610 MISCELLANEOUS PAPERS. 



vals between them ; but even if that i.s denied, the evidence is now 

 overwhelming that matter is not homogeneous, but possesses a grain 

 of some kind, regularly repeated at intervals which can not be greater 

 than one two millionth nor less than one four-hundred millionth of an 

 inch. Accordingly, we must picture our standard bar as a conglomer- 

 ation of grains of some kind or other, having magnitudes of the order 

 specified, and all in ceaseless motion, the amplitude of which depends 

 upon the temperature of the bar. To our mental vision the polished 

 terminals are therefore like the surtace of a pot of boiling water, and 

 "we recognize that there must be a limit to the accuracy with which the 

 interval between them can be measured. As a basis for estimating 

 how near this limit we have approached it will suffice to say that for 

 fifty years past it has been customary to state comparisons of standards 

 of length to one one-millionth of an inch. Nevertheless, most authori- 

 ties agree that although one one-hundred thousandth of an inch can 

 be distinguished in the comparators, one twenty five thousandth of an 

 inch is about the limit of accuracy attainable in comparing standards. 

 Possibly such a limit may be reached under the most tavorable circum- 

 stances, but in the case of the yard and the meter, which are standard 

 at different temperatures, the following values of the meter by observers 

 of the highest repute render it doubtful if anything like that accuracy 

 has yet been attained.* 



Inches. 



1818. Captain Heury Kater 39.37079 



1866. General A. R. Clarke 39.37043 



1883. Prof. William A. Rogers 39. 37027 



1885. General C. B. Comstock 39,36985 



The earliest standard of English weight of which we have any very 

 definite knowledge is the mint pound of the Tower of Loudon. It 

 weighed 5,400 troy grains, and the coinage was regulated by it up to 

 the year 1527, when it was abolished in favor of the troy pound of 

 5,760 grains. Contemporaneously with the tower pound there was also 

 the merchant's pound, whose exact weight is now involved in so much 

 doubt that it is impossible to decide whether it consisted of 6,750 or of 

 7,200 grains. The tower pound and the troy pound were used for 

 weighing only gold, silver, and drugs, while all other commodities were 

 weighed by the merchant's pound until the thirteenth or fourteenth 

 century, and after that by the avoirdupois pound. It is not certainly 

 known when the troy and avoirdupois pounds were introduced into 

 England, and there is no evidence of any relation between them when 

 they first became standards. The present avoirdupois pound can be 

 clearly proved to be of similar weight to the standard avoirdupois pound 

 of Edward III (A. D. 1327-1377), and there is good reason for believing 

 that no substantial change has occurred either in its weight or in that 

 of the troy pound since their respective establishment as standards in 

 England. 



* See Note C, at the end of tbe Address. 



