1 46 Royal Astronomical Society. 



care on the part of the operator, hut tending to injure his health 

 hy the labours required in it, and thus not admitting of frequent re- 

 petition. The necessary cost of an instrument produced by such 

 an amount of severe labour is also another very serious objection. 

 The author had long been of opinion, that to copy the divisions of 

 a circle which had been graduated with extraordinary care, upon 

 work of smaller dimensions, would in general be more satisfactory 

 than original graduation. The latter process consists of several suc- 

 cessive steps, in either or all of which a certain amount of error 

 may escape detection, which in general may go far to balance one 

 another, although there will be parts in almost every work where 

 errors appear arising from an accumulation of those minute quantities. 



The author had long since determined, as soon as he could obtain 

 sufficient leisure, to construct an engine sufficiently large for the 

 graduation of all circles, excepting those of the largest class, and 

 the object of this paper is to lay before the Society a brief notice of 

 the successful termination of the work. 



The engine, in general arrangement and construction, is similar 

 to that made by Mr. Edward Troughton, in the author's possession, 

 though there are several additions and peculiarities which are pointed 

 out by him. The circle or engine-plate is of gun-metal, 46 inches 

 in diameter, and was cast in one entire piece by Messrs. Maudslay 

 and Field, teeth being ratched upon its edge. The centre of the 

 engine-plate is so arranged that it can be entered by the axis of the 

 instrument to be divided, and the work by this means brought down 

 to bear upon the surface of the engine- plate, which arrangement pre- 

 vents the necessity of separating the part intended to receive the divi- 

 sions from its axis, &c. — a process both troublesome and dangerous. 



Upon the surface, and not far from the edge of the engine-plate, 

 are two sets of divisions to spaces of five minutes, one set being in 

 silver and the other strongly cut upon the gun-metal face. There 

 are also as many teeth upon the edge as there are divisions upon 

 the face of the engine-plate, namely, 4320, and consequently one 

 revolution of the endless screw moves through a space of five 

 minutes. The silver ring was divided according to Troughton's 

 method with some slight variations. In this operation it seemed to 

 the author the safer course to divide the circle completely, and then 

 to use a single cutter for ratching the edge ; and he believes that 

 the teeth upon the edge have been cut as truly as the original divi- 

 sions themselves. 



Another very important arrangement is, that the engine is self- 

 acting and requires no personal exertion or superintendence, nothing 

 being necessary but the winding up of the machine, or rather the 

 raising of a weight, which, by its descent, communicates motion to 

 the dividing engine. The machinery is so arranged that it can be 

 used or dispensed with at pleasure, there being some cases in which 

 a superintending hand is desirable. 



The author then proceeds with a description of the machinery, as 

 represented in the drawings accompanying his paper, and draws 

 particular attention to the contrivance by which the engine can dis- 

 charge itself from action when it has completed its work. 



