Mr. Troughton on dividing Instruments, 107 
instruments, the tools liable to objection might be dispensed 
with. The means of doing this was first suggested, by see- 
ing the action of the perambulator, or measuring wheel ; the 
surface of the earth presenting itself as the edge of the nw 
strument to be divided,' and the wheel of the perambulator as 
a narrow roller acting on that edge ; and hence arose an idea 
that some easy contrivance might be devised, for marking off' 
the revolutions and parts of the roller upon the instrument. 
Since the year above-mentioned, several persons have pro- 
posed 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 occa- 
sion, however, to have made an apology for acting upon a 
thought, which, unknown to me, had been previously con- 
reived by others ; for it will be seen in the sequel, how little 
the roller has to do in the result, and with what extreme cau- 
tion it is found necessary to employ it. 
When a roller is properly proportioned to the radius of the 
circle to be divided, and with its edge made a small matter coni- 
cal, so that one side may be too great, and the other side too little, 
it may be adjusted so exactly, that it may be carried several 
times around the 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 high num- 
bers. Yet, such is the imperfection of the edges of the circle 
and roller, that, when worked with the greatest care, the in- 
termediate parts, on a radius of two feet, will sometimes be 
P 2 
