SECOND LECTURE. 



CONSTRUCTION OF ROADS. 



Having examined briefly the principles which govern the engineer 

 in determining the general line of a road, we shall now consider the 

 rules to be followed in the construction. 



In the first place a regular cross section of the road bed is import- 

 ant, with a smooth hard surface, and sufficient width to accommodate 

 the traffic expected. 



In a new, sparsely settled country, the road should be quite narrow, 

 since it is then much more easily kept in repair ; a width of sixteen or 

 eighteen feet is quite sufficient. Near large cities roads should have 

 a width of fifty to sixty feet, or even more. The surface must be such 

 as will remain smooth, and not be easily affected by the weather. If, 

 as is usually the case in new countries, we make use of the material 

 found on the spot, for the road, such as clay, gravel, &c, we may 

 make a very good road by paying strict attention to the drainage. 

 In fact water or dampness is the great enemy of the engineer; it acts 

 in the destruction of the road in three ways. In large quantities, as 

 during heavy rains, it washes the surface of the road into gullies, and 

 undermining the banks causes serious and expensive accidents. In 

 smaller quantities it percolates into the material, and converts the 

 earth into a pasty mud, which yields to the horses feet and to the 

 wheels, and sometimes slips out of place, so that an embankment will 

 melt away into a shapeless mound. In winter it freezes and throws 

 up the earth which has been soaked with it to the destruction of the 

 surface of the road. 



Drainage is then one of the first objects of the engineer. The sur- 

 face water must be carefully and quickly led away by ample ditches 

 on each side of the road, which turn it into the natural water courses, 

 <or discharge it where it can do no harm. 



These same ditches, when properly placed, and sometimes aided by 

 secondary ones, or by drains, will serve to keep the whole mass of ma- 

 terial dry, and prevent accident from the two other causes mentioned. 



Almost any material will make a good road if it is properly drained ; 

 all will give trouble if drainage is not attended to. Sand, as we find 

 it in the neighborhood of the sea, is, to a certain extent, an exception 

 to this rule. 



Every precaution must be taken, therefore, to carry off the water 

 which falls upon the surface. To effect this the road should be slightly 

 sloped transversely from the center each way to throw the water into 

 the ditches. 



