104 POPULAR LECTURES AND ADDRESSES. 



length is merely the length of a certain definite 

 piece of brass, or other solid substance used for 

 a measuring-rod, or the length between two marks 

 upon it ; it may be an inch, or a foot, or a yard, 

 or a metre, or a centimetre the principle is the 

 same. The metre, it is true, was made originally 

 as nearly as possible equal to the ten-millionth 

 of the length of a certain quadrant of the earth, 

 estimated as accurately as possible from the 

 geodetic operations of MM. Mechain and 

 Delambre in 1792, performed for the founda- 

 tion of the metrical system. But this merely 

 gave the original metre measure, and what is 

 meant by the metre now is a length equal to it, 

 or to some authentic copy which has been made 

 from it as accurately as possible ; and the one- 

 hundredth part of the metre thus defined is the 

 centimetre which we definitively adopt as the unit 

 of length. 



Thus our unit of length is independent of the 

 earth, and is perfectly portable, so that the 

 scientific traveller roaming over the universe 

 carries his measuring-rod with him ; and need 



