On dividing Instruments. 83 



juvenile conceit, fallacious as I afterwards found it, fur- 

 nished the first train of thoughts, which led to the method 

 about to be described ; for it occurred to me, that if I could 

 by any means apply the principle of turning to the art of 

 divi<ling instruments, the tools liable to objection might be 

 dispensed with. The means of doing this was first sug- 

 gested, bv seeino- the action of the perambulator, or mea- 

 suring wheel ; the surface of the earth presenting itself as the 

 edge of the instrument to be divided, and the wheel of the 

 peramb'.ilator as a narrow roller acting on that edge; and 

 hence arose an idea that some easy contrivance might be de- 

 vised, for marking oft' the revolutions and parts of ilie roller 

 upon the instrument. Since the year above mentioned, se- 

 veral persons have proposed to me, as new, dividing by the 

 roller, and I have been told, that it also occurred long ago 

 to Hook, Sisson, and others; but, as Hatton on watch- 

 making says, " I do not consider the man an inventor, 

 who merely thinks of a thing; to be an inventor, in my 

 opinion, he must act successfully upon the thought, so as 

 to make it useful." I had no occasion, however, to have 

 made an apology for acting upon a thought, which, un- 

 known to mt, had been previously conceived by others ; for 

 it will be seen in the sequel, how little the roller has to do 

 in the result, and with what extreme caution it is found 

 necessary to employ it. 



When a roller is properly proportioned to ihe radius of 

 the circle to be divided, and with Its edge made a small 

 matter conical, so that one side may be too great, and the 

 other side too little, it Jiiay be ad)vi.slcd so exactly, that it 

 may be carried several times around jhe circle, without 

 the error of a single second ; and it acts with so much 

 steadiness, that it may not unaptly be considered as a wheel 

 and pinion of indefinitely higlt numbers. Yet, such i$ 

 the imperfection of the edges of the circle and roller, that, 

 when worked with the greatest care, the intermediate parts^ 

 on a radius of two feet, will tjometimis be unequal to 

 the value of half a minute or more. After having found 

 the terminatino!; point of a quadrant or circle so perma- 

 nent, allhougiri was not prepared to expect perfect equa- 

 lity throughout, yet I was much mortified to find the 

 errors so great, at least ten times as much as I expec-tedj 

 whic!) fact indicated, beyond a doubt, that if the roller 

 id to be trusted at all, it must only be trusted through a 

 very short arc. H:id there been any thing slippery in the 

 artioii, which woiiUl have been indicated 1)y measuring tlic 

 SAnie part at different times dilTerently, there would have 

 F 2 b«n 



