MBNAI BRIDGE. 403 



more than 500 tons ; the bridge is therefore 

 not loaded by its own weight with more than 



-i. th of what it ought theoretically to bear : the 



horizontal force too acting at the top of the pier 

 to overturn or derange it is then ordmarily only 



1 fifi 2 



y th of what is given above, or y = 9 yds 



tons. This force is very small compared with 



the pressure on the pillar, it being only -^ 



of that pressure ; but when it is considered 

 that it is constantly acting at the end of a lever 

 equal in length to the height of the pillar 

 (perhaps 200 feet high), and that frequently 

 by jerks as in windy weather, and when 

 there are heavy weights on the bridge, it may 

 not possibly be without effect. It was not 

 however considered of much consequence by 

 the able Engineer of the bridge, or the tendency 

 might have been easily avoided, by making 

 the angles on each side of the pillar equal. 



We have reckoned the strength of the iron 

 in the chains at 27 tons per inch, but with from 



I 2 



y to yds of this weight it generally begins to 



stretch permanently ; which point may be con- 

 sidered as the extent to which it should ever 

 be used in practice. As to the relative strengths 

 of the parts of the chains, the top link should 



be about ^nd stronger than the bottom one 

 3 E 2 



