Declinometer. 



XV 



9. From the adopted values of the long and short scale divisions, the co-effi- 

 cient for reducing divisions of the short scale to the same value as the divisions of 



the long scale, is wfo^ ~ 1*115 ; the reciprocal = 0"897. 



10. Determination of the scale readings at the magnetic axis of the declinome- 

 ter magnet. 



For this purpose, the scale reading is first observed with the magnet in its 

 usual position (direct), and next with the magnet inverted, the stirrup being made 

 with prolonged sides to admit of this inversion. Simultaneous observations of the 

 unifilar magnetometer are made in order to eliminate the changes of declination 

 during the intervals. When these eliminations are performed, the mean of the 

 reading with the magnet, direct and inverted, is the reading at the magnetic axis. 

 Increasing readings (the magnet being direct), indicate decreasing westerly decli- 

 nation. 



1. Observations with the Long Scale. 



May 5. 1843. 1 inversal (the copper ring used) gives the zero reading, 256*87 



May 6. 1843. 3 257-30 



July 19. 1843. 6 256-88 



Oct. 16. 1844. In the following series the copper ring was not used. 



Table 3. — Long Scale Reading at the Magnetic Axis of the Declination Magnet. 









Unifilar, minus 

 129 Sc. Div., 



Declinometer Reading. 



Mean of each 

 two direct, 



Reading for 



Magnetic 



Position of 



Declinometer 



Unifilar 



reduced to the 







and of each 







Magnet. 



Reading. 



Reading. 



Declinometer 



Direct 



Inverted 



two 



Axis. 









== u. 



minus u. 



plus u. 



inverted. 







Sc. Div. 



Sc. Div. 



Sc. Div. 



Sc. Div. 



Sc. Div. 



Sc. Div. 



Sc. Div. 



Direct 



269-67 



129-75 



+ 0-84 



268-83 









Inverted 



245-10 



129-58 



0-65 





245-75 



268-65 



257-20 



Direct 



268-89 



129-38 



0-43 



268-46 





245-69 



257-07 



Inverted 



245-30 



129-30 



0-33 





245-63 



268-53 



257-08 



Direct 



269-67 



129-95 



106 



268-61 





245-62 



25711 



Inverted 



243-92 



130-52 



1-69 





245-61 



268-69 



257-15 



Direct 



271-33 



131-29 



2-55 



268-78 





245-57 



257-17 



Inverted 



242-19 



132-00 



3-34 





245-53 



268-72 



257-12 



Direct 



272-22 



132-19 



3-56 



268-66 





245-66 



257-16 



Inverted 



241-59 



132-77 



4-20 





245-79 



268-70 



257-24 



Direct 



273-31 



133-10 



4-57 



268-74 









The mean reading of the long scale for the magnetic axis = 257-14. 



This determination has been adopted, as the varying position of the copper 

 ring in the other series was apt to render the results uncertain. (See No. 12.) 



