APPENDIX 561 



the instant when the sun's center crosses the meridian of 

 any place and the instant of its next passage over the same 

 meridian is not uniform, chiefly because the motion of the 

 earth in its orbit about the sun varies from day to day. 

 The mean solar day is the average length of all the variable 

 solar days throughout the year. It is divided into 24 X 

 60 X 60 = 86,400 seconds of mean solar time, the time re- 

 corded' by clocks and watches. The sidereal day used in 

 astronomy is nearly four minutes shorter than the mean 

 solar day. 



The Three Fundamental Units. Just as the measure- 

 ment of areas and of volumes reduces simply to the measure- 

 ment of length, so it has been found that the measurement 

 of most other physical quantities, such as the speed of a ship, 

 the pressure of water in the mains, the energy consumed by 

 an electric lamp, and the horse power of an engine, may be 

 made in terms of the units of length, mass, and time. For 

 this reason these three are considered fundamental units 

 to distinguish them from all others, which are called derived 

 units. 



The system now in general use in the physical sciences 

 employs the centimeter as the unit of length, the gram as 

 the unit of mass, and the second as the unit of time. It 

 is accordingly known as the c. g. s. (centimeter-gram-second) 

 system. 



