280 0?i a Method of examining 



performed. The process is precisely the same in both cases^ 

 and will of course be described in the stme words. 



The first point to be examined is th.it. of \m°, uliich 

 must be done in the usual way, by bringing the j)oints of 

 O and 180° to the moveable wires of the opposite micro- 

 iTietcrs, and then turning the circle half-way round, and 

 bisecting the points again with the moveable wires ; and 

 lastly, taking half the difference betwixt the distances of 

 the wires in the two positions of the circle for the error at 

 the point of 180^. Having now bisected the point of zero 

 with the moveable wire of the micrometer, which »s in- 

 tended to be used in the rest of the process (for wc shall 

 have no further occasion for both), we must slide the mi- 

 croscopo along the arc, till by moving the wire a little wc 

 can bisect the point of QO'', and then the micrometer must 

 be firmly clamped to the iirc. The circle must then be 

 turned til! the point of 180^ is brought to the microscojx?, 

 and that of 90" to the micromcicfr, so that we may be able 

 to bisect each by a slight motion of their respective wires. 

 This being done, we must observe, from the positions of 

 the wires, how much the interval betwixt them has in- 

 creased or decreased in the measurement of the new arc ; 

 and this increase or decrease must be noted down with a 

 4- or — acc6rdmgly. In the same manner wc must pro- 

 ceed through the remaining two arcs of 90", observing and 

 noting down the diflerence betwixt each and the original 

 arc. 



The point of zero must now be brought again to the 

 micrometer^ and bisected by the moveable wire, and the 

 microscope be made to slide back along the arc, till by 

 moving the wire a little we can bisect the point of 60^ 5 and 

 uhen this is done, ihe microscope must be clamped. We 

 must then measure the arc of 60^ against every succeeding 

 arc of 60" in the circle, precisely in the same way that we 

 measured the first arc of 90" against the other three. The 

 arc of 45*^ is next to be measured against every succeeding 

 arc of 45°, and this will complete all that is necessary to 

 be done in the early part of the morning before the heat of 

 the sun can have affected the ten^perature of the instru- 

 ment. The rest may be performed at onr leisure. 



You will immediately perceive the object of this kind 

 of measurement. It enables us to determine, with any 

 degree of accuracy that may be required, the proportion 

 which the first and every succeeding arc of the circle, con- 

 tained betwixt the micrometer and the microscope, hears 

 ^o the whotp circle; and of course the absolute length of 



th« 



