DRAINAGE OF BOGS. 731 



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 onl}'' 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. 



8. Drains for Cutting off the Water from Bogs. 



There are frequently small watercourses which run into a bog, 

 or water runs down a slope into it. If then trenches can be dug 

 so as to cut oif the water-supply from the bog, they are very 

 serviceable as an aid to ordinary drains, but will not alone suffice 

 to drain the bog. 



4. Collecting-Drains and Tanks. 



A large number of bogs are supplied with water by infiltration 

 from neighbouring watercourses. If then the bog lies above the 

 level of the water it is possible to drain it in the ordinary 

 manner ; this cannot be done if it is on about the same level 

 as the water. More extensive works are then usually required 

 (which are too costly where peat-digging is concerned) in order 

 to exclude inundations from the bog, or remove the water from 

 collecting drains by means of pumps and hydraulic engines. 

 Only when the inlet of water is inconsiderable can water which 

 collects in the drains during the night be removed by manual 

 labour. The construction of a sufiicieutly large tank near the 

 bog to receive the water can be only exceptionally undertaken. 



5. Piercing an Impcrmeahlc Pan. 



If a bog should rest on a thin bed of loam or clay, below 

 which is an impermeable stratum, or pan, of gravel or sand, the 

 simplest method of draining it is often to bore or break through 

 the pan and thus allow the water to sink belo\v it. If, however, 



