34 THE REALITIES OF MODERN SCIENCE 



ciple to the familiar sandglass. They were large 

 water bottles, and the unit of time was that required 

 for the water to run out. In Greece, where public 

 speaking reached perhaps its highest point in history, 

 such clocks, called clepsydra, were used to time the 

 speakers. In one of the orations of Demosthenes 

 appear the words, "you, there, stop the water." 



To-day the unit of time which is commonly adopted 

 is the second, being the sixtieth part of a minute, 

 which in turn is the sixtieth part of an hour, which is 

 one twenty-fourth of a mean solar day. Now a solar 

 day is to be found as follows. As the earth revolves 

 the stars seem to an observer to revolve about an axis 

 which is drawn from the center of the earth to a point 

 very close to the so-called polestar, or North Star. A 

 transit is set up so that the telescope lies in the plane 

 formed by the axis of the celestial sphere, which we 

 have just described, and the vertical line from the transit 

 instrument to the center of the earth. This plane 

 is the meridian plane at the location of the instrument. 

 At noon the sun appears to cross this meridian. The 

 time between two successive transits of the sun is called 

 a solar day. Because the earth's movement about the 

 sun is along an ellipse the solar day differs in duration 

 from day to day. Its average duration is the funda- 

 mental unit of time from which we obtain the more 

 convenient and shorter unit of the second. 



