THE EIFFEL TOWER. 739 



above the earth. The swayiug of the long curved uprights will not be 

 felt much by people at the summit. The height of the tower is such 

 that the nature of the motion is gradual and less observable than in 

 light-houses constructed of masonry, in which the elasticity is some- 

 times remarkable, owing to the quality of the mortar used. It is in 

 recent years only that metallic beams have been made that enable 

 engineers to erect structures to a height of 200 feet. Still further ad- 

 vances in the manufacture of iron make it now easy to attain 250 or 

 even 350 feet. So many unknown quantities require consideration in a 

 tower 1,000 feet high that the problem becomes serious and hard to 

 solve. M. Eiffel points out the signiti(;ant tact that the obstacles re- 

 semble those met with in extending a bridge from 500 feet to twice that 

 distance horizontally, because of the great and accumulating side pres- 

 sure of the wind exerted upon high vertical structures. It is thus seen 

 that the construction is a greater achievement than would be at first 

 imagined. It was desirable, while estimating the tremendous wind 

 ]nessure, to avoid the multiplication of upright beams, involving diag- 

 onal braces more than 300 feet in length, which would result in an im- 

 mense ugly iron frame-work resembling an elongated cage, or trestle- 

 work railway bridge set up on end, with a deplorable architectural 

 effect. Clumsy masses of beams and braces were necessarily omitted. 

 The curved lattice-work before mentioned disposed of this question. 



The corner i)illars narrow from about 50 feet on a side at the base to 

 16 feet near the summit. They are anchored on solid foundation walls, 

 and above, are bound together by horizontal girders, which serve as 

 supports for several large halls or assembly rooms at different heights. 

 These floors increase the security of the structure. The uncertainty of 

 the wind force and its extent as calculated has led M. Eiffel to be pecu- 

 liarly prudent in his methods of construction. He assumes for purposes 

 of safety that the force goes on increasing from the base to the summit 

 until the pressure is doubled. In making estimates of resistance the 

 iron lattice-work was considered a solid wall taking the full force of 

 the wind. In the more open parts of the tower the actual surface of 

 the iron was multiplied by four to secure safety from the effects of a 

 severe tempest. The wind in Paris ordinarily exerts a strain of from 

 13 to 15 pounds for each square meter.* A pressure of 22 pounds is 

 allowed for in Germany, and Austria, in metallic frame works not sub- 

 jected to the tremors of passing trains. This rule also holds in France. 

 But it becomes necessary to provide for a much severer strain when 

 only one end of the structure is supported, as in the Eiffel tower. 



The inclination of the stone-work supporting each corner is at an 

 angle of 50^. In extending upward the slanting ponderous iron-work, 

 it was very difficult to maintain absolute stability, especially before the 

 masses had been made secure by girders at the hrst gallery. As the 

 work progressed this danger of displacement (requiring the utmost 



*3P.37 inches oa a side. 



