UNITS. 



I. Standards of Length and Mass. 



The only systems of units used extensively at the present day are the British 

 and metric. The fundamental units in these systems are those of time, length, 

 and mass. From these all other units are derived. The unit of time, the mean 

 solar second, is common to both systems. 



The standard unit of length in the British system is the Imperial Yard, which 

 is defined to be the distance between two marks on a metallic bar, kept in the 

 Tower of London, when the temperature of the bar is 60° F. 



The standard unit of mass in the British system is the Imperial Pound Avoirdu- 

 pois. It is a cylindrical mass of platinum marked " P. S. 1844, i lb.," preserved 

 in the office of the Exchequer at Westminster. 



In the metric system the standard unit of length is the Metre, now represented 

 by numerous platinum iridium Prototypes prepared by the International Bureau 

 of Weights and Measures. 



The standard of mass in the metric system is the Kilogramme, now represented 

 by numerous platinum iridium Prototypes prepared by the International Bureau 

 of Weights and Measures. 



Both systems of units have been legalized by the United States. Virtually, how- 

 ever, the material standards of length and mass of the United States are cer- 

 tain Prototype Metres and certain Prototype Kilogrammes. The present status 

 of the two systems of units so far as it relates to the United States is set forth 

 in the following statement from the Superintendent of Standard Weights and 

 Measures, bearing the date April 5, 1893. 



Fundamental Standards of Length and Mass.* 



" While the Constitution of the United States authorizes Congress to ' fix the 

 standard of weights and measures,' this power has never been definitely exer- 

 cised, and but little legislation has been enacted upon the subject. Washington 

 regarded the matter of sufficient importance to justify a special reference to it in 

 his first annual message to Congress (January, 1790), and Jefferson, while Secre- 

 tary of State, prepared a report at the request of the House of Representatives, in 

 which he proposed (July, 1790) 'to reduce every branch to the decimal ratio 

 already established for coins, and thus bring the calculation of the principal 

 affairs of life within the arithmetic of every man who can multiply and divide.' 

 The consideration of the subject being again urged by Washington, a committee 



* Bulletin 26, U. S. Coast and Geodetic Survey. Washington : Government Printing Office, 

 1893. Published here by permission of Dr. T. C. Mendenhall, Superintendent Coast and Geo- 

 detic Survey. 



