OF ARTS AND SCIENCES. 



XV. 



ON THE PRESENT STATE OF THE QUESTION OF 

 STANDARDS OF LENGTH. 



By W. A. Rogers. 



Presented April 14, 1880. 



It is not my intention in this paper to enter into a minute account 

 of the construction and comparison of the various standards of length 

 which have been made the basis of measurements, either in trigo- 

 nometrical surveys of the earth's surface, or in more strictly physical 

 investigations. Many of these possess a certain historical interest, 

 even when they have but little inherent value. For information of 

 this kind, the reader is referred to the references at the end of this 

 paper. 



I shall confine myself to a consideration of those standards of length 

 which are in actual use, and which have the authority and sanction of 

 either national or international law. 



Three natural units have been proposed as the basis of a standard 

 of length, as follows : — 



I." The length of a pendulum beating seconds in a vacuum at the 

 level of the sea in the latitude of London. 



II. One ten-millionth part of a quadrant of the earth's surface. 



III. The length of a wave of light of given refrangibility. 



It is generally supposed that the yard of Great Britain was founded 

 upon the first of the natural units named, but it will be seen from the 

 act of Parliament legalizing the standards prepared by the Royal Com- 

 mission, signed June 17, 1824, that the reference of the standard of 

 length to this unit refers to its restoration in case of loss or destruction, 

 and not to its original construction. Notwithstanding many experi- 

 ments were made at this time by Kater and others for the purpose of 

 ascertaining the length of the standard expressed in terms of the 

 length of a seconds pendulum, the yard actually legalized was con- 

 vol. xv. (n. s. vii.) 18 



