354 SAND DUNES. 



The undulations, of whatever size, generally show a 

 gradual slope towards the wind; while the driven sand 

 keeps constantly pushing upwards; and then flowing 

 over the crest it falls down away from the wind on 

 the opposite side in a much steeper slope. From a 

 number of measurements taken with a clinometer in 

 the deserts of Sinai and Egypt the angle of rest for 

 dry running sand has been ascertained to be 3 1 degrees. * 

 Careful observations of drifting sands, when carried out 

 during a series of years, seem to show that it is never 

 at rest ; and that the whole mass is, in many places, 

 kept slowly moving forward under the influence of 

 the prevailing winds. The sand dunes upon the West 

 Coasts of Ireland and France furnish good examples 

 of this, and French observations seem to show that 

 the forward movement of the sand dunes on the French 

 coasts amounts to about twenty to twenty-five metres 

 per annum, f This rate of progress must, however, we 

 think, be somewhat exceptional; because if no steps 

 were taken to prevent it the sand would advance 

 something like a quarter of a mile in twenty years. 

 Nevertheless that sand dunes do advance with greater 

 or less speed, is a well ascertained fact; and great 

 injury has sometimes been done by them ; houses, lands, 

 and even cities, being engulfed, and buried beneath the 

 drifting sand. 



In the desert of Gobi for instance the remains of 

 several ancient towns or cities have been discovered, 

 which were buried in the drifting sands at periods of 

 unknown antiquity; and in one or two cases these 



* See The Engineer of June 14, 1889. 



f M. Le Comte D'Escayrac de Lauture, Le Desert et Le Soudan, 

 1853, footnote to p. 34. 



