Mr. Troughton on dividing Instruments . 137 
of errors. The apparatus, remaining entire in the possession 
of the workman, with its primitive dots, the table of errors, 
&c. is ready for dividing another standard, which will be pre- 
cisely similar to others that have been, or may be, divided 
from it. It may be considered, indeed, as a kind of engine ; 
and, as it is not vitiated by the coarse operation of racking 
with a screw, but performed by only looking at the work, the 
method will command about three times the accuracy that 
can be derived from the usual straight-line dividing engine. 
Should it be asked, if an engine thus appointed would succeed 
for dividing circles ? I answer, Yes ; but I would not recom- 
mend it; because, beyond a certain extent of radius, it is not 
necessary ; for the errors, which would be introduced into the 
work by the violence of racking a large wheel, are suffici- 
ently reduced by the comparative shortness of the radius of 
such instruments as we divide by that method : And, what is 
still more to the purpose, the dividing engine is four times 
more expeditious, and bears rough usage better. I cannot 
quit the subject of dividing straight lines without observing, 
that I never had my apparatus complete. The standard which 
I made for Sir George Shuckburg Evelyn in 1 796 was done 
by a mere make-shift contrivance, upon the principle of divid- 
mg by the eye ; how I succeeded may be seen in Sir George's 
papers on Weights and Measures (Phil. Trans, for 1798). I 
made a second, some years after, for Professor Pictet of 
Geneva, which became the subject of comparison with the 
new measure of France, before the National Institute; and 
their report, drawn up by Mr. Pictet, has been ably re-stated 
and corrected by Dr. Young, as published in the Journals of 
the Royal Institution. I made a third for the Magistrates of 
mdcccix. T 
