74 



for each month and hour computed, after the omission from the 

 record of all the hourly directions which deviated 3' '3 from their 

 final normals, the differences were taken between each of the re- 

 maining hourly directions and the final normal of the same mouth 

 and hour, arid were entered afresh in lunar monthly tables, having 

 the lunar days in successive horizontal lines, and the twenty-four 

 lunar hours in vertical columns, each difference being placed under 

 the lunar hour to which it most nearly approximated. The entries 

 in these tables should consequently represent directly the lunar in- 

 fluence at the different lunar hours, subject only to minor disturb- 

 ances ; the effects of the solar-diurnal variation as well as of the 

 larger disturbances having been eliminated. The differences were 

 marked with a + sign when the north end of the magnet was east 

 of its mean direction, and with the sign when west of the same. 

 The differences were then summed up, and hourly, monthly, and 

 annual means taken by the non-commissioned officers of the Royal 

 Artillery employed at Woolwich, under the superintendence of 

 Mr. Magrath. 



Having in the former paper exhibited the results of the disturb- 

 ances at Kew in comparison with those at Hobarton, I propose to 

 do the same with the lunar-diurnal variation treated of in this com- 

 munication ; believing that such comparisons are very conducive to 

 a just appreciation of the systematic character and natural reality of 

 the results, and instructive both by the agreements and disagree- 

 ments which they exhibit. The lunar-diurnal variation at Hobarton 

 has been obtained for the purpose of this comparison, by a similar 

 process to that which has been described above, from observations at 

 every solar hour during five years (Sundays excepted), from Oct. 1, 

 1843 to Sept. 30, 1848 ; omitting as disturbed such observations as 

 deviated 2'* 13 from their respective final normals. The total number 

 of hourly observations was 36,832 ; the disturbed observations 2606 ; 

 and the number employed in the lunar-diurnal variation 34,226. As 

 it has been customary to represent such periodical variations by 

 formulae of well-known character, the results at Kew and Hobarton 

 are here represented by formulae in which , corresponding to x (the 

 lunar time for which the lunar- diurnal variation is desired), is counted 

 in hours and parts of an hour, multiplied by 15, from the epoch of 

 the moon's upper culmination. The + sign corresponds (as before) 



