142 Ml* Buchanan's Description and Users of FrotnActing Hahtt, 



to the eircuinfereuee, and the intermediate spaces connected 

 by brass of open frame work. See plan of framing and brass 

 wheel. The exterior circle has been very accurately divided 

 by Mr Adie on his dividing engine into degrees and half de- 

 grees, and these again by a nonius into minutes, each of which 

 is most easily distinguishable by the naked eye. The instru- 

 ment can be set by the hand very nearly to a minute ; but for 

 greater accuracy, there is a tangent screw by which this can 

 be accomplished for very nice purposes, and which, by a 

 simple and ingenious arrangement, can be at once attached or 

 detached at pleasure. 



The parallel ruler forms a peculiar feature in this instru- 

 ment. In other methods of protracting, this is shifted about 

 to different parts of the paper, and to different angles, whereby 

 it becomes difficult to preserve the parallelism with accuracy. 

 In the present instrument, the ruler continues invariably on 

 one parallel or meridian, while the various degrees of angular 

 deviation from the meridian, as already stated, are produced 

 by the rotation of the whole board round its centre, and the 

 parallel ruler at the same time ranging with a parallel motion 

 over the whole extent of the board, the angles can in this 

 manner be transferred with readiness to any part of the plan. 

 The ruler being thus fixed in angular position, its parallelism 

 can be preserved with great accuracy, and this is done by 

 means of a roller fixed in the centre of the frame to which the 

 straight edge is attached, with a toothed wheel fixed at right 

 angles to the roller at each extremity ; these wheels running 

 in two brass racks fixed in frames at right angles to the roller 

 on each side of the board. These wheels and racks being ac- 

 curately divided and cut, we obtain a degree, of parallelism in 

 the instrument which is almost perfect, and which could not 

 be obtained so simply by any other means that I am aware of. 

 So long as the wheels continue to traverse in the same teeth, 

 there can hardly be an error in the parallelism to the extent 

 of many seconds. Now, the weight of the ruler preserves the 

 wheels in the same teeth ; but should any accidental motion 

 or concussion of the instrument drive them out of their true 

 position, the error is soon intimated by a want of correspon- 

 dence in the different angles which form a check to one an- 



