144 THE EIFFEL TOWER. 



Thus the tower is 325 yards, or 975 feet high, and a body 

 would take over eight seconds in falling from its summit. 



2. Foundations. 



The nature of the subsoil was satisfactory at two of the 

 corners, those, namely, remote from the Seine {Fig. 1). But 

 towards the river there was a tilt in the interfaces of the 

 various beds, which required deep excavations and the 

 use of caissons to ^11 up, before the foundations could be 

 trusted. 



The argile plastique has an average thickness of 16 

 metres, and rests on the chalk of the Paris basin. It can 

 safely bear a load of from 40 to 50 pounds per square inch. 



The foundations remote from the Seine offered little 

 difficulty. They were filled up with concrete which dried 

 as it was deposited. For the others, iron caissons worked 

 by compressed air were sunk to a depth of 14 metres. The 

 workmen inside dug away the loose damp earth which the 

 caisson encountered in its descent, and as soon as a homo- 

 geneous surface was reached, filled up the vacant space with 

 concrete to a height of seven metres. The system of caisson- 

 foundations is expensive, but rapid and secure. It has been 

 also used for the Forth Bridge, the Brooklyn Bridge, and 

 the Magasins du Printemps in Paris. 



The weight of the tower is 9,000,000 kilos, or 9,000 tons. 

 (N.B. — English ton = French tonne, nearly = 1,000 kilos.) 



The area of the concrete is such that when the above 

 pressure is distributed over it, it only amounts to 30 pounds 

 per square inch. Above the concrete rest stone piers, 

 shaped like oblique pyramids, which bring the foundations 

 level with the upper ground. The pressure on these, owing 

 to the reduction in area, increases up to 420 pounds on the 

 square inch, or only one-fortieth of the maximum load at 



