13d 



which account, I imagmc, that the resistance to the several 

 annuh is to be taken as proportional to the pressures ti)ey 

 sustain, and measured by them, i. e. by their magnitudes or 

 areas, or the number of particles in them, to which a motion 

 qs imparted; which were stated to be as their respective radii 

 or distances from the center: and, consequently, I suppose 

 the resistance to be the inverse of this, or as the distances 

 of the annuli from the outer edge of the polisher; which 

 distances measure the direct resistance, or the quantity of 

 pitch, to which equal motion, with that in the respective 

 anmdi, is communicated. 



And from hence it follows, that, if a mirror, previously 

 ground to a, spherical figure, Avere to be polished on such 

 a polisher as this: the resistance and friction of the pitch, 

 being greatest, and increasing to a maximum at the cen- 

 ter, and diminishing towards the extremity, would wear 

 down and polish the mirror, most in the central part, and 

 least towards its edges; thus giving to it a curvature, the 

 reverse of a conoid, which it ought to have, and which it 

 can never at first acquire correctly, by any other mode 

 of polishing, but that of wearing it most down (and thus 

 reducing its curvature), towards its extremities.* 



Secondl}'. When there is a hole made through the 



center of the polisher, or a void space left there, un- 



coated with pitch. -f- 



In 



* It will be hereafter shewn, for what particular purpose, solelj-, such a 

 polisher may be employed. 



t Tliere ought always to be a hole made through the polisher, to pre- 

 vent the confinement of air or water, near the center of it. 



