538 THE POPULAR SCIENCE MONTHLY. 



tlie name of drift or flying sand. While single grains have l)ut 

 little cohesive force, sand behaves toward the wind like water; 

 and the method of the formation of dunes is undoubtedly very 

 similar to that of the formation of waves. If the sand was quite 

 even and horizontal and the wind blew regularly in the same 

 direction, it would not get at the sand. But the surface of a sand- 

 bed is not even; it consists of the roundish heads of the sand- 

 grains that form the upper layer. The wind blowing over them 

 moves them out of their place, and, the individual grains being 

 roundish, they roll. The continuous pressure of the wind extends 

 their movement, and these grains striking upon the projecting 

 grains that are still at rest, disturb them, and the movement 

 spreads more and more. The smaller grains at last no longer 

 touch the ground, and only the heavy ones retain the springing 

 character of their motion, till the wind is restrained and weakened 

 by some fixed objects — plants or buildings — and is compelled to 

 let part or all of its load fall. These objects are thus exposed to 

 be submerged in sand ; and hence it is that we so often see fields 

 of low plants and even villages overwhelmed. It is interesting 

 to observe the different ways in which different objects receive 

 the wind. A tight wall does not catch the sand immediately in 

 front of itself. A furrow is formed just before it, through the 

 generation of side-currents, which receive the sand from actual 

 contact with the wall. There is then formed a sand-ridge parallel 

 to the wall, but at first separated from it by a hollow ; but after- 

 ward, when the ridge has become high enough to shield the wall 

 from the wind, the side-currents are extinguished, and the sand 

 advances to it. The eand driven over the wall by the wind falls 

 at a considerable distance behind it. A striking illustration of 

 this process was formerly to be seen at the church of Altpillau, on 

 the Baltic. The village, which had previously surrounded the 

 church, was removed farther to the east on account of the pres- 

 ence of the sand, but the church had to be left where it was. A 

 sand-ridge some twelve or twenty feet high was formed around 

 it, but nowhere reached the walls of the building ; and while the 

 congregation were obliged to climb over the ridge, they never 

 found the church-doors buried. A broken wall, an open fence, or 

 a quick-set hedge behaves quite differently toward the advancing 

 sand. ISTo furrow is formed in front of it. The air-current forces 

 a large part of its load through the openings, but is so weakened 

 by the obstruction that it drops it before and behind the fence. 

 A little wall is formed around a tree-trunk, which is not, how- 

 ever, of great extent behind it. In isolated bushes and tufts of 

 herbage, the intervals between the single stalks are filled up with 

 sand, and a little mound is gradually formed. 



Like the inland drift-sands, the dunes of the coast also migrate. 



