UNCLE IN ENGLAND. 221 



sharp grains of which are carried forward by the 

 jagged teeth of the saw, and are thus made to 

 tear away the slate. 



" It is on this principle," said my uncle, " that 

 precious stones are cut by a thin circular plate 

 of iron, with emery, or diamond powder. And 

 a seal engraver's apparatus is only a sort of lathe, 

 to which he can attach small copper-wheels that 

 are made to revolve with great rapidity. To the 

 plain edge of one of these wheels, he applies oil, 

 with a little diamond powder, which soon cuts 

 into the hardest stone ; and thus by the form 

 and size of the wheel, and the direction in which 

 the stone is pressed against it, he can accom- 

 plish any device either in relief or intaglio. In 

 all these cases, the particles of sand, emery, or 

 diamond, bed themselves in the soft metal, and 

 grind away the harder surface opposed to them ; 

 and, what will appear rather singular at first 

 sight, when two hard substances rub against 

 each other, it is the hardest which wears away 

 the most. For instance, the highly tempered 

 steel knife-edges, by which some pendulums are 

 suspended, for experimental purposes, are less 

 liable to wear than the still harder agate planes, 

 on which they work : for the minute atoms of 

 dust, conveyed by the air, adhere to the steel, 

 and in the course of time act upon the agate." 



But to return to our mill. Solid blocks, thick 

 enough to make about twenty slates, are thus 



u 3 



