On dividing Instruments. 16.5 



I have always divided the sector from the engine, becanse 

 that is the readiest method, and interior to none in point of 

 accuracy, where the radius is very short ; but, as it is more 

 liable than any other to centrical error, the adjustment of 

 the arc by the screw E becomes necessary: by that' adjust- 

 ment, also, any undue run in the action of the roller may 

 be reduced to an insensible q.uantitv *. 



When the utmost degree of accuracy is required, I give 

 the preference to dividing by lines, because they are made 

 with a less forcible effort than dots are ; and also because, 

 if any small defect in the contexture of the metal causes the 

 cutter to deviate, it will, after passing the defective part, 

 proceed figain in its proper course, and a partial crooked- 

 ness in the line will be the only consequence ; whereas a 

 dot, under similar circumstances, would be altogether dis- 

 placed. But, on the other hand, where accuracy has been 

 out of the qtiestion, and only neatness required, I have used 

 dots ; and I have done so, because I know that when a dot 

 and the wire which is to bisect it are in due proportion to 

 each other, (the wire covering about two-thirds of the dot,) 

 the nicest comparison possible mav be obtained. It may 

 be further observed, that division by lines is complete ia 

 itself; whereas that by dots requires lines to distinguish 

 their value. 



On the upper side of Fig. 1. is represented the apparatus 

 for cutting the divisions. It consists of three pieces J KL, 

 jomted together so as to give to the cutter an easy motion 

 for drawing lines directly radiating from the centre, but in- 

 flexible with respect to lateral pressure ; dd are its handles. 

 The cutting' point is hidden below the microscope H; it is 

 of a conical form, and were it used as a dotting point, it 

 would make a puncture of an elliptical shape, whose longer 

 diameter would point towards the centre. This beautiful 

 contrivance, now well known, we owe to the ingenuity of 

 the late Mr. Hindiey of York ; it was borrowed by Mr. 

 llamsdenfj and applied with the beet effect to his dividing 

 engine. 



ft might have been mentioned sooner, that in the instance 

 which I liave selected as an example of my dividing, the 

 operation took place when the season of the year, and the 

 smoke of London, had reduced the day to scarcely six hours 

 of effective light ; and rather than confine my labours 



• Sec note page 1C6". 



t 'I'liis I learned from that most accurate artiyt Mr, John Stajiclifle, who 

 wai hiiuicU apprentice to Hindlcy. 



L 3 wiihia 



