556 DRAINAGE OF LAND FOR [Jan. 



in the system represented in Fig. 5, in which main, sub-main, cand 

 minor are set forth. 



Here the principal depression, let us suppose, is indicated by the 

 broad line, and here the main must be constructed (as in the plan) ; 

 and where hollows on a smaller scale occur, striking the course of 

 the main, and forming an independent area or basin, the sub-main 

 must be inserted, communicating with the main at a right, or more 

 advantageously, an oblique angle. In this case, let us suppose that 

 one of such hollows exists on either hand, as shown by the lines of 

 medium breadth in the plan : these point out the direction of the 

 sub-mains, into which minors are led in the same way as they have 

 hitherto been shown to empty themselves into the mains only. 



The volume of water thus collected is in its turn thrown into the 

 main, and discharged in due course at the ordinary outlet. 



An example differing entirely from any of those already dealt with, 

 is represented in Fig. 6. The method employed for surface-draining 

 an isolated hill is here shown — a somewhat conventional hill no 

 doubt, and which in nature would probably be free from excessive 

 moisture ; but exaggerated though it be, it will best serve that pur- 

 pose as showing a specimen of the system which may be applied in 

 more or less modified forms to ground of a kindred nature. 



Here the minor or parallel drains cut across the face of the steep 

 slope, and cannot fail to intercept and convey to the sub-mains, 

 which in this case are cut vertically to the fall of the ground, such 

 moisture as is not retained in the soil. These, as before, discharge 

 themselves into the main drains (as in the plan), for which, in such 

 ground, a suitable depression ought without difficulty to be found. 



A combination of the different systems mentioned in the course 

 of this paper is given in Fig. 7. 



Here an area of considerable extent is dealt with, and one which, 

 let us suppose, requires thorough drainage, happily an uncommon 

 occurrence in such ground as that of which we now speak. For 

 convenience' sake the lowest ground within this area has been taken 

 at 100 ft. above tlie sea-level, ranging to 180 ft. at the highest 

 point, the intermediate elevations being indicated here and there by 

 figures ; and it will be easily seen that ground of such conformation 

 necessitates tlie division of the whole into various drainage areas or 

 basins — six in this case. As to the outlets, convenient points must 

 be selected, each one of course at a lower level than the deepest part 

 of any drain belonging to its own particular system. In other 

 w^ords, the elevation of the outlets vary according to the elevation 

 of the areas to which they appertain. 



Here three such outlets have been found sufficient, each of which, 

 for the sake of simplicity, has been put down at an elevation of lOQ 



