MISCELLANEOUS PAPEKS. 

 TABLE OF DATA FOR UNDERDRAINING ROADS. 



411 



FALL OF DKAIJiT. 



The fall that is given the drain may be varied a considerable ; it should not 

 be so great that the water will tend to folloAV along on the outside of the tile 

 and undermine it, nor so little that the drain will not discharge freely. These 

 limits must be determined to a great extent by the nature of the ground, but in 

 general the extreme limits for ordinary workmen will be from eight feet to three 

 inches fall in 100 feet measured horizontally. It is hardly safe to depend upon 

 a drain laid with slopes greater or less than these extremes, though drains have 

 been laid and performed satisfactory work with a fall of one-tenth of a foot in 

 1,000 by exercising great care in keeping the fall uniform and the joints tight. 

 In soil difficult to wash it is probable that drains with a slope greater than 8 in 

 100 or 1 in 12^ could be made and laid so as to retain their positions ; but in 

 most soils it would not be advisable to try a slope even as steep as 8 feet in 100. 

 The surface of the road should never rise more than one foot in going 25 hori- 

 zontally, so there will be no need of caution for getting drains too steep ; but 

 in some cases it may liappen that a sufficient fall can only be obtained by mak- 

 ing the outlet deeper than the end of the tile. Suppose the road lie across a 

 level marsh one mile in width as an extreme case ; suppose farther that outlets 

 can be provided in but six places and are one-sixth of a mile apart, the two 

 outer of which are one-twelfth of a mile from the edge of the marsh ; this will 

 require each drain to be one-twelfth of a mile long ; if the outlet be made four 

 and one-tenth feet deep and the extremity three feet deep, a fall of three inches 

 in 100 feet will be secured, and the water of the subsoil will be conveyed rapidly 

 away. In such a case as this last the outlets will be very expensive. They 

 should consist of ditches with a sufficient capacity to convey the water rapidly 

 away from the mouths of the tile and also from the open side ditches. The 

 case in which outlets could not be provided has already been discussed under 



