440 SAND.PLAINS OF BELGIUM. [April 



consideration, and his reasonings upon the whole. And such a 

 teacher we find in Wessely. Herr Wessely, in the volume I have 

 cited, Die Europdische Fluijsand unci seine Cultur, in calling attention 

 to the fact of a covering of vegetation binding down and giving 

 fixation to sand dunes and plains, and advocating a liberal plant- 

 ing of these with appropriate grasses and herbage, and this as 

 effectual means — and the only means — at our command, of effecting 

 the fixation of sand-drifts, intimates that if, in employing this means 

 for effecting this end, we would follow nature, we must make our- 

 selves acquainted with all the workings of nature. He alleges that 

 in the sand-drift the main motor powers in action are the wind and 

 gravitation ; and he states that the movement is within certain limits 

 analogous to that of water and flood, and to that of a drifting snow- 

 storm, and that its laws may be studied in these, allowance being 

 made for consequences resulting from difference in the specific gravity 

 of the substances transported. Beckoning the specific gravity of 

 water as 1, that of snow as 0-9, and that of drift-sand 2'6 to 2-9, 

 in like proportions does the gravitation of the substance operate, 

 and limit the effect of the wind. 



The ever-varying deviation and force and duration of the wind pro- 

 duce subordinate efiects, and the effects are further limited in various 

 ways. It is only the dry and superficial layer of sand which is 

 mobile ; deeper-lying damp layers only come under the influence 

 of the wind as they in succession become uncovered and dried. 



But besides these main motive powers creating and limiting 

 the drift, there are others which are not without their influence in 

 determining the forms the drift-sand formations shall assume, such 

 as the vegetation on the dunes or adjacent ground, and other local 

 bodies, amongst which may be reckoned more especially the courses 

 by which the rainfall drains away. 



The following may be considered a free translation of statements 

 made by Wessely. 



Were the surface of the field of sand perfectly horizontal, and 

 the current of wind the same, the latter would not produce upon 

 the former the effect which it does, but the very projections of the 

 courses or rounded heads of the grains of sand gave a hold to the 

 wind before which they flee or roll, or advance with long leaps. 

 The finest sand is carried farthest, and the coarsest grains may be 

 scarcely moved, if moved at all, from their position, and a slight 

 breeze may scarcely disturb the outline of the surface, while a 

 strong wind may raise the sand the height of a man, and make it 

 .spring in leaps well-nigh 100 feet long. Moving quicksand would 

 be a more appropriate description of the phenomenon than flying 

 drift-sand. The prevalent idea that the sand is carried in cloud-like 



