730 DIGGING AND PREPARATION OF PEAT. 



2. OnUiiary Drains. 



The usual method of draining is to lead the water from 

 the bog in ordinary open drains. It is then essential that 

 some land near the bog is on a lower level than its bed ; this 

 generally occurs. The levels taken of the bog and the 

 immediately surrounding country show the difference of altitude 

 between the lowest point of the bed of the bog and that of the 

 external land, and the gradient of the line joining these two 

 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 out- 

 side 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 sur- 

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



