NATURAL FENCES. 145 



wood, on the very ridge of the heath, take some time to 

 think whether it will grow there or not. We know, 

 however, of some pine plantations, which have stood 

 more than fifty years, that have not yet attained the 

 height of six feet, and have, to all appearance, been 

 getting smaller for the last twenty years ; yet these, 

 though wholly without value as timber, have had some 

 effect in breaking the fury of the wind. 



The enclosures of tall plants that generally surround 

 marshes, have a similar use ; and they afford shelter 

 and nesting places for many of the aquatic birds, and 

 furnish cover for the little spoilers that hawk and fish 

 there. But those screens, whether natural or artificial, 

 have, when they separate surfaces of a different nature, 

 a double utility. It must not be supposed that all the 

 matters which are wafted from place to place by the 

 winds of the season, are of a fertilizing or beneficial 

 nature. The transfer is general ; and while the good 

 soils give out a part to ameliorate the bad, the bad repay 

 them by sending substances that are pernicious. Pasture 

 lands that are situated near bogs and quagmires, receive 

 in abundance from those places the light seeds of 

 mosses and lichens; and heath will very soon overrun a 

 portion of upland that has been cleared, not from the 

 sprouting of the old roots, for it will happen although 

 those have been cleared away with the greatest care, 

 but from the carrying of the seeds of heath by winds 

 that blow from the heath to the cultivated field, as 

 soon as the surface of the latter becomes so much 

 heated as to cause the air over it to ascend, and the 

 air from colder places comes in to supplyjts place. 

 Want of attention to this sowing by the winds of plants 

 o 



