DRAINAGE. 71 



tural importance, I will give you these in a rough way, sufficient 

 to make the matter intelligible at a glance.&quot; 



&quot; Suppose A to be the section of the bed of a brook of living 

 water, running through a marsh, which ordinarily keeps within 

 its banks, and that this section contains any given number of 

 square feet say 30. Then estimate, from the best authority, 

 what section would convey its highest floods, and suppose it, for 

 example, to be four times the usual bed, or 120 square feet. 

 Then, as the banks should not be calculated to hold more than 

 four feet depth of water, they must be placed at --, or 40 feet 

 apart ; and the back drains, B and C, will be placed so as to 

 form the embankment D and E, thrown up from their excava 

 tion at the required distance from each other. It is. however, 

 better in practice to give most ample room between these, as it 

 is much safer, in times of flood, to permit the water to spread over 

 more land, and be shallow against the banks, than to spare the 

 land, and have deep water ready to take fearful advantage of any 

 flow or derangement in them, consequent upon cracks from Ions: 

 periods of dry and hot weather, or the burrowing of water-rats 

 and moles, &c.&quot; 



11 The meandering nature of the rivers and brooks, in low 

 marshes, often leads them into courses disadvantageous for con 

 veying the whole mass of the dead water, collected in the back 

 drains, from the lands drained into them ; and although it is best 

 to let them pretty strictly accompany the rivers or brooks, yet, 

 when there occurs a better outlet at the termination of the 

 drainage on one side of the river or brook than on the other, it 

 may be necessary to connect these drains, under the brook or 

 river, by a circular or other brick tunnel. Cases also occur 

 where brooks, with their accompanying embankments and side 

 drains, have their natural course into rivers with the same 

 accompaniments as in the sketch&quot; on the next page. 



&quot; Here it becomes necessary to lay a tunnel of sufficient size to 



