76 



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 1 2 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 of 

 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 

 the Table, we see that the lunar times when the moon's influence pro- 

 duces no deflection (or the times when the variation is zero), are 

 four, and are nearly the same at Kew and at Hobarton, two of them being 

 a little more than an hour before the moon's passage of the meridian, 

 both at her upper and lower culminations, and the other two inter- 

 mediate. So far the two stations are alike ; but in regard to the 

 direction towards which the magnet is deflected (if in conformky 

 with general usage we speak in both hemispheres of the north end 

 of the magnet, as is done in the Table), we see that the variation 

 becomes west at Kew when it becomes east at Hobarton, and vice versd\ 

 the phases, while agreeing in hours at the two stations, having through- 

 out opposite signs. 



By extending the comparison of the lunar hours at which the lunar 

 variation passes through its zero-points to other stations than Kew 

 and Hobarton, we are made aware of differences which appear to 



