LINEAR MEASUREMENT. 223 



Standards Commission for restoring the British Standards destroyed 

 in 1834. 



The ancient standards of length, the yard of Henry VII. and the 

 yard of Queen Elizabeth, were both end-standards. 



The Parliamentary standard yard, legalised in 1824, and destroyed 

 in 1834, was defined by two dots marked on a brass bar. 



The ancient Prussian standard of length was a line-standard, but in 

 1838 there was substituted for it an end-standard, constructed by 

 Bessel. He gave as his reasons 



1. That if a flexible bar be supported on two points, the extreme 

 length of the bar from the centre of one end to the centre of the other 

 end is not sensibly altered by its flexure, whilst the distance between 

 two points or lines upon the upper surface may be sensibly altered. 



2. That the principle of end-measure is more convenient than that 

 of line-measure for the production of copies, in other words, for 

 comparisons. 



As to the first of these points, the Standards Commission remarked 

 in their report, that Bessel himself admitted this objection would be 

 removed if the lines were engraved on surfaces depressed to the middle 

 of the thickness of the bar. And they stated, moreover, that the ten- 

 dency to an alteration of the apparent length, either at the surface or 

 mid-depth of the bar, might be obviated by proper adjustment of the 

 points of support, and still more surely by supporting the bars at 

 numerous points on lever frames, with equal supporting forces at all 

 the points, or by floating the bar in quicksilver. 



As to the second point, the Commission considered that line- 

 measures had been invariably adopted for measuring British geodetic 

 bases ; and their use led the Commission to form a high estimate of 

 the convenience of the line principle, and to consider that a standard 

 which was intended to apply to them should be constructed on the 

 same principle. They considered, also, that the construction of defining 

 lines on a standard bar was more simple and easy than that of defining 

 ends ; and that the tendency of an end-standard to an alteration of 

 length was by wear in only one direction, whilst a line-standard was 

 practically invariable. 



The end-standards are connected with spherical ends, the radius oi 

 the spherical curve being half the length of the bar. 



