108 



ON DIVIDING ASTRONOMICAL INSTRUMENTS. 



brass, three quarters of an inch thick, and not less than 

 three inches broad ; six feet may do very well for the length ; 

 Aparatus for it may be laid ujion a deal plank strengthed by another 

 tte purpose, picnic screwed edgewise 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, the circumference 

 of which 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 

 intervalsof 0"4of an inch each : The angle of the subdividing 

 sector shoxild of course be 11" 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 wante d.The revolu- 

 tions of the roller and its 3'^ 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 disectional number that is 

 contained in the length; i. e. 128 intervals or 51*2 inches, 

 with the standard measure; noting thedilt'erence as indica- 

 ted by the micrameter heads. The examination and con- 

 struction of the table of errours 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 be- 

 fore,- to the erroneous dots on the bar, corrected by a corres- 

 ponding table of errours. The apparatus, remaining en- 

 tire in the possession of the workman, with its primitive 

 dots, the table of errours, &c., is ready for dividing another 

 standard, which will be precisely similar to others that have 

 been, or may be, divided from it. It may be considered, in- 

 deed, 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 th^e 

 usual straight-line dividing engine. Should it be asked, if 



am 



Method of 



applying it 



