NAVIGATION. 



equinoxes. If the earth revolved under guidance 

 of a mechanism such as this model, the circum- 

 ference of its rolling pivot-shaft would be 5! feet 

 and that of the fixed ring or hoop on which it 

 rolls, 52,000,000 feet. 



FIG. 3. Precessional Model. 



15. Our astronomical knowledge of the pre- 

 cession of the equinoxes has given a most 

 interesting and marvellous assistance to historians 

 in estimating the date of the pyramid-building 

 of Egypt. In six of the pyramids of Gizeh and 



