494 



APPENDIX. 



Fig. 268. 



described; but the mahogany slab, a, is of a different form, 

 having a brass pin fitted into its upper surface, the free end of 

 which is seen at 6 ; on this pin a small brass disc, e, turns, 

 carrying 12 objects. Through the disc, c, near to its margin 

 and at regular distances, twelve small holes are drilled and 

 tapped, one of which is shown at <? ; a cell having been removed 

 for the purpose, a larger hole is drilled through the boss, e, at 

 its centre, through which the pin, b, passes. 



The glass cells employed are made in a mould, the bottom 

 and sides being of one piece ; each cell is mounted with 

 marine glue on a Small disc or button of brass, furnished with 

 a solid shank, which is tapped to form a screw, so as to admit 

 of its being fastened into the small hole, d, in the disc, c. 



