1 70 On dividing Instruments . 



supports tiie circle; its position- must be at the vertex, 

 where also I must have mv station ; atid the instrument it- 

 self must be turned arouiui its axis, in its proper vertical 

 posiiion, as tlie work proceeds. The above may sutfice, 

 for tl>e present, l.o graliiy those who feel themselves iuter- 

 esied upon a subject which will be better understood, if I 

 sh:udd hereafter have the honour of laying before the Royal 

 Societv a particular description of the mstrument here al- 

 luded to; a task which I mean to undertake, when, after 

 heinsr fixed in the place designed for it, which I hope will 

 be effected at no very distant period, it shall be found com- 

 pletelv to answer the purp(\scs intended. 



Should it be required to divide a circle according to the 

 centesimal division of the quadrant, as now recommended 

 and used in France, we shall have noditficulty. 'I'he 100^ 

 of the quitdrant may be conveniently subdivided into 10 

 each, making 4000 divisions in the whole round. The.256 

 biseclional uitcrvals, the two tables of errors, and the man- 

 ner of proceeding and acting upon them, will be exactly the 

 same as beiore, until we come to cut the divisions ; and for 

 this purpose wo must have another line divided upon the 

 sector. For -^-jrVo^th part of the circle being equal to 3'-4 of 



1° 24' 22 '5 



the usual angular measure tt" — = 15|lhs divisions; 



=" 5 '4 



and just so many will be equivalent to one of the intervals 

 of the circle. The value of one of the great divisions of the 

 sector will be 1° 2G' 24", and that of the ^th parts, which 

 are to be annexed to the right and left as before, will be 

 lO' 48"^ therefore divisible by the engine, hhould any 

 astronomer choose to have both graduations upon his in- 

 strument, the additional cost would be a mere trifle, pro- 

 vided both were done at the same time. 



It must already have been anticipated, that dividing by 

 the eve is equally applicable to straight lines as it is to circles. 

 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 edgewise on its lower surhtce. The 

 bar should be planed, on both its edges antl 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 

 i'or the sake of adjustment. The roller n)ay be divided into 



32 parts, 



