212 REPORT— 1895. 



' quiet ' days varied arbitrarily, so that the positions of the two midnights 

 of a ' quiet ' day on the photographic sheets were constantly being inter- 

 changed. It may also be added that the phenomenon is not peculiar to 

 Kew, but may be seen in the published Greenwich and Falmouth results 

 for the same epoch. 



§ 5. The phenomena seem not unlikely to be only another phase of 

 phenomena observed many years ago by General Sabine and Dr. Lloyd. 

 The former from a careful study of what he termed ' disturbances ' con- 

 cluded that the aggregates of easterly and westerly declination disturbances 

 at Kew during 1858-62 nearly balanced, 'there being in some years a 

 slight preponderance of westerly, and in other years of easterly deflection ; ' ' 

 at the same time he found ^ ' a slight preponderance of easterly values on 

 the average of the four years (1858-61).' As regards the horizontal force 

 he deduced^ from 5,932 disturbed observations at Kew during 1858-64 

 that ' the ratio of the value of the disturbances decreasing the (horizontal) 

 force to those which increased it was nearly as 3'23 to 1.' In agreement 

 with this last result Dr. Lloyd found ^ from an examination of the 335 (?) 

 most disturbed days at Dublin during the ten years 1841-50 that 'the 

 mean efiect of disturbances is to diminish that (the horizontal) force.' 



The significance of these results, when taken along with the tendency 

 suggested by Table II. to a slight abnormal increase of westerly declina- 

 tion on quiet days, and the clear evidence of an abnormal increase of 

 horizontal force on these days, need not be dwelt on. Although the 

 vertical foi'ce lies outside our present inquiry, it may not be amiss to 

 mention that evidence of a connection of ths kind foreshadowed above is 

 also supplied by it. The results, in fact, from the last five years at Kew 

 show a distinct abnormal decrease of vertical force on ' quiet ' days, while 

 General Sabine observed a very decided preponderance of disturbances to 

 be associated with increasing vertical force. 



Whether the species of opposition in the phenomena exhibited by the 

 horizontal and vertical forces is to be associated with the fact that at pre- 

 sent the former element is increasing the latter decreasing seems an inter- 

 esting speculation, but it lies outside our present inquiry. 



Method of Treating the Non-cyclic Diurnal Element. 



§ 6. General Sabine got out tables according to which ' disturbances ' 

 at Kew were by no means uniformly distributed over the twenty-four hours ; 

 further he found that disturbances associated with easterly deflections 

 had diflferent hours for maxima and minima from disturbances associated 

 with westerly deflections, and a like difference appeared between disturb- 

 ances associated with increasing and those associated with decreasing 

 horizontal force. If this be so, and if the non-cyclic phenomena observed 

 on ' quiet ' days be in any way complementary to the disturbed phenomena, 

 then there is certainly ground for suspecting that the abnormal increase, 

 say, of H.F. observed in the mean ' quiet' day of a certain month is not 

 uniformly distributed over the twenty- four hours. While fully recognising 

 the uncertainty of the hypothesis of uniform distribution of the non-cyclic 

 element, I have unhesitatingly adopted it provisionally. What General 



' Phil. Trans, for 1863, p. 282. 

 " Prnc. Boy. Soc, vol. xi. p. .590. 

 « Phil. Tf'ans. for 1871, p. 310. 

 * -1 Treatise on Magnetism, p. 211. 



