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was more easy and successful, when I used pitch of nearly 

 the common consistence, than when I employed such as 

 was made very hard, by long boiling- it, or by the addi- 

 tion of much resin. Such softer pitch will admit moi-e' 

 than one application of the polishing powder, Avithout 

 scratching the metal, or spoiling its previous polish; by 

 which means, the process will be more expeditious. It 

 wUl instantly accommodate itself ta the successive altera- 

 tions in the form of the metal; as this, by wearing down 

 towards its edges, gradually changes, from a spherical, 

 to a conoidal shape: and it will promote this effect, by 

 opposing a greater resistance to the metal, and greater fric- 

 tion towards its extremities, when its previous disposition 

 on the polisher has been judiciously provided, in the man- 

 ner before explained. 



But, to fulfil these intentions effectuall^^^ a certain kind 

 of motion, of the mirror on the polisher, must be carefully 

 observed, during the operation: for, as the softer pitch 

 will continually yield, and sink under the pressure of the 

 metal; so, the form of the polisher, degenerating in every 

 stroke, must be recovered, and preserved correct. Ac- 

 cording to the principles before laid down, the face of 

 the polisher must be considerably larger than that of the 

 metal, in order to afford a greater resistance to the spe- 

 culum, towards its extremities : so that, as the metal co- 

 vers only a part of the polisher, if the former were to be 

 •confined in its motion, the pitch, sinking under it, would 



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