533 



Baily's apparatus. The whole of this work is of the most elaborate 

 kind. 



Section VII. gives the results of comparisons of numerous bars, 

 from 1851 to 1855, and describes the grounds for suspicion of 

 change in Bronze 28, and the removal of the suspicion. Mr. Sheep- 

 shanks's death occurred at the end of the observations relating to the 

 suspected change ; and the operations on bars, defining the yard by 

 line-measure, were closed. 



Section VIII. gives an account of the formation of End-measure 

 Bars, which had been begun by Mr. Sheepshanks, but was com- 

 pleted by Mr. Simms, under the superintendence of the Astronomer 

 Royal. The general principle is this. If two end-bars have each 

 a defining mark almost equally distant, in the two bars, from the 

 middle of its length ; and if the two bars are placed end to end, the 

 longer segment of the one touching the shorter segment of the other, 

 the distance between the two lines can be compared, by microscopes, 

 with a line-standard. If the contacts be now made by the other 

 ends, a similar comparison can be made. If the two results be added 

 together, we have a comparison of the sum of the entire lengths of 

 the two end-standards with double the length of the line-standard. 

 This operation being performed, so as to effect a comparison of the 

 three pairs which can be made from three end-standards (the sum of 

 each pair being compared with the double line-standard), we have 

 three simple equations from which the lengths of the three end- 

 standards can be deduced. The end-bars are constructed, some of 

 bronze, some of iron or steel ; but in all, the ends are of agate, ground 

 to the curvature of a large sphere, whose centre is the middle point 

 of the bar. The lengths of three bronze end-bars, and of four iron or 

 steel end-bars, were determined by this process. 



Section IX. gives a statement of the closing proceedings of official 

 character, with extracts from the Final Report of the Commission, 

 and extracts from the Act of Parliament legalizing the new Standard ; 

 a table of standard temperatures for the compared bars ; and an 

 account of the disposal of the bars. The Act of Parliament (18 and 

 19 Victorise, cap. 72) recognizes the Bar deposited at the Exchequer 

 Office, and numbered 1, as bearing "the genuine Standard of that 

 measure of length called a Yard," and recognizes four copies as avail- 

 able for restoration of the Standard in case of loss. These copies 



