Disturbances of Magnetic Declination in the Kew Observatory. 313 



The westerly and easterly deflections in the British Islands, as 

 represented by the automatic records at Kew, are obviously governed, 

 as in all other parts of the globe where the phenomena have been 

 analysed, by distinct laws. The westerly deflections have their chief 

 prevalence from 5 a.m. to 5 p.m., or during the hours of the day ; 

 the easterly deflections, on the other hand, prevail chiefly during the 

 hours of the night, the ratios being above unity from 7 p.m. to 3 A.M., 

 and below unity at all other hours. The easterly have one decided 

 maximum, viz. at 11 p.m., towards which they steadily and con- 

 tinuously progress from 5 p.m., and from which they as steadily and 

 continuously recede until 5 a.m. the following morning. The 

 westerly deflections appear to have two epochs of maximum, one 

 from 6 to 7 a.m., the other about 3 p.m., progressing regularly 

 towards the first named from 3 a.m., and receding from it to 9 a.m. ; 

 at 9, 10, and 1 1 a.m. the ratios remain almost sensibly the same, but 

 towards noon they begin to increase afresh, and continue to do so 

 progressively to the second maximum at 3 p.m., from which hour 

 they progressively decrease to 7 p.m. Those ratios which are less 

 than unity, viz. those of the westerly deflections from 6 p.m. to 

 4 a.m., and of the easterly from 4 a.m. to 6 p.m., do not in either 

 case exhibit the same decided tendency to one or two well-marked 

 minima, as the ratios which are above unity do in both cases towards 

 their maxima. It is possible, however, that this may in some degree 

 be explained by the following consideration : — 



The aggregate values of the disturbances prevailing at the different 

 hours, as stated in the Table, are those which have prevailed, not 

 only over the forces which would retain the magnet in its mean po- 

 sition, but also over any disturbing influences in an opposite direc- 

 tion, which may be conceived to have existed contemporaneously ; 

 and we cannot but suppose that as both westerly and easterly dis- 

 turbances do record themselves as prevailing at the same hours on 

 different days, that these opposite influences may sometimes coexist, 

 neutralizing each other and not appearing in the record. We may 

 reasonably suppose that the degree in which the aggregate values in 

 the Table, both westerly and easterly, may be diminished thereby at 

 the different hours, may be in some measure indicated by the dis- 

 parity, or the reverse, in the amount of the aggregate values of dis- 

 turbance in the opposite directions at those hours. Thus we may 

 suppose that at a particular hour, 11 p.m. for example, when the 

 amount of westerly deflections is very small, and of easterly very 

 great, the diminution of the aggregate values of either by mutual 

 counterbalance may be extremely small, while of equal absolute 

 amount in both. Now a very small amount deducted from the large 

 aggregate easterly value will scarcely have any effect whatsoever on 

 the ratio at that hour to its unit or mean hourly value ; whereas the 

 same small amount deducted from the far less aggregate westerly 

 value at the same hour would have a far more sensible effect upou 

 its ratio. Assuming, therefore, the probability that westerly and 

 easterly disturbing influences do sometimes coexist and neutralize 

 each other in the record, and that we may in some degree judge of 

 the respective amounts of the conflicting influences at the several 

 Phil Mag. S. 4. Vol. 22. No. 147. Oct. 1861. Y 



