Royal Society. 311 



means have received the name of " Final Normals," and may be de- 

 fined as being the mean directions of the magnet in every month and 

 every hour, after the omission from the record of every entry which 

 differed from the mean a certain amount either in excess or in defect. 



In this process there is nothing indefinite ; and nothing arbitrary 

 save the assignment of the particular amount of difference from the 

 normal which shall be held to constitute the measure of a large dis- 

 turbance, and which, for distinction sake, we may call " the separating 

 value," It must be an amount which will separate a sufficient body 

 of disturbed observations to permit their laws to be satisfactorily 

 ascertained ; but in other respects its precise value is of minor sig- 

 nificancy ; and the limits within which a selection may be made, 

 without materially affecting the results, are usually by no means 

 narrow ; for it has been found experimentally on several occasions, 

 that the Ratios by which the periodical variations of disturbance in 

 different years, months and hours are characterized and expressed, do 

 not undergo any material change by even considerable differences in 

 the amount of the separating value. The separating value must ne- 

 cessarily be larger at some stations than at others, because the abso- 

 lute magnitude of the disturbance-variation itself is very different in 

 different parts of the globe, as well as its comparative magnitude in 

 relation to the more regular solar-diurnal variation ; but it must be 

 a constant quantity throughout at one and the same station, or it 

 will not truly show the relative proportion of disturbance in different 

 years and different months. 



The strength of the Kew establishment being insufficient for 

 the complete work of a magnetic observatory, the tabulation of 

 the hourly directions from the photographic records has been per- 

 formed by the non-commissioned officers of the Royal Artillery, 

 employed under my direction at "Woolwich, where this work has been 

 superintended by Mr. John Magrath, the principal clerk, as have 

 been also the several reductions and calculations, which have been 

 made on the same plan as those of the Colonial Observatories. 



In the scale on which the changes of direction of the declination- 

 magnet are recorded in the Kew photographs, one inch of space is 

 equivalent to 22'-04 of arc. On a general view and consideration of 

 the photographs during 1858 and 1859, 0*15 inch, or 3'"31 of 

 arc appeared to be a suitable amount for the separating value to be 

 adopted at that station ; consequently every tabulated value which 

 differed 3'-3l or more, either in excess or defect from the final nor- 

 mal of the same month and hour, has been regarded as one of the 

 larger disturbances, and separated accordingly. The number of dis- 

 turbed observations in the two years was 2-124 (viz. 121 i in 1858, 

 and 1213 in 1859), being between one-seventh and one-eighth of the 

 whole body of hourly directions tabulated from the photographs, of 

 which the number was 17,319. The aggregate value of disturbance 

 in the 2424 observations, was 14,901 minutes of arc ; of which 7207 

 minutes were deflections of the north end of the magnet to the west, 

 and 70*94 to the east ; the easterly deflections thus having a slight 

 preponderance. The number of the disturbed observations, as well 

 as their aggregate values, approximated very closely in each of the 

 two years, 1859 being very slightly in excess. The decennial period 



