13 6 Mr. Troughtgn on dividing Instruments. 
An apparatus for this purpose should consist of a bar of brass, 
three quarters of an inch thick, and not less than three inches 
broad ; six feet may do very well for the length ; it may be laid 
upon a deal plank strengthened by another plank screwed edge- 
wise on its lower surface. The bar should be planed, on both 
its edges and on its surface, with the greatest exactness ; and 
it will be better, if it has a narrow slip of silver, inlaid through 
its whole length, for receiving the dots. An apparatus nearly 
similar to the other should slide along its surface, carrying a 
roller, whose circumference is 12,8 inches, and turned a little 
conical for the sake of adjustment. The roller may be divided 
into 32 parts, each of which when transferred to the bar will 
give intervals of 0,4 of an inch each : The angle of the sub- 
dividing sector should of course be 1 1° 15', and subdivided into 
four parts, which will divide the inch into tenths : The surface 
may also receive other lines, with subdivisions suited to the 
different purposes for which it may be wanted. The revolu- 
tions of the roller and its ~ parts must be dotted upon the 
bar ; taking care, by sizing the roller, to come as near the 
true standard measure as possible : When this is done, com- 
pare the extent of the greater bisectional number that is con- 
tained in the length; i.e. 128 intervals or 51,2 inches, with 
the standard measure ; noting the difference as indicated by 
the micrometer heads : The examination and construction of 
the table of errors may then be conducted just as was done for 
the circle. 
Being now ready for the performance of its work, the scale 
to be divided must be laid alongside of the bar, and the true 
divisions must be cut upon it by an appeal, as before, to the 
erroneous dots on the bar, corrected by a corresponding table 
