\^ 



442 LIEUT.-GENERAL SABINE ON THE RESULTS OF THE 



of computing the monthly means for each of the twenty-four hours in every month ; 

 and second, of marking (for subsequent exclusion) every hourly position which differed 

 from the mean of the same hour in the same month, not less than a certain specified 

 and definite amount ; such difference being regarded as evidence of the presence of a 

 magnetic disturbance. The amount of difference thus adopted as a criterion (known 

 commonly by the name of " the separating value") was constant throughout ; being 

 0'150 of a scale-division in the Declination, and 0"106 of a scale-division in each of the 

 other elements ; equivalent to 3'- 3 of arc in the Declination, -00109 part of the horizontal 

 force at Kew, and '000269 part of the vertical force at Kew. The number of hourly 

 ])ositions thus separated was, in the Declination 10271, or about 1 in 5*9 of the whole 

 body ; in the horizontal force 11*747, or about 1 in 5*1 of the whole ; and in the vertical 

 force 13'562, or about 1 in 3-8 of the whole. The record of the hourly positions, 

 exclusive of those thus separated, was then rearranged and rewritten in Lunar Tables, 

 according to the Lunar hours to which each position most nearly corresponded; and 

 from these the mean variation in each lunar hour in each year was derived, as shown in 

 the subsequent Tables, Nos. II., III., and IV. 



The tabulation from the photograms and the subsequent calculations were executed 

 at the Magnetic Office at Woolwich, under the superintendence of Mr. John Mageath. 



I have been thus particular in stating the processes which the photographic records 

 have undergone, first, because that which forms the subject matter of the investigation, 

 viz. the moon's action on the magnetism of the globe, is measurable only by very minute 

 quantities, and requires consequently peculiar cai'e and suitable arrangements for its satis- 

 factory detection and determination ; and secondly, because the results, now submitted 

 to the Royal Society, present a variation which, small as it is in amount, is far too regular 

 and too systematic to be ascribed to accidental causes, and affords a strong indica- 

 tion of the existence of a general law, the complete development of which has yet to be 

 looked for from the extension of similar investigations in other localities. A great 

 encouragement to the prosecution of the research is supplied by the remarkable corre- 

 spondence of the phenomena of the lunar-diurnal variation at Kew, as now made known 

 to us, with those at Hobarton, shown by the results of the observations at that Obser- 

 vatory between the years 1843 and 1848. The general aspect of the variation produced 

 by the moon's influence at the two stations is the same, viz. a double progression in 

 every twenty-four lunar hours, producing extreme deflections of the same character at 

 opposite points of the moon's diurnal course ; the turning-points of the variation taking 

 place nearly at the same lunar hour at each of the two stations, and the amounts of the 

 variation being approximately the same at both. The stations (Kew and Hobarton) being 

 in opposite magnetic hemispheres, we might naturally expect what we actually find to be 

 the case, that the lunar hours of the maxima of Easterly deflection in the one hemisphere 

 are those of Westerly deflection in the other hemisphere, and vice versd ; and that the 

 lunar hours of the maxima and minima of the north Dip and of the northern Total Force 

 at Kew are the same as those of the maxima and minima of south Dip and of the southern 



