VOL. LXVII.] PHILOSOPHICAL TRANSACTIONS^ if) I 



cut across its surface with a graver, in the manner represented in fig. 1, pi. 2. 

 These grooves will serve to lodge the emery, and enable the tool to cut a great 

 deal faster. There is no occasion to fear any alteration in the convexity of this 

 tool by working the metal on it, for the emery will bed itself in the lead, and so 

 far arm its surface, that it will preserve its figure and cut the metal very fast. 

 Any kind of low handle, fixed on the back of the metal with soft cement, will 

 be sufficient ; but it should cover two-thirds of its back to prevent its bending. 

 This way of working will cut the metal faster, and with more truth, than the 

 method described by Dr. Smith ; for should the surface and rough parts be at- 

 tempted to be ground off by a common grind-stone by hand, though you did it 

 as near the gage as possible, yet the metal would be so much out of truth when 

 applied to the succeeding tool, that no time would be saved by it. I used to 

 employ a common labourer for this purpose, who soon acquired such a dexterity 

 at working on this tool, that in two hours time he would give a metal of 4 

 inches diameter so good a face and figure, as even to fit it for the hones. When 

 all the sand-holes and irregularities on the face of the metal are ground off, and 

 the whole surface is smooth and regularly figured, the speculum is then ready 

 for the brass grinder, and must be laid aside for the present. 



The manner of forming the brass-grinding tool. — ^The following is the method 

 I have always pursued. Procure a round stout piece of Hamburgh brass, at 

 most a 6th part larger than the metal to be polished; and let it be well ham- 

 mered into a degree of convexity (by the assistance of the gage) suitable to the 

 intended speculum. Having done this, scrape and clean the concave side so 

 thoroughly, that it may be well tinned all over; then cast on it, after it has 

 been pressed a proper depth into the sand, the former composition of tin and 

 lead, in such quantity, that it may, for a speculum of 4 inches diameter, be at 

 least an inch and a half thick, and with a base considerably broader than the 

 top, that may stand firmly on the bench in the manner hereafter to be described. 

 It must then be fixed and turned in the lathe with great care, and of such a 

 convexity as exactly to suit the concave gage, which we suppose already made. 

 It will be necessary to be more careful in forming this than the former tool, and 

 especially that no rings be left from the turning; then the succeeding hone tool 

 will not require so much exactness, as any defects in turning, will, by a method 

 hereafter mentioned, be easily remedied ; but any inequality or want of truth in 

 the brass tool will occasion a great deal of trouble before it can be ground out 

 by the emery. This tool must have a hole, somewhat less than that in the 

 metal to be worked on it, in the middle, quite through to the bottom. 

 When this tool is finished off in the lathe, its diameter should be an 8th wider 

 than the metal. 



How to form the bed of hones, or the third tool. — Having chosen the kind of 



VOL. XIV. y 



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