

224 SECTIONMECHANICS. 



That although a single comparison by end-measure was, perhaps, 

 more accurate than a single comparison by line-measure, yet there was 

 no doubt that by repeating the comparisons, unexceptionable accuracy 

 might be given to observations of line measures. 



Upon full consideration, the Commission unanimously preferred to 

 adopt a line-measure for the new standard of length, the defining line 

 to be marked at the mid-depth of the bar. ^ < H**U\*v(^ <*J*s) 



Specimens of both the line-standard and end-standard yards, con- 

 structed under the superintendance of the Standards Commission, and 

 of the lever-supports, are now exhibited. 



The same question was also fully considered by the International 

 Metric Commission at Paris, and a similar conclusion was arrived at 

 for the construction of the international standard metres. 



There can be no question of the advantages of the form of end- 

 measures, adopted by Sir Joseph Whitworth, with the defining ends 

 constructed as true planes normal to the measuring axis; and more 

 particularly to his application of the principle to guages, to which, 

 indeed, the principle of line-measure is clearly inapplicable. But this 

 mode of application of the principle of end-measure is no.t so well 

 adapted to a standard unit of length, such as our standard yard, from 

 which secondary standards of multiples and parts of this unit are to be 

 derived. If a measure of length, whether an end-standard or a line- 

 standard, is to be subdivided into parts, these parts can only be marked 

 upon it by defining lines. 



f There are now in existence no less than ten standard Egyptian 

 / measures of the home of the Pharaohs. These are standard cubits, 

 \ end measures, divided by lines into palms and digits. A model of the- 

 \ Jbest specimen of these ancient standard cubits~is~now exhibited. 



It appears to me, however, that one great practical objection to an 

 end-measure, as used by Sir Joseph Whitworth, with a contact or 

 feeling apparatus, consists in alterations of its length by variations of 

 temperature, and in the difficulty, if not impossibility, of determining 

 the extent of this influence. In comparisons made with a micrometer 

 microscope, on the other hand, whether of line-measures or of end- 

 measures, the actual temperature of the measuring axis of the bar may 

 be maintained nearly constant, and may be ascertained, and allowance 

 made for its effect. There is really no practical difficulty in the accurate 



