800 DIGGING AND PREPARATION OF PEAT. 



points. This is the line of greatest fall, and should be the 

 direction of the principal drain. 



It should be noted that a steep gradient is desirable only 

 outside the bog ; within the bog the gradient of the drains should 

 be less the more water the bog contains. Digging the principal 

 drain is commenced at its lowest point outside the bog ; it 

 often suffices to continue this drain up to the bog, but, as a rule, 

 it should be conducted to the lowest point within the bog. In 

 case a brook runs through the bog it may often be used as the 

 principal drain after some cuts have been made in it to improve 

 the flow of water. If the bed of the bog slopes down towards a 

 neighbouring river or brook, this slope affords the best 

 gradient for the drainage. If, however, the bog lies in a 

 depression surrounded by higher land, it is a question of expense 

 whether to cut through the latter or construct a tunnel to serve 

 as a drain. The dimensions of the main drain depend on 

 the gradient and the quantity of water to be removed. It is 

 not generally necessary to drain down to the bed of the bog. 

 Too broad or deep drains often injuriously dry up the bog and 

 are extremely costly both in construction and maintenance. 

 Where the drain leaves the bog a simple sluice-gate should be 

 constructed in order to retain sufficient water in the bog during 

 winter. In the case of small bogs and drains, instead of a sluice- 

 gate, the inlet into the main drain is blocked in autumn with peat. 



If there is much change of gradient in the bed of a large bog, 

 several draining trenches are cut through the latter. It is often 

 advisable to cut these drains from a certain point in the bog, 

 and then lead them outwards in diverging directions, which 

 generally cross one another at right angles. 



Whilst generally the main drain is completed once for all, the 

 subsidiary drains are dug gradually during the progress of 

 removal of the peat. They are generally at right angles to the 

 main drains and are intended to drain only that portion of the 

 bog which is being worked. They are naturally smaller than 

 the main drain. 



In the extensive bogs of Holland, Friesland and Bremen the 

 main drains serve not only for drainage, but also for the purpose 

 of communication by barges and conveyance of the peat ; they 

 are frequently 26 to 32 feet broad. 



