ON COMPARING AND REDUCING MAGNETIC OBSERVATIONS. 



71 



The days accepted as undisturbed are the following 



IV. Eejyort hij G. M. Whipple. 



In the Report of the Committee presented to last year's meeting, 

 Dr. Wild, of St. Petersburg, .submitted a Table showing the different 

 values of the solar-diurnal vai'iation of the declination in Pawlowski for 

 October 1882 and March 1883, as derived from the photographic records of 

 that observatory, after treatment by the two methods of Sabine and Wild. 



He found that the difference in the value of the declination at any 

 hour of the day for the two months in question varied from +08 to — 0'9 

 in a range of 8-1, or 21 per cent., of the whole. 



At the request of the Committee I have jirepared tables showing the 

 mean daily variation of declination at the Kew Observatory for the three 

 years (1870-1872) and have contrasted the values obtained there, by 

 Sabine's and Wild's methods, both for the whole year, as well as for the 

 sammer and winter semi-annual periods. I have, in addition, compared 

 both sets of values with those published by the Royal Observatory, 

 Greenwich, for the same periods. 



The results given in Table III. would show that the differences in the 

 values of the diurnal range of the declination magnet at the Kew Obser- 

 vatory, as determined by Sabine's or Wild's methods, vary to an extent of 

 0*7' in a total range of 12', or may equal G per cent, of the whole ; whilst 

 in the summer half-year (Table I.) the extreme difference amounts to 

 11 in a range of 15', or 7 per cent., and in winter to an extreme difference 

 of 0'8 in a range of 8"7, equal to 9 per cent. 



The greater diurnal range is afforded in every instance by Sabine's 

 method of treatment of the observations, although the difference isbutsmall. 



Contrasting the Kew results with those of Greenwich, we may fairly 

 consider the difference to be due in some measure to instrumental causes, 

 the construction of the magnetographs being dissimilar at the two obser- 

 vatoi'ies. The slight difference in position of the two observatories may 

 likewise have some influence. 



Accordingly, Table III. shows that the normal daily range at 

 Greenwich differs from that at Kew, as deduced by Sabine's method, by 

 1'4', or from that derived by Wild's method by 1'8', in a range of 11'6', 

 the percentages being 12 and 15 I'espectively. 



In the summer half-year (Table I.) we get differences of 1 -i , or 10 

 per cent., by Sabine's, and the same percentage by Wild's method, in a 

 range of 14"7' ; whilst in the winter we similarly obtain differences of 

 1"3' by Sabine's, and 2'0' by Wild's method in a range of 84', or percent- 

 ages of 16 and 25 respectively. G. M. Whipple. 

 Kew Observatory, Jali/ 1886. 



' Owing to imperfections of the record only these two days were eventually used. 

 - Apparatus destroyed by fire. . ' Kecord begins Nov. 1 2. 



