On the Lunar-diurnal Variation of the Magnetic Declination. 481 

 Table I. — Lunar-diurnal Variation at Kew and Hobarton. 



Lunar 

 Hours. 



Kew. 



Hobarton. 



Lunar 

 Hours. 



Observed. 



Computed. 



Observed. 



Computed. 



Col. 1. 



Col. 2. 



Col. 3. 



Col. 4. 



Col. 5. 



Col. 6. 







-'g-o 



-'8-0 



4-'4-8 



:+"4-3 







1 



-11-4 



-10-0 



4- 6-1 



4- 6-4 



1 



2 



- 8-6 



- 93 



4- 5-2 



+ 6-8 



2 



3 



- 50 



- 6-2 



4- 5-9 



4- 5-4 



3 



4 



- 3-2 



- 1-7 



4- 4-2 



4- 2-7 



4 



5 



+ 1-4 



+ 3-0 



0-0 



- 0-6 



5 



6 



+ 5-4 



+ 6-5 



- 4-9 



- 3-7 



6 



7 



+ 7-6 



+ 8-0 



- 6-1 



- 5-5 



7 



8 



+ 8-6 



+ 7-0 



- 4-9 



- 5-6 



8 



9 



+ 4-3 



4- 3-9 



- 3-3 



- 4-1 



9 



10 



+ 2-8 



- 0-4 



- 3-2 



- 4-3 



10 



11 



- 3-0 



- 4-6 



+ 3-6 



+ 2-0 



11 



12 



-106 



— 7*5 



+ 4-9 



+ 4-9 



12 



13 



-104 



— 8-2 



4- 6-6 



+ 6-4 



13 



14 



- 7-0 



- 6-3 



+ 5-9 



+ 6-2 



14 



15 



- 2-2 



- 2-3 



+ 4-1 



4- 4-3 



15 



16 



+ 4-8 



4- 3-0 



+ 1-4 



-1- 1-1 



16 



17 



+10-4 



4- 8-0 



- 3-4 



- 2-7 



17 



18 



+13-2 



+11-6 



- 6-4 



- 5-9 



18 



19 



+12-6 



4-127 



- 6-5 



- 7-7 



19 



20 



+ 7-2 



4-n-l 



- 6-6 



- 7-8 



20 



21 



+ 6-2 



+ 7-1 



- 8-4 



- 6-0 



21 



22 



- 0-4 



+ 1-7 



- 1-9 



- 2-9 



22 



23 



- 1-4 



-4-6 



4- 0-8 



4- 0-9 



23 



The aspect of the lunar-diurnal variation at Kew and Hobarton 

 presents features of great simplicity as well as accord. The form at 

 both stations is a division of the 24 lunar hours into four equal or 

 nearly equal portions, in which the magnet is attracted alternately to 

 the east and to the west of its mean position, which is passed through 

 four times in the progress of the magnet towards two extreme east- 

 erly and two extreme westerly deflections : the easterly extremes 

 are about 12 hours apart, and the westerly the same. As far as our 

 present experience goes, this appears to be the general form of the 

 lunar-diurnal variation of the declination at all the stations at which 

 it has been examined ; it is also that of the corresponding variations 

 of the Dip and Total force. At Hobarton, where the results are ob- 

 tained from five years of observation, there is scarcely any difference 

 deserving of notice between the amplitudes of the extremes on either 

 side of the upper culmination and those on either side of the lower 

 culmination. At Kew, where the results are obtained from only 

 three years, the extreme deflections are not quite so symmetrical 

 in amount, but they may become more so as additional years are 

 brought into the account. The amplitude of the oscillation on a mean 

 of the two alternations is 9"' 74 at Kew and 6"-8 at Hobarton, a differ- 

 ence in correspondence with the difference in the opposite direction cf 

 the antagonistic retaining force of the earth's magnetism at the two 

 stations, which is 3' 7 at Kew and 4*5 at Hobarton. On inspecting 



