ELISHA MITCH KLL SCIEXTIl-IC SOCIETY. 4:> 



It is respectfully submitted to constructors that most of 

 the formulas in current use are inadequate to give a proper 

 depth of key for the very heavy rolling loads of to-day, 

 although it is thought that such formulas may still serve 

 the purpose of a rough guide in the design of common 

 road bridges, unless heavy concentrated loads, as steam 

 road-rollers, are to pass over them. In all cases it is best to 

 use the formulas for an assumed key and then by a theo- 

 retical treatment determine the proper key by one or two 

 trials. 



