ROADS AND BRIDGES 



341 



Quantities of crushed rocks for different widths and depths of road 



Continued 



Tons of 

 Width in feet Depth in inches 



17 4 



17 6 



17 



will require 



17 

 17 

 18 

 18 

 18 

 18 

 18 

 19 

 19 

 19 

 19 

 19 

 20 

 20 

 20 

 20 

 20 



8 



10 



12 



4 



6 



8 



10 



12 



4 



6 



8 



10 



12 



4 



6 



8 



10 

 12 



stone per mile 

 1,859 

 2,789 

 3,718 



4,648 

 5,578 

 1,968 

 2,953 

 3,937 

 4,921 

 5,906 

 2,078 

 3,117 

 4,156 

 5,195 

 6,234 

 2,187 

 3,281 

 4,375 

 5,468 

 6,562 



Under some conditions hard roads can best be made 

 of paving bricks, granite blocks, cement blocks, flat rocks, 

 or cobblestones, and under other conditions even iron 

 wagon tracks may be used. 



The discovery of immense quantities of easily mined 

 iron, the improvements in iron smelting and in the manu- 

 facture of steel plates, together with the cheapened 

 transportation, have brought iron rails almost within 

 the possibility of large use in road making. It is not 

 likely, however, that they will come into prominent use for 

 roadways, except possibly across much-traveled bridges, 

 where they will receive and endure the wear which would 

 cause boards or asphalt to be so rapidly worn out as to 

 be more expensive than the steel tracks. One thing 

 in favor of steel tracks on roads very much traveled is 

 the saving on draft on teams ; there being very little 

 friction, the force required to draw the load is very light. 



The following table, according to Prof. King, shows 



