30 



grades should be used. Where the declivity is very great, as upon 

 the springy sides of many New England hills of hard-pan, the 

 lateral drains should as a rule run diagonally with the slope, instead 

 of in the direction of most rapid descent, so as to more effectually 

 cut off springs and underground water veins, which otherwise might 

 appear at the surface between the laterals. 



The larger the volume of the flow, and consequently the greater 

 the size of the drain to carry it, the flatter may be the grade at 

 which it may safely be laid. Thus, while the fall of two-inch and 

 three-inch drains should rarely be less than three inches per hun- 

 dred feet, a four-inch drain may with equal safety have a fall of 

 two and one-half inches only, and six-inch and eight-inch drains 

 of two inches per hundred feet. It should be borne in mind, how- 

 ever, that an obstruction in a large or main drain is a much more 

 serious matter than in a lateral, and consequently such flat grades 

 should be permitted only in cases of actual necessity. 



Laying out and Construction of Drains. 



The laying out of drains, especially at flat grades, can be prop- 

 erly done only with the aid of accurate levelling instruments, 

 which work can be done by an engineer or surveyor of ordinary 

 skill and capacity. The expense of sucli services is trifling, com- 

 pared with the facility, convenience and certainty of successful 

 results thereby insured in the work. 



The following suggestions may aid in laying out and construct- 

 ing the work. 



Having secured the proper outlet for the main drain, mark tlie 

 position of all mains and laterals by driving into the ground at 

 each end of each line, and also at every point where there is a 

 change either in grade or direction, a grade peg or hub level with 

 the surface. Beside it drive a reference stake, leaving one foot 

 exposed, and mark thereon the depth at which the drain is to be 

 laid below the top of the peg. 



Upon opposite sides of each peg, across the line of tlie drain 

 and far enough to clear the trench, drive two stakes firmly into 

 the ground, leaving two or three feet out, and at some uniform 

 height above the established grade of the drain nail across each 

 pair of stakes a batter board made level from end to end, and 

 mark thereon the centre line of the trench by a nail or notch in 

 the upper edge plumb over the peg. Assume, for example, that 

 all the batter boards are to be set at the uniform height of six feet 

 above grade. If the cut or the depth of the trench marked on the 

 reference stake at the lower end of any given drain is 4.25 feet, 

 then the batter board nmst be placed 1 . 75 feet above the peg ; 



