MOTIONS OF THE EARTH 69 



b. 1303 : 8-11. Latitude and Longitude. 



c. 1305 : 297-299. Effect of Latitude on Climate. 



d. 1308 : 100-101. Effect of Latitude on Climate. 



e. 1310 : 311-313. Latitude and Longitude. 



/. 1312 : 24-28. Localization of Places, Longitude and 

 Latitude. 



Experiment 23. To Locate the North by Means of the 



Sun. 



Apparatus: Straight stick, string. 



a. Tie a stone to the string and use it as a plumb bob. 

 Drive the stick into the ground so that it is vertical, using 

 your plumb bob to test your work. At twenty minutes be- 

 fore noon begin marking where the end of the shadow falls. 

 Repeat every three minutes until the shadow begins to 

 lengthen. A line from the stick to the mark, which indicates 

 the shortest shadow, points north. This shadow is cast at 

 real noon. See Section 51, Time. 



Experiment 24. To Locate the South by Means of a 

 Watch. 



At any time of day point the hour hand of the watch at the 

 sun. Halfway between the hour hand and the figure twelve, 

 on the watch dial, is the south. Repeat at different times of 

 day and satisfy yourself that this method is correct. Explain 

 why this method is correct. 



50. MOTIONS OF THE EARTH 



The earth rotates on its axis once in about twenty-four 

 hours, and revolves around the sun once a year. A point on 

 the equator moves about one thousand miles an hour, while 

 the whole earth moves around the sun at the rate of eighteen 

 and five-tenths miles a second. 



