

ELECTRICAL UNITS OF MEASUREMENT. 107 



philosophy of measurement, an understanding 

 of what we mean by measurement, and which is 

 essential to the intelligent practice of the mere art 

 of measuring, is to cut off all connection with the 

 earth, and think what we must then do, to make 

 measurements which shall be definitely compar- 

 able with those which we now actually make, in 

 our terrestrial workshops and laboratories. Sup- 

 pose, then, the traveller to have lost his watch 

 and his tuning-fork and his measuring-rod ; but 

 to have kept his scientific books, or at all events 

 to have in his mind a full recollection and under- 

 standing of their contents : how is he to recover 

 his centimetre, and his mean solar second ? 



Let us consider the recovery of the centimetre 

 first. Wherever he is let him make a piece of 

 glass, like this which I hold in my hand, out of 

 materials which he is sure to find, in whatever 

 habitable region of the universe he may chance 

 to be ; and let him with a diamond, or with a 

 piece of hard steel, or with a piece of flint, en- 

 grave on it one thousand equidistant parallel lines, 

 upon a space which may be about the breadth 



