MAGNETIC OBSERVATIONS AT THE KEW OBSERVATORY. 443 



Total Force at Hobarton. The approximate accordance of the numerical values of the 

 lunar influence at the two stations, and in each of the three elements, is also the more 

 remarkable when we take into view the shortness of the time during which the investi- 

 gation has as yet been prosecuted, and the minuteness of the quantities which are 

 involved — measured as they are by a few seconds of arc in the Declination and Incli- 

 nation, and by millionth parts of the terrestrial magnetic force. Such an accordance 

 cannot be viewed otherwise than as a great encouragement to the continuance of the 

 research where it is already in progress, and to its adoption elsewhere. 



The conclusions in regard to the Declination are those which will perhaps be generally 

 regarded as entitled to principal consideration, inasmuch as their derivation is much more 

 direct than in the cases either of the Inclination or of the Total Force. To this it may 

 be added that, in comparing the declination-phenomena at Kew and Hobarton, we have 

 the advantage of a somewhat larger series of observations than is the case either in the 

 Inclination or in the Total Force, as the hourly observations of the Declination at 

 Hobarton in 1841 and 1842 are available for the purpose, in addition to those of the 

 subsequent years 1843-1848 ; making together eight years of the Declination, whilst we 

 have only six years of either the Horizontal or the Vertical Force. In all cases, how- 

 ever, and whether from Kew, Hobarton, or elsewhere, the numerical values which we 

 may derive in regard to the lunar influence can only be regarded as approximations ; 

 and as indicating generally what we may expect will be accomplished by a further per- 

 severance, rather than as preferring a claim to present or immediate precision. With 

 this reservation we may view the facts regarding the moon's influence on the magnetic 

 declination at Kew and Hobarton as placing beyond doubt the existence, at this parti- 

 cular epoch in the great cycle of the variations of the terrestrial magnetism, of a lunar- 

 diumal variation which has two equal or very nearly equal progressions, both in time 

 and in amount; producing consequently two easterly and two westerly maxima of 

 deflection in every lunar day, with four nodal epochs, occurring also at nearly equal 

 intervals of lunar time, in which the direction of the magnet due to other causes is 

 undisturbed by the moon's influence. The lunar hours of extreme deflection at Kew 

 and Hobarton are 1, 7, 13, and 19 ; 1 and 13 being the westerly extremes at Kew and 

 easterly at Hobarton ; the nodal hours, or those in which the lunar influence is inope- 

 rative in producing deflection, are, as nearly as can be judged, strictly intermediate 

 between the times of extreme deflection; viz., between 4 and 5, 10 and 11, 16 and 17, 

 22 and 23 hours. The amounts of the extreme deflections, measured by the mean of all 

 the observations hitherto, are at Kew, westerly, ll"'l at 1 hour, 9"-4 at 13 hours; 

 easterly, ll"-6 at 7 hours, 10"-8 at 19 hours: at Hobarton, easterly, 8"-4 at 1 hour, 

 9"-l at 13 hours; westerly, 7''-3 at 7 hours, and 9"-l at 19 hours. The antagonistic 

 terrestrial magnetic force by which the deflecting action of the moon on the horizontal 

 magnet is opposed, is (approximately) 4'5 at Hobarton, and 3*8 at Kew, expressed in 

 British units. 



If we now extend this examination to other stations in the middle latitudes whefe 



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