260 Mr. Louis Bell on the Absolute 



usually remained constant within half that amount. The 

 bars under comparison were side by side, symmetrically 

 placed with reference to the illumination, and were at tem- 

 peratures very near to 16°' 67, at which they were standard. 



The fact?, then, concerning the speculum-metal bars are 

 these : — In about two and a half years S a 2 has shortened by 

 very nearly l'Oyu, and S a t by a little over that amount. In 

 S a 2 this change has taken place exclusively in the last deci- 

 metre, and in S'\ it has been confined to the first decimetre. 



The apparent slight increase in Dn^S^ and Dm 2 S a i I do 

 not regard as beyond the effect of the experimental errors. 

 The changes in the lengths of the subdivisions of these 

 standards are very curious, and some explanation may be 

 offered by the fact that the bars were cast in a nearly vertical 

 position and annealed in sawdust, a method hardly sufficient 

 for a material so strongly crystalline as speculum metal. I 

 think, however, one is justified in drawing the conclusion 

 that speculum metal, so tempting on account of its beautiful 

 surface and the exquisite sharpness of the graduations drawn 

 upon it, is a material thoroughly unsuitable for standards of 

 length by reason of its tendency to change with time. I have 

 thus entered into somewhat minute details in the case of 

 these bars, because the whole question of changes in standards 

 of length is in a somewhat unsettled state, and it seems 

 desirable to put on record this case, which has been investi- 

 gated with more than ordinary care by both Prof. Rogers 

 and myself, and in which the changes found have taken place 

 within a comparatively short time. 



It is quite well known that in 1855 this question was 

 raised by Mr. Sheepshanks, then engaged in constructing 

 the new British standards. Discrepancies amounting some- 

 times to 2 or 3.U- appeared in his measurements; but after a 

 considerable amount of study, these differences appeared to 

 be too irregular to be fairly, ascribable to actual changes. 

 Slight variations of temperature, especially when the standards 

 compared were of different materials, the lagging of the real 

 temperatures of the bars behind the thermometer indications, 

 and particularly the effect of coarse and sometimes unsym- 

 metrical defining lines, are perhaps enough to account for the 

 observation. 



The work, however, done on the U.S. bar " Bronze 11," 

 as reported in the report of the Coast Survey for 1877, seems 

 to show genuine change in that standard. 



A long series of comparisons with the Imperial Yard and 

 its copies in 1878, showed systematically a shortening relative 

 to the Imperial Yard of over 4fi. Although further measure- 



