46 



are cut square by a large diamond, which moves 

 in a wooden frame, and they are then carried to 

 the grinding room. There each plate is laid on 

 a table, covered with a large slate or flag ; and to 

 keep the glass "steady it is bedded on the slate 

 in wet plaster of Paris, which you know has the 

 property of setting, or becoming hard, in a few 

 minutes. A smaller plate of glass is then laid 

 on the larger one, and being properly loaded 

 and drawn forwards and backwards, with a con- 

 stant supply of fine sharp sand and water, the 

 two glasses grind each other to a smooth even sur- 

 face. A ledge round the lower glass prevents the 

 sand and water from running off; and the upper 

 or moveable glass has a strong plank cemented 

 to it on which the weights are laid. An upright 

 pin is fixed to this plank, to which a handle, 

 like a coach wheel, is attached for the workmen 

 to give motion to the glass, and much skill is 

 required to vary this motion in every possible 

 direction ; for if they were frequently to repeat 

 the same stroke, the glasses would grind each 

 other into furrows. But no matter what pains 

 are taken to vary this motion, the two surfaces 

 have always a tendency to become slightly sphe- 

 rical, one convex and the other concave ; and 

 to prevent this, the upper glasses of the different 

 grinding tables are occasionally changed, so 

 that two convex or two concave plates mutually 

 correct each other. 



