222 SECTION MECHANICS. 



possible to appreciate : and that Sir J. Whitworth's process is the only 

 one which allows of this appreciation of small quantities. 



In the proceedings which are being carried on by the members of the 

 International Commission on the Metre, it will be as much as they can 

 <io if they succeed in appreciating, by the most delicate optical means, 

 a quantity four times as great, viz., the ten-thousandth part of a milli- 

 metre (miilimetre=*o3,937 inch). 



Sir J. Whitworth's process is then the most exact which is as yet 

 known, and whenever short distances are concerned, it can be adopted 

 with perfect security. 



The " Metre " Commission has nevertheless adopted the " metre a 

 traits " as being more easily comparable. In it the true length is in 

 reality merely defined by the medium of the thickness of a line, which 

 is not to be less tha five-thousandths of a millimetre in breadth. 

 The fact is, that for long distances, the direct measurement between 

 two trials (touches) is much less precise, unless recourse is had to a 

 complemental standard of the same perfection as those of Sir J. 

 Whitworth ; but then the true measurement is further complicated by 

 two contacts, and it is absolutely necessary, in measures of precision, 

 to take particular precautions with regard to all that concerns the 

 influence of the temperature and of flexion. 



In order to satisfy these two conditions in the best way possible, a 

 proposal of mine has recently been adopted. It consists of a form of 

 set section ruler having the shape of an X, in which the neutral fibre is 

 apparent. As the international metres must be "mesures a traits," 

 recourse will exclusively be had to optical processes, in order to 

 compare them ; proceedings which alone can guard against contacts 

 and the wear of the faces, which might result rom frequent use. 

 We could not, however, in any case, hops to reach the degree of 

 exactness attained by Sir J. Whitworth's English inch standard, which 

 he has made with so much care and trouble, and which must be 

 considered, until the definitive adoption of the metrical system, as 

 the type of the most perfect precision, which mechanical means permit 

 of attaining. 



Mr. CHISHOLM : The question whether a measure of length was 

 best defined by the whole length of a bar, or oy the distance between 

 two points or lines marked upon it, was carefully considered by the 



