THE EIFFEL TOWEE. 147 



as the product of the height and the distance of the centre 

 of pressure above the ground, i.e. as the square of the height 

 roughly. . Hence this wind action acquires special import- 

 ance, and some way must be found to provide against it. 

 The peculiar and characteristic curve of the tower is the 

 result. 



Theory of the Curve. — We can show that any force can 

 be resolved into three components along three arbitrary 

 straight lines in its plane (Fig. 2) ; for if the force P cut 

 one of the straight lines BC in D, then P may be resolved 

 along BD and AD, and AD in its turn may be resolved along 

 the other sides AB and AC. 



In the case where P passes through an angular point B, it 

 resolves at once along BA and BC alone, and we get a zero- 

 component along AC. 



Now, in the tower let ABCD {Fig. 3) represent any one 

 of the storeys, and EF all the rest above it. Let the action 



Fig. 3. 



of the wind on this latter section be collected at P. Then 

 this force can, as above, be taken by the three girders AB, 

 BD, CD. 



And to allow for wind from the contrary direction we 

 shall require another girder joining CA. But if it should 



