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 
V o 0 0 
the long scale, is = 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. 
Position of 
Magnet. 
Declinometer 
Reading. 
Unifilar 
Reading. 
Unifilar, minus 
129 Sc. Div., 
reduced to the 
Declinometer 
: - u. 
Declinometer Reading. 
Mean of each 
two direct, 
and of each 
two 
inverted. 
Reading for 
Direct 
minus u. 
Inverted 
plus u. 
Magnetic 
Axis. 
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 
1-06 
268-61 
245-62 
257-11 
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.) 
