Whenever a magnetic meridian is required, it may be laid off 

 by attaching a Declinatoire to the side of the standard as shown in 

 Fig. 79. With the circles set to zero, the instrument can be 

 maneuvered on the azimuth axis with the lower tangent screw 

 until the needle floats freely in line with the indices. Open the 

 upper clamp and, directing the telescope to the objective point, the 

 bearing of any course can then be read directly to the near- 

 est minute from the graduated circles. 



The manufacturer is required to set the N-S line of the compass 

 in the same vertical plane with the transit telescope and is also re- 

 quired to set the index line of the Declinatoire so that it will be 

 parallel with the line of collimation. 



Adjustment 



This can be accomplished only by placing the theodolite at 

 one end of a previously determined solar or stellar meridian. With 

 the vernier set at zero, the telescope is set in the meridian. The 

 needle's declination at the point of observation must be known 

 (see p. 90). Lay this off to the K. or W., as the case may be. 

 Turn the telescope in that direction and measure the amount on 

 the vernier plates. Unclamp the needle and see if it swings freely 

 in alignment with the indices at the end of the box. If not, it will 

 be permissible to remove the set screws and rub down one of the 

 bearings with a piece of emery paper. By repeated trials a satisfac- 

 tory fitting and adjustment may be accomplished. 



We give these instructions in case an instrument should receive 

 a blow and thus derange the relationship secured between the 

 attachment and the standards. The instruments used by the Can. 

 Top. Survey are regularly tested at the Dominion Magnetic Obser. 

 vatory where it has been decided that the amount of the index error 

 is likely to change, not only from accident, but from weak stand- 

 ards, improper packing, etc., and that on account of the diurnal 

 variation (see p. 90) a perfect adjustment within 5' or 10' is not 

 possible. 



If the adjustment as suggested above is not undertaken, the 

 constant error of deviation should be known and applied to each 

 magnetic observation where actual values are sought. In the case 

 of Alidades it is a matter of only relative or scant importance to 

 have the magnetic axis parallel, or coincident, with the collimation 

 axis; for a constant error applied to each observation will not destroy 

 the proportion of the traverse. 



To re-balance the needle, for different latitudes, proceed as 

 directed on p. 48. Handle the needle with great care and do not 

 return it to the pivot until it has been wiped dry from the moisture 

 of the fingers. The frequent shitting of the counter weight by 

 inexperienced hands should be avoided. Previously it was the 

 common practice to provide a graduated arc of 5 or 10 at 

 each end of the needle box, but nothing is accomplished by this 

 plan and the size of the accessot !y reduced by the present 



arrangements. 



141 



