152 POPULAR LECTURES AND ADDRESSES. 



a circle of a centimetre, and the millimetre, which 

 is i/ioth of a centimetre (or in round numbers 

 i/4Oth of an inch), and a circle of a millimetre. 

 (For convenience the woodcut Fig. 33, representing 

 the diagram in question, shews the relative dimen- 

 sion of the centimetre and millimetre only.) We 



One centimetre. One millimetre. 



FIG. 33- 



will adhere however to one simple system, for it 

 is only because we are in England that the yard 

 and inch are put before you at all, among the 

 metres and centimetres. You see on the diagram 

 then the metre, the centimetre, the millimetre, with 

 circles of the same diameter. Somebody tells me 

 the millimetre is not there. I cannot see it, but 

 it certainly is there, and a circle whose diameter 

 is a millimetre, both accurately painted in black. 

 I say there is a millimetre, and you cannot see 

 it. And now imagine there is i/ioth of a 

 millimetre, and there i/iooth of a millimetre, 



