Drainage. 



68 



speedily discharge itself. But except in the case of iron pipes, socket 

 pipes, or pipe and collar, a rapid descent is undesirable, as in the 

 event of the slightest derangement in the bedding of the drain, the 

 channel washes away and stoppage ensues. Of the two evils it is 

 better to err on the side of over-capacity. It may here be observed 

 that it is never desirable to fill a drain quite to the top with stone, as 

 in such a case the water carries down sand and particles of other 

 matter, which soon cause it to silt up. 



The following, which is taken from a treatise on '' Land Culture" by 

 G. A. Dean, Esq., shows the approximate number of acres which main 

 drains of various sizes are capable of draining, the annual rainfall 

 being estimated at 30 inches : — 



In order to render the above arrangement of main drains as applied 

 to various areas more intelligible, we give the preceding illustration 

 (Fig. 1) of an oblong field of heavy clay land, 60 acres in extent, and 

 which can be drained by one outlet only, and that into the river at F, 

 the mains being in every case laid along the hollows, and being pro- 

 portioned throughout to the quantity of water they may have to 

 discharge. 



From A to a, A to b, A to d, o to p, D to g, h to i, E to k = 3 inches. 

 „ A A to B, C to 0, E to h = 4 inches. 

 „ B to C = G inches ; C to D = 7 inches. 

 „ D to E=8 inches; E to F= 9 inches. 



By carefully following the arrangement in the above plan it will be 

 seen that the capacity of the main drains gradually increases from A 

 to F. And it should be observed that the larger the pipe used and 

 the greater the pressure of water, the less may the fall be made. 



The laying out of drains should never be attempted by the eye 

 alone ; but the whole thing should be done in a systematic way by 

 the aid of a spirit level. Stumps being driven in all along the mains, 

 at distances of one chain apart, or less if required, and the whole field 

 carefully laid down to a scale, the plan should be put aside for 

 reference in case of any stoppage or future derangement. And before 



