210 PHYSIOGRAPHY 



longitude and time. Since the earth turns through 360 in 24 

 hours, it turns 15 in one hour, or 15' of longitude in one minute 

 of time. The sun therefore rises one hour earlier at a place in 

 longitude than in a place in the same latitude in longitude 15 

 W., and one hour later than at a place in the same latitude in longi- 

 tude 15 E. Similarly, noon comes an hour earlier in longitude 

 than in longitude 15 W., and an hour later than in longitude 15 

 E. All places on a given meridian have noon and midnight at the 

 same time, and such places are said to have the same time; but 

 places on different meridians have different times. St. Louis is 

 about 15 farther west than Philadelphia, and Denver is about 15 

 west of St. Louis. When it is noon at Philadelphia it is about 

 eleven o'clock at St. Louis and ten at Denver. When it is one 

 o'clock at Philadelphia it is noon at St. Louis and eleven o'clock 

 at Denver, and when it is noon at Denver it is one o'clock at St. 

 Louis and two at Philadelphia. 



The variations of time with changes of longitude become ap- 

 parent when long journeys are made either east or west. Thus 

 a watch which has the correct time in New York has not the cor- 

 rect time when it is carried to Chicago. To avoid the difficulties 

 of time-keeping growing out of travel, the railroads of the United 

 States have adopted a system of standard time. Under this system 

 the country is divided into north-south belts, about 15 wide 

 (Fig. 205), and in each belt all railways use the same time. The 

 railway time in adjacent belts differs by one hour. By this system 

 the clocks and watches do not show correct local time except on 

 one meridian of each belt. 



Lengths of Degrees 



The length of a degree of longitude, as measured on the surface 

 of the earth, is the -$$-$ part of a parallel. Since the parallels are 

 very much shorter near the poles than near the equator, the length 

 of a degree of longitude is less in high than in low latitudes. At 

 the poles, where the length of the parallel becomes zero, the length 

 of a degree of longitude also becomes zero. At the equator, the 

 length of a degree of longitude is a little more than 69 (69.16) 

 miles. 



