266 MANIPULATION. 



but if the first stroke fail, the operation must be repeated 

 until a scratch- is made, as it often happens that, from the 

 hardness of the surface of the unannealed glass, a plough- 

 diamond will not readily mark it. 



To Cut Circular and Oval Covers. — This may be done 

 either by a machine in which a plane surface of wood, on 

 which the glass is laid, is made to revolve in a circle or oval 

 underneath a diamond-point, or by the writing-diamond ; 

 in which latter case a model of the size of the cover will be 

 required. In order to cut circles, the following plan has 

 been adopted with great success by the author: — Six or 

 more flat pieces of brass, each about 1^ inch square and -^ of 

 an inch thick, are to be provided, each having a hole in 

 its centre, the smallest hole to be about \ an inch in diameter, 

 that in a second Jj of an inch larger, and so on, increasing by 

 the same amount up to the sixth ; each piece of brass should be 

 marked with a number. One of these being laid upon the thin 

 glass, and pressed firmly down with the finger, the diamond 

 is to be passed round the margin of the disc, care being taken 

 that, in the passing round, it be made to revolve on its own 

 axis, otherwise the beginning and end of the cut will not 

 join, a little practice wiU soon overcome this obstacle. By 

 this plan any odd pieces of glass may be cut up and put away 

 in boxes, each being marked with the number of the brass em- 

 ployed in the cutting. When oval covers are required, 

 pieces of brass with oval holes may be used in the same 

 manner as the others. When a number of discs of the same 

 size are required, it is advisable to have the thin glass cut into 

 strips a little broader than the circles are to be, the circles 

 when cut can then be readily separated from the surrounding 

 glass ; but should any difficulty arise, a few strokes made by 

 the diamond through the largest and most attached parts, will 

 readily cause their separation. Much of the thin glass used 

 for covers is slightly curved, care, therefore, is required to 

 select those pieces that are the flattest, especially when the 

 covers are destined for cells containing fluid ; if they be not 

 flat, the gold-size, or other material employed to cement their 

 edges, will be certain to run underneath them. When the 



