FORM, MOTIONS, LATITUDE, AND LONGITUDE 211 



Degrees of latitude are measured on meridians. They also 

 vary in length. The length of a degree of latitude is about 68% 

 miles in India, while in Sweden, the most northerly point where a 



Fig. 205. Map showing the standard-time zones in the United States. 



degree has been measured, it is 69*4 miles. All measurements which 

 have been made show that the length of a degree of latitude, meas- 

 ured on the earth's surface, increases as the poles are approached. 

 At the poles it is calculated that a degree must be about 69^ miles. 

 In the United States, the average length is about 69 miles. The 

 increase of length of the degree toward the poles means that the 

 earth is flattened at the poles. 



The actual measurement of the length of a degree of latitude is a difficult 

 matter, but the principle of the measurement is easily understood. At any 

 given point in the northern hemisphere the north star is a certain number 

 of degrees above the horizon. When the observer, starting from any point, 

 has gone northward until this star appears 1 higher above the horizon, he 

 has gone one degree. In practice, the measurement is difficult, for the degree 

 is to be measured at sea-level, and on a smooth surface. Since the land is 

 above sea-level, the actual measurement must be corrected for elevation 

 above sea-level, and for unevenness of surface. 



