Xviii INTRODUCTION. 



Fundamental Units of Length and Mass. — It is desirable that as few 

 different kinds of unit quantities as possible should be introduced into our measure- 

 ments, and since it has been found possible and convenient to express a large 

 number of physical quantities in terms of length or mass or time units and com- 

 binations of these, they have been very generally adopted as fundamental units. 

 Two systems of such units are used in this country for scientific measurements, 

 namely, the customary, and the French or metric, systems. Tables of conversion 

 factors are given in the book for facilitating comparisons between quantities ex- 

 pressed in terms of one system with similar quantities expressed in the other. In 

 the customary system the standard unit of length is the yard and is now defined 

 as 3600/3937 meter. The unit of mass is the avoirdupois pound and is defined 

 as 1/2.20462 kilogram. 



The British yard is defined as the " straight line or distance (at 62° F.) between 

 the transverse lines in the two gold plugs in the bronze bar deposited in the office 

 of the exchequer." The British standard of mass is the pound avoirdupois and 

 is the mass of a piece of platinum marked "P. S. 1844, i lb.," preserved in the 

 exchequer office. 



In the metric system the standard of length is the meter and is defined as the 

 distance between two lines at 0° Centrigrade on a platinum iridium bar deposited 

 at the International Bureau of Weights and Measures. This bar is known as the 

 International Prototype Meter, and its length was derived from the " metre des 

 Archives," which was made by Borda. Copies of the International Prototype 

 Meter are possessed by the various governments, and are called " National 

 Prototypes." 



Borda, Delambre, Laplace, and others, acting as a committee of the French 

 Academy, recommended that the standard unit of length should be the ten mil- 

 lionth part of the length, from the equator to the pole, of the meridian passing 

 through Paris. In 1795 the French Republic passed a decree making this the 

 legal standard of length, and an arc of the meridian extending from Dunkirk to 

 Barcelona was measured by Delambre and Mechain for the purpose of realizing 

 the standard. From the results of that measurement the meter bar was made 

 by Borda. The meter is not now defined in terms of the meridian length, and 

 hence subsequent measurements of the length of the meridian have not affected 

 the length of the meter. 



The metric standard of mass is the kilogram and is defined as the mass of a 

 piece of platinum-iridium deposited at the International Bureau of Weights and 

 Measures. This standard is known as the International Prototype Kilogram. 

 Its mass is equal to that of the older standard, the "kilogramme des Archives," 

 ■made by Borda and intended to have the same mass as a cubic decimeter of dis- 

 tilled water at the temperature of 4° C. Copies of the International Prototype 

 Kilogram are possessed by the various governments, and as in the case of the 

 meter standards are called National Prototypes. 



