266 Introduction to the Study of Science 



Let us assume, for the sake of keeping the number within 

 easy comprehension, that a horse exerts a pulling or tractive 

 effort of 100 pounds, although a horse usually has greater 

 pulling force than this. On a soft, dirt road of the usual coun- 

 try type, such a tractive effort will haul a load of about 400 

 pounds. On a macadamized road, smooth and level, it will 

 haul a load of about 3000 pounds. On steel rails and a level 

 track it will haul a load of approximately 20,000 pounds. This 

 illustrates clearly why the heaviest hauling is done on rails. 

 Speed of movement may be considered in the same connection ; 

 but it will not essentially modify the numbers given for the 

 load and the pulling force. 



III. RAILROADS 



111. Conditions determining location of railroads. Several 

 features of railroad construction are worthy of careful study 

 because of the mechanical principles which they illustrate. 

 A railroad is located in a new country where it will secure the 

 greatest natural advantages in construction and hauling, and 

 also obtain whatever traffic the region may afford through its 

 mining, agriculture, and other industries. Formerly railroads 

 were run in a very winding way through a hilly country to 

 avoid many heavy grades, tunneling, and expensive bridge- 

 work. To-day speed, hauling capacity, and economy of power 

 are necessary, and not merely ease and economy of construc- 

 tion ; consequently roads are built directly through mountains 

 by tunnels and across rivers and canyons by bridges and via- 

 ducts. As instances of such road building we may refer to the 

 Chicago, Milwaukee, and St. Paul, and the Union Pacific rail- 

 roads, which save several hundred miles of trackage, reducing 

 by several hours the running time between Chicago and the 

 Pacific Coast, and the expense of operation which is almost 

 enough to balance the additional expense of construction. 



Railroads, moreover, are expending millions of dollars to 

 shorten the routes and reduce the grades and curves -of old 



