OP THE UNITED STATES. 377 



ber of concentric circles of proper diameters could be drawn 

 upon the plate. 



The axis of the engines being generally very strong, would 

 easily admit the necessary aperture in the centre; and it 

 might for that purpose have an outer diameter of seven 

 inches, without impeding its easy motion in the manner in 

 whicli the engine is otiiciwise made and used. All otiier 

 parts of the engine would be exactly similar to that of Mr. 

 Troughton, as described in the Encyclopedia of Brewster; 

 Art. Graduation. 



The circle intended for dividing being laid upon tlie en- 

 gine, with its axis last to it, and centred approximately by any 

 one of the circles, a compound micronieter microscope is 

 placed between the two bars, at tlic place of the cutting tool, 

 having its wires so placed as to form one or more tangents 

 to the circle drawn on the instrument for the limits of the 

 divisions. The instrument will evidently be centred, when 

 the wire of the nncroscope touches this circle always in the 

 revolution of the engine upon its axis. As long therefore as 

 this docs not take place, the position is to be corrected, half 

 by moving the instrument properly upon the engine, and half 

 by !)ringing the microscope, or its wires, nearer to, or fartlicr 

 from the centre ; and the different circles upon the instru- 

 ment may serve to verify each other. 



When the instrument is thus adjusted upon the engine, it 

 may be fastened by various means. If melted wax Ije cast 

 about the parts by which the instrument is supi)ortcd, it will 

 be sufficient to keep it steady during the time required for 

 dividing, and will not derange it by any unequal pressure. 



In the present improved state of the dividing engine of 

 Mr. Troughton, accidental errors of the divisions arc scarcely 

 possible, if due attention be paid to the proper stability of 

 all the moveable parts, and the legulaiity of treadinir. so that 

 no tooth of tlie racking wheel may be passed over, and that 

 the tracer may not admit any vacillation in its adjustments. 



The tracer of Mr. 'I'roughtnn is better tlian the roimd point 

 comn\only used. It is groimd below to an elliptic section, of 



