Conclusions from the Kew Photographic Records. 545 



1867 on the other. Hence we see the importance of -maintaining, 

 during the remaining portion of the decennial period, the photogra- 

 phic records of the Kew Observatory, with as little change as may 

 be practicable in the instruments and methods which have been 

 employed during the first portion. 



The Table which is printed in Phil. Mag. vol. xxii. p. 312, shows 

 the aggregate values of the disturbances in 1858 and 1859 distri- 

 buted into the several solar hours of their occurrence, and distin- 

 guishing between the disturbances which produce westerly and those 

 which produce easterly deflections of the compass-needle. It also 

 exhibits the ratios of disturbance at the several hours to the mean 

 of the 24 hours taken as the unit. The subjoined Table contains the 

 same particulars for the four years, 1858 to 1861, inclusive. It has 

 of course a somewhat higher authority than the earlier table, inas- 

 much as ratios obtained from the records of four years are to be pre- 

 ferred to those derived from two years only. But the principal point 

 of interest in comparing them with each other is the evidence which 

 their correspondence affords, of the substantial truth of the two first 

 of the three general conclusions adverted to in the commencement of 

 the present communication, viz., the periodicity of the disturbances in 

 respect to the several hours of solar time, and the distinct character of 

 the laws which regulate the disturbances producing westerly deflec- 

 tions, and those producing easterly deflections. The principal features 

 of both classes of disturbance are the same, whether viewed in the 

 record of the two or of the four years. Regarded from either point 

 of view, both classes follow progressions manifestly dependent upon 

 the hours of solar time, the progressions ot the westerly and those of 

 the easterly deflections being as manifestly governed by distinct and 

 different laws. The westerly deflections have their chief prevalence 

 from 5 a.m. to 5 p.m., or during the hours of the day, the ratios at all 

 the other hours being below unity. The easterly deflections, on the 

 other hand, prevail chiefly during the hours of the night, the ratios 

 being for the most part below unity at the hours when the westerly 

 are above unity, and, conversely, above when the westerly are below. 

 The easterly have one decided maximum at 11 p.m., towards which 

 they steadily and continuously progress from 5 p.m., and from which 

 they as steadily, and continuously, recede until 5 a.m. the following- 

 morning. The westerly appear in both records to have a double 

 maximum, one about 6 or 7 a.m., the other about 2 or 3 p.m. 



The main object of the Table is to exhibit the amounts of disturb- 

 ance and the ratios at the several hours, derived from the photo- 

 graphic records of the four years ; but, in order to facilitate the 

 examination of the correspondence in these respects of the results 

 severally deducible from the two and from the four years, the ratios 

 of westerly and of easterly disturbance at the different hours which 

 were derived from the photographic records in 1858 and 1859 are 

 added, being reproduced from the Table, Phil. Mag. vol. xxii. p. 312. 



Phil. Mag. S. 4. No. 164. Suppl Vol. 24. 2 



