^ MATHTSMiTICAL DIVIPINO ENGIltE. 



pamphlet, published by order of the Commissioners of 

 Longitude ; also,- in the article engine, in Dr. Rees's New 

 Cyclopaedia, as well as some other works of a similar nature; 

 it therefore becomes unnecessary tor the Society to give any 

 more of Mr. Allan's engine in their drawings than is expla- 

 natory of the improvement, the engine being used in the 

 same manner as Rarasden's ; this part is the great circle, 

 upon which the arch to be divided is placed, and the circle 

 turned about a determinate quantity at each division, by 

 means of a screw, the threads of which engage fine teeth, 

 cut around the periphery of the circle. The improvement 

 by Mr, Allan consists in the method of cutting or racking 

 these teeth, to ensure their being perfectly of equal size, iu 

 all parts of the circle. 

 Description of The plan, f^g, 1, inp late II, represents the upper surface 

 the plate. of a belmetal cjrcle mounted upon an axis, A, fig. 3, and 

 its surface made truly plane, and perpendicular to the axis; 

 the section shows the figure of the axis, and the central 

 ring B, to give the greatest strength to .the circle ; C is a 

 section of a portion of the frame of the engine ; and D a 

 socket into which the axis A is fitted ; the circumference of 

 the large circle is turned to such a figure as to receive a ring 

 of brass, o, fig. 3, which is united firmly to it by a number 

 of pins, one of which is shown in the figure. Upon this ring 

 a second, 5, isplaced,the two makingthe samethicknessas thft 

 circle. The inside of the ring b, and the outside of the bel- 

 metal circle, are fitted to each other with theutmost accuracy, 

 and great care taken to turn the said fitting truly concentric 

 with the axis of the circle; the brass rings a and b are held 

 together by twenty four screws, as shown in the plan ; and 

 a groove, corresponding to the curvature of the screw which 

 moves the circle, is turned in the outside of the two ; in this 

 state the racking of the teeth is performed by a screw 

 similar to that afterward used to turn the circle to its divi- 

 sions, but notched across the threads, so that it cuts like a 

 saw, when pressed against the circle and turned round, and 

 removes the metal from the spaces between the teeth, which 

 are by this means formed around the edge of the circle ; 

 wKen tl«s has been perforniied all round, two fine lin^ are 

 drawn adrosiJ the brtsaf and belmetal circles, diametrically 



opposite 



