146 



THE EIFFEL TOWER. 



dreamed of before. It may be we have not seen the end of 

 this improvement. 



4. Shape and Stability. 



In section the tower was bound to be a regular polygon. 

 A circular shape, or many-angled polygon, seems the most 

 symmetrical ; but as each angle means one more ascending 

 rib in the tower, economy of weight demands as few angles 

 as possible. The triangle has of all plane figures the fewest 



Fig. 2. 



angles, but such a section would have entailed great loss of 

 room on the various platforms, because its angles are so 

 sharp. A square was therefore the next to suggest itself. 



The question of the stability of the Eiffel Tower, on 

 account of its great height, involves considerations that do 

 not arise in ordinary buildings. For the base of the tower 

 remaining constant, it is clear that the moment of stability 

 (weight X \ length of base) varies, roughly speaking, as the 

 height, whereas the overtiiming moment of the wind varies 



