Page:Popular Science Monthly Volume 33.djvu/555

Rh Fortunately, it is only the dry sand that is destitute of cohesion. Were this not the case, a whole dune might be taken up and removed to another spot during a very heavy storm. Sand possesses considerable capillarity, by virtue of which the ground-water at its bottom rises through its substance; so that an apparently dry dune is so moist only a few feet below its surface, as to form a compact mass. Rain, however, only temporarily lends it a certain degree of fixedness. The air present between the grains of sand permits to rain-water a slow percolation, so that it has been observed, particularly with fine sand, that water ascends in it from below more rapidly than it descends from above. Rain-water, not being all sucked up by the sand, has to run down the slope, and, therefore, not rarely washes deep furrows in the mass. Sand that has been moistened by rain is more tractable after drying than before it was wet, because the interposition of the water has separated the grains, and they are more easily moved by the wind. The wind can only wear away the surface of a dune. It therefore takes the direction of the ascent of the dune, and carries the sand with it. A space free from wind is formed upon the top of the dune, where the larger grains fall upon the ground and run down on its other side, forming a nook in which they are enabled by their cohesion to remain, while the finer grains are carried farther by the wind. The dunes thus maintain their general forms while slowly advancing. The progress of the dunes has been frequently observed, and attempts have been made to measure it. A series of observations for twenty-three years in the barrens of the Banat gives an average of two metres a year. It is estimated, according to Count Adelbert Baudissen, on the island of Sylt, that the dunes are moving from west to east at the rate of four metres a year. Hagen names a rate of five and a half meters a year for two dunes on the Friesian lowlands, and Krause, four metres a year for another one. Elie de Beaumont describes dunes in Brittany that have moved since 1666 at the rate of seven metres a year; and Behrendt gives the average annual progress as from five to six metres.

A traveling dune is stopped by no obstacles. With the irresistibility of an element only slower than water or fire, it presses forward, burying field and wood, and even whole villages. The spires of church-towers may still be seen projecting out of the sandy sea of Brittany, testifying to the presence of former dwelling-places, there. The whole northern point of Jutland has been • given up by man to the advancing sand.

In the seventeenth century an old churchyard was found, over which a dune had taken its course, on the Courland lowlands north of Kranz. A sand hill that separated the hamlet Sarkan from the parish-village Rositte, and which had to be crossed by