370 SCIENCE IN SHORT CHAPTERS. 



such, for example, as the rocks forming the earth's crust. 

 In the " Black Country" of South Staffordshire, which is 

 undermined by the great ten-yard coal-seam, cottages, 

 chimney-shafts, and other buildings may be seen leaning 

 over most grotesquely, houses split down the middle by the 

 subsidence or inclination of one side, great hollows in fields 

 or across roads that were once flat, and a variety of other 

 distortions, due to the gradual sinking of the rock-strata 

 that have been undermined by the colliery workings. In 

 some cases the rocks are split, but usually the subsidence is 

 a bending or flowing down of the rocks to fill up the 

 vacuity, as water fills a hollow, or "finds its own level." 



I have seen many cases of the downward curvature of 

 thereof of a coal-pit, and have been told that in some cases 

 the surrounding pressure causes the floor to curve upwards, 

 but have not seen this. 



Earthquakes afford another example. The so-called 

 solid crust of the earth is upheaved, and cast into positive 

 billows that wave away on all sides from the centre of dis- 

 turbance. The earth-billows of the great Lisbon earth- 

 quake of 1755 traveled to this country, and when they 

 reached Loch Lomond, were still of sufficient magnitude to 

 raise and lower its banks through a perpendicular range of 

 two feet four inches. 



It is quite possible, or, I may say, probable, that there 

 are tides of the earth as well as of the waters, and the 

 subject has occupied much attention and raised some dis- 

 cussion among mathematicians. If the earth has a fluid 

 centre, and only a comparatively thin crust, as some suppose, 

 there must be such tides, produced by the gravitation of the 

 moon and sun. * 



Ice presents some interesting results of this viscosity. 

 At a certain height, varying with latitude, aspect, etc., we 

 reach the "snow line" of mountain slopes, above which the 

 snow of winter remains unmelted during summer, and, in 

 most cases, goes on accumulating. It soon loses its floccu- 

 lent, flaky character, and becomes coherent, clear blue ice 

 by the pressure of its own weight. 



A rather complex theory has been propounded to ex- 

 plain this change the theory of regelation i.e., re-freez- 



