120 



GEOLOGY. 



Fig. 46. 



C. A portion of the steam 

 is at first condensed into 

 water, while the tempera- 

 ture of the water is eleva- 

 ted by the latent heat thus 

 evolved, till at last the low- 

 er part of the cavity is filled 

 with boiling water, and the 

 upper part with steam un- 

 der high pressure. The ex- 

 pansive force of the steam 

 becomes at length so great 

 that the water is forced up the fissure or pipe E B, and 

 runs over the rim of the basin. When the pressure is 

 thus diminished, the steam in the upper part of the cav- 

 ity A expands until all the water, D, is driven into the 

 pipe ; and when this happens, the steam, being the light- 

 er of the two fluids, rushes up through the water with 

 great velocity. If the pipe be choked up, even for a 

 few minutes, a great increase of heat must take place ; 

 for it is prevented from escaping in a latent form in 

 steam, so that the water is made to boil more violently, 

 and this brings on an eruption." 



205. Sand Moved by Wind. In some parts of the 

 world great changes are effected by clouds of sand. 

 The sand-hills produced in this way are called dunes or 

 downs. Very commonly they come from sand that has 

 been washed up upon the shore by the sea, which, on 

 being dried, is carried inland by the winds. There are 

 many dunes of this kind in Cornwall, England, and on 

 the coast of France a great number of villages have been 

 entirely destroyed by them. They are quite common on 

 some parts of the coast of the United States, and espe- 

 cially on Cape Cod. Sometimes dunes occur in the inte- 

 rior of a country. In Egypt the westerly winds have 

 blown the sand over almost the whole of the country 

 west of the Nile, making it a desert, and the remains of 



