220 BERTHA'S VISIT TO HER 



elasticity, and that a small marble ball will re- 

 bound to a considerable height, if dropped on a 

 hard substance. Some kinds of stone have a 

 disposition to warp or bend permanently, as he 

 made me recollect was the case in one of the 

 slabs of marCle in the dining-room fire-place at 

 Fernhurst ; and, he says, that the flags in many 

 of the streets of London, are hollow on the 

 upper surface from their having been originally 

 too thin, and from being supported only at the 

 edges, they have yielded in the middle. 



After the slates are split, they are squared and 

 cut to the various shapes and sizes used in roof- 

 ing ; this is generally done in a rough but expe- 

 ditious manner with a sort of a chopper, but 

 some of the larger and finer kinds are cut with 

 frame-saws, so as to be precisely of the same di- 

 mensions, and to have nice smooth edges. These 

 are called milled slates, because the saws are 

 worked by a water-mill. Of course, we went to 

 see this operation : a fine mountain stream turns 

 the wheel which gives motion to more than a 

 dozen pair of long frame-saws ; each pair is set 

 to the distance required for the length or the 

 breadth of the slate, so that the parallel sides are 

 cut by the same stroke ; and, as the saws move 

 forward and backward, water is kept constantly 

 dripping into the cut, and sand is thrown in by 

 boys. The saws, we were told, would make but 

 slow progress without the assistance of sand the 



