idian; 

 that is, of a circle supposed to be drawn on the surface 

 of the earth through both poles. The metre is divided 

 into 10 decimetres, into 100 centimetres, and into 1,000 

 millimetres (l m = 10 decim = I00 cm = l,000 mm ). In order 

 to be able to compare the magnitude of surfaces and the 

 volumes of bodies, square and cubic measures are required. 

 In measuring a length we ask, how many times it con- 

 tains a certain other length, called the unit of length ; 

 and according to the magnitude of the length to be 

 measured we choose as our unit the metre, decimetre, 

 centimetre, or millimetre. Likewise, in measuring a 

 surface, we have to ascertain how many times it contains 

 another definite surface, which represents the unit of 

 superficial measurement. This unit is always a square ; 

 that is, a plane rectangular and equilateral surface of 

 which each side is equal to the unit of length. We have 

 therefore square metres, square decimetres, etc. 



The magnitude of a superficial area can, however, only 

 he found indirectly, by a calculation, from the measure 



