MISCELLANEOUS PAPERS. 401 



that of clearing a road of water, is best secured by selecting a course for the 

 road which is not horizontally level, so that the surface of the road may in its 

 longitudinal section form in some degree an inclined plane ; and when this can- 

 not be obtained, owing to the extreme flatness of the country, an artificial incli- 

 nation may generally be made. When a road is so formed, every wheel-track 

 that is made being in the line of the inclination, becomes a channel for carry- 

 ing off the water much more effectively than can be done by a curvature of the 

 road without the danger or other disadvantages which necessarily attend the 

 rounding of the road much in the middle. Walker considered a fall of about 

 one inch and a half in ten feet to be about a minimum in this case, if it is at- 

 tainable without a great deal of expense." 



It may be remarked concerning the theory proposed by Walker for surface 

 drainage, that though it might effectually secure that object, it would, if the 

 road be too steep, not only increase the draught of vehicles passing over, but 

 the surface Avould suffer greatly from being washed. For instance, the min- 

 imum inclination allowed by him of one and one-half inch in ten feet, or of one 

 foot in eighty, would give a velocity to the water flowing down of about one foot 

 per second, provided the water is one-eighth of an inch in depth, which would 

 be sufficient to move fine gravel, and consequently injure the surface very 

 much, while a greater inclination would affect it much worse. Altliough the 

 grade proposed is lighter than many that are retained on good roads, still when 

 the choice lies betAveen level roads and roads of this inclination, level roads are 

 always to be preferred, since the draught over them is one-eightieth lighter than 

 over roads having this inclination, and other methods equally effectual can be 

 found of removing the surface water. 



The methods previously described of constructing roads as prescribed by 

 McAdani and Telford have been adopted and carried out to a great extent in 

 making the roads in Great Britain and in some portions of this country. Long 

 and constant use has demonstrated that the principles promulgated by McAdam 

 are sound and reliable only to a certain extent. It is to be noticed further that 

 in McAdam' s directions for preparing a road-bed he makes slight provision for 

 drainage, a condition whicli he admits to be of much im^wrtance when stating 

 the principles on which he thinks the construction of good roads depends ; Tel- 

 ford, on the other hand, though saying very little about the great importance 

 ■of drainage, makes thorough provision for it by his pavement of large stone, 

 through which the water passes as though it were a sieve, leaving tlie road high 

 and dry. Again, it is very much to be doubted that a road covering entirely 

 imi^ervious to water can be formed as McAdani in his directions indicates. 

 McAdam gives one or two instances where the road covering has preserved the 

 road-bed beneath perfectly dry. But from some statements he makes regard- 

 ing the comparative merits of the roads constructed by him and by his rival 

 Telford, we must conclude that he changed his mind respecting the power of 

 broken stone to resist the permeating force of water. Telford prepares a differ- 

 ent foundation, but uses a road covering in every essential the same as that 

 proposed by McAdam. Yet of these roads McAdam says the water passes 

 through as readily as through a sieve, a statement entirely incompatible with 

 the one "that a broken stone road is impermeable to water," Although no 

 road covering is entirely impenetrable to Avater, yet a good compact covering of 

 either broken stone or gravel, properly rounded in the center, will convey a very 

 large per centage of the water that falls upon it to the sides of the road, while 

 .a flat surface of either broken stone or gravel will soon absorb water and become 

 51 



