FARM MECHANICS. 



CHAPTER XX 

 PEINCIPLES OF DEAPT. 



If it were possible to construct a perfect road its length 

 would be the shortest distance between the places con- 

 nected, and it would offer no resistance to movement over 

 it A pair of parallel, level, smooth and rigid steel rails, 

 well bedded, constitutes the nearest approach to the perfect 

 road yet devised, and how vastly superior the steel track of 

 the railroad is to the best paved street is shown by the 

 enormous loads moved and high speed attained over them. 



529. How the Draft Increases With the Grade. A pull of 

 2,000 Ibs. is required to lift a ton vertically, but to simply 

 move it horizontally only the friction of the carriage and 

 the resistance of the air need be overcome. The more 

 nearly level that roads are built, therefore, the heavier and 

 the faster may loads be moved over them. If the road- 

 bed rises one foot in 100 feet it is said to have a one 

 per cent, grade, and this amount of slope will increase the 

 draft one per cent, of the weight of the load over what it 

 would be on the same road-bed level. A two per cent, grade 

 rises two feet in every 100 feet and the draft is increased 

 by it two per cent, of the load ; a ten per cent, grade rises 

 ten feet in every 100 feet and will increase the draft of a 

 ton 200 Ibs. over what it is on a level road of the same char- 

 acter. The heavier the loads to be moved, therefore, the 



