RAILROADS AND HIGHWAYS 165 



up grade, and most of us are familiar with the struggling 

 horse and the puffing locomotive. For this reason engineers, 

 wherever possible, level down the steep places, and reduce 

 the strain as far as possible. 



The slope of the road is called its grade, and the grade 

 itself is simply the number of feet the hill rises per mile. A 



FlG. 106. A long, gradual ascent is better than a shorter, steeper one. 



road a mile long (5280 feet) has a grade of 132 if the crest of 

 the hill is 132 feet above the level at which the road started. 



In such an incline, the ratio of length to height is 5280-1- 132, 

 or 40; and hence in order to pull a train of cars to the summit, 

 the engine would need to exert a continuous pull equal to 

 one fortieth of the combined resistance of the train. 



If, on the other hand, the ascent had been gradual, so that 

 the grade was 66 feet per mile, a pull from the engine of one 

 eightieth of the combined resistance would have sufficed to 

 land the train of cars at the crest of the grade. 



