General Methods of Work 17 



with the necessary mark and circle readings before and after the observation for 

 the reference of the magnetic meridian to the astronomical meridian. The plane 

 of detorsion of the suspension is of course first carefully determined and made to 

 coincide with the vertical plane through the collimation axis of the instrument. A 

 complete specimen set is given on page'37. 



From the mean circle readings for each distance in the deflection observations 

 and from the fact that the deflecting magnet is inverted during the second set, a 

 value of the magnetic south meridian is obtained which, in connection with mark 

 readings made before and after the observations, may be used in deriving an 

 approximate value of the magnetic declination. Such values must be corrected, 

 however, for index and collimation error as determined by comparisons with the 

 standard magnetometer. The data derived thus are used only where, for some 

 reason, the regular declination observations were either not obtained at all or 

 appear defective. 



The attempt is made to eliminate instrumental errors, as far as possible, by 

 diflferent orientations of the instrument for the various sets obtained, e. g., different 

 orientations of footscrews, thus causing readings to fall on dififerent parts of the 

 circle, etc. This is also the case with regard to the dip circle. 



Dip circle observations. ^The usual dip circle observations comprise (o) decli- 

 nation by compass attachment, from which also for ordinary latitudes the mag- 

 netic meridian reading for dip work is at once obtained; (b) dip with two or more 

 needles (in the order of observation: polarity A north, needle No. 1, polarity A 

 north, needle No. 2, etc., reversal of polarities of the needles, observations of 

 polarity B north, in the reverse order of the needles used; the mean results will 

 thus apply practically to the same mean time). In addition, the following dip 

 circle observations are made whenever there is no magnetometer equipment 

 available, or when, as is the case in high magnetic latitudes, the usual horizontal 

 intensity observ^ations present difiiculties and the intensity work, in consequence, 

 either requires strengthening or must be confined wholly to total intensity determi- 

 nations: (c) loaded dip; {d) deflections with suspended needle "direct" or face of 

 needle towards face of vertical circle or of observer; {e) deflections with suspended 

 needle "reversed," or face of needle away from that of vertical circle; (/) loaded 

 dip. In the case of an instrument provided with two deflecting distances deflections 

 are obtained for suspended needle both direct and reversed for each distance, with 

 a loaded dip observation between deflections for the two distances. 



Care is always exercised that the deflecting needle is invariably mounted in 

 the same relative position to its case. For land work in regions where the dip is 

 from to 75 and where the equipment includes a magnetometer, the total 

 intensity observations are not made except when, for some reason, specially pre- 

 scribed. See specimen observations, pp. 36 and 38-39. 



A single set of declination observations with the compass attachment of the 

 dip circle comprises the following: (c) mark readings with peep sights (or with 

 telescope if one is provided), direct and reversed position of instrument, the needle 

 being clamped during this operation; {b) the needle released and examined to see 



