SURVEYING INSTRUMENTS. 73 



will appear either nearly in contact, or overlapping each 

 other ; then perfect the contact, by moving the tangent-screw, 

 and call the minutes and seconds denoted by the vernier, the 

 reading on the arc. Next, place the index about the same 

 quantity to the right of zero, or on the arc of excess, and 

 make the contact of the two images perfect as before, and 

 call the minutes and seconds on the arc of excess the read- 

 ing off the arc ; and half the difference of these numbers is 

 the index error ; additive when the reading on the arc of 

 excess is greater than that on the limb, and subtractive 

 when the contrary is the case. 



EXAMPLE. , 



Reading on the arc - - 31 56 



off the arc - 31 22 



Difference - - - - 0,34 



Index error - = 017 



In this case, the reading on the arc being greater than 

 that on the arc of excess, the index error, = 17 seconds, 

 must be subtracted from all observations taken with the 

 instrument, until it be found, by a similar process, that the 

 index error has altered. One observation on each side of 

 zero is seldom considered enough to give the index error 

 with sufficient exactness for particular purposes : it is usual 

 to take several measures each way ; " and half the differ- 

 ence of their means will give a result more to be depended 

 on than one deduced from a single observation only on each 

 side of zero." 



To make the Line of Collimation of the Telescope parallel 

 to the Plane of the Sextant. This is known to be correct, 

 when the sun and moon, having a distance of 90 degrees or 

 more, are brought into contact just at the wire of the tele- 

 scope which is nearest the plane of the sextant, fixing the 

 index, and altering the position of the instrument to make 

 the objects appear on the other wire ; if the contact still 

 remains perfect, the axis of the telescope is in proper ad- 

 justment ; if not, it must be altered by moving the two 

 screws which fasten, to the up-and-down piece, the collar 

 into which the telescope screws. This adjustment is not 

 very liable to be deranged. 



Of the sextant, it has been said, that it is in itself a port- 



