296 SCIENCE IN SHOUT CHAPTERS. 



a few times as rapidly as possible, but without breaking it ; then, 

 without loss of time, .feel the portion that has been bent. It is hot- 

 painfully so if the experiment is smartly made. How may this be 

 explained ? 



It is evident that in the act of bending there must have been a dis- 

 placement of the relative positions of the particles of the metal, and 

 the force demanded for the bending indicated their resistance to this 

 movement upon each other ; or, in other words, that there was 

 friction between them, or something equivalent to such internal 

 friction, and thus the mechanical force exerted in the bending was 

 converted into heat-force. 



Here, then, was fluidity, according to the above definition ; not 

 perfect fluidity, but fluidity attended with resistance to flow, or what 

 we have agreed to call viscosity. But water also offers such resist- 

 ance to flow or viscosity, therefore the difference between iron or 

 copper wire and liquid water as regards their fluidity is only a dif- 

 ference of degree, and not of kind ; the demarcation between solids 

 and liquids is not a broad, clearly-defined line, but a band of blend- 

 ing shade, the depths of tint representing varying degrees of vis- 

 cosity. 



Multitudes of examples may be cited illustrating the viscosity of 

 bodies that we usually regard as types of solidity, 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 lean- 

 ing over most grotesquely, houses split down the middle by the sub- 

 sidence 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 the roof of 

 ft coal-pit, and have been told that in some cases the surrounding 

 pressure causes the floor to curve upward, 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 disturbance. The earth billows of the 

 great Lisbon earthquake of 1755 travelled 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 discussion 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 flocculent, flaky character, and becomes coherent, clear blue 

 ice by the pressure of its own weight. 



