Transportation on Land 265 



roads lead you to suppose that they will last as long as the 

 Roman roads? Explain, giving examples of different kinds 

 of roads. 



109. Kinds of roads. In cities, where traffic is excep- 

 tionally heavy, streets are macadamized, are paved with bricks 

 or wooden blocks, or have a concrete base covered with a layer 

 of asphaltum and sand. Asphaltum, like petroleum, is a 

 natural product, obtained mainly from the asphaltum lakes in 

 Trinidad, South America. In rural districts roads are often 

 dirt-covered, more or less stone being mixed with the dirt. 

 Sometimes country roads are made by covering a layer of coarse, 

 well-packed rock with finer rock, which is bound together by 

 tar, crude petroleum, or asphaltum. Dirt roads may be 

 merely oiled with petroleum, or they may be plowed up and 

 mixed with crude oil and then packed until they become 

 solid and smooth. What kinds of roads are found in your 

 vicinity ? 



A good road is higher in the center, sloping gradually towards 

 the sides. What is the advantage of this surface form? Cul- 

 verts under the road take care of temporary streams, while 

 bridges are built over permanent waterways. Hollows or 

 ravines may be filled in or bridged. Hills are graded down or 

 cut through to secure relatively light grades. 



110. Roads and motive force. The use of steam, electricity, 

 and gasoline as motive forces has made necessary radical 

 changes and improvements in the kind and nature of the road- 

 way. Solid dirt or macadamized roads are fairly useful for 

 slow transportation of small tonnage ; but with the present 

 industrial and commercial activities such roadways are en- 

 tirely inadequate except for a limited tonnage over short dis- 

 tances, as from farm to station or elevator. The chief con- 

 ditions in road making to-day are determined by the available 

 motive forces, that is, the pulling power and the speed. The 

 importance of the roadway to motive force may be illustrated 

 in the following example. 



