OF ARTS AND SCIENCES. 297 



VI. The Whitworth Steel Yard, designated W. 



This yard is an end-measure standard. Tt was purchased of Sir 

 Joseph Whitworth & Co., in 1880, at the cost of about $70. No 

 statement of the correction required for the length of the yard at 62° 

 accompanied it. One may infer, therefore, that it was considered 

 standard at that temperature. The terminal surfaces which define the 

 yard are one half-inch in diameter. All the observatious which have 

 been made indicate that these surfaces are exactly parallel. The steel 

 has been tempered at each end, the temper extending about one inch. 



This bar has been included in the present list of prototyjies on 

 account of the relation which it may be supposed to bear to the 

 system of gauges universally adopted in Great Britain. The original 

 from which the Whitworth steel standards were constructed seems to 

 have been a bronze bar made by Troughton and Sims at the same time 

 the national standards were made by these distinguished mechanicians. 

 It does not appear that this yard was included among those compared 

 by Sheepshanks. Probably, however, its relation to them was deter- 

 mined by Mr. Sims. I have been unable to find any record of a 

 direct comparison with the Imperial Yard, but without question the 

 correspondence must be very close. The transfer of this yard to steel 

 involves many considerations which do not enter into a comparison of 

 standards having the same composition as the Imperial Yard. The 

 great service which Whitworth has rendered to the science of metrol- 

 ogy would lead us to expect that the steel yard which is the result of 

 his researches truly represents the bronze yard when converted into 

 its equivalent upon steel at 62°. Nor is this expectation disappointed. 

 A certain allowance must always be made for the different conditions 

 under which independent investigations are made. It will not do to 

 say that the error is all on one side. But admitting that the Whit- 

 worth steel standard really differs from the steel standard yard R^ by 

 the amount indicated by this discussion, it yet remains true that this 

 difference will disappear from the exact subdivisions of the yard with 

 the units which are required in exact mechanical operations, viz. the 

 inch and its multiples. 



The important data with respect to the prototypes described above 

 are presented in the following table. 



