TERRESTRIAL MAGNETISM. 179 



This brings me to my next point, namely, that if we are to draw con- 

 chisions from the minor differences between measnrements of secular 

 or diurnal change made in the observatories, it is not only necessary 

 that we should know whether the instruments are strictly comparable 

 and constant, but the observations must be reduced by precisely the 

 same methods. 



In 1886 the late Mr. Whipple drew the attention of the British Asso- 

 ciation to the fact that there was a systematic difference between the 

 diurnal ranges of declination at Greenwich and Kew. His results were 

 based on the three years 1870-1872. In 1890 two of my students, Messrs. 

 Eobson. and S. W. J. Smith, extended the comparison to three more 

 recent years (1883, 1886, 1887), and obtained results in complete accord 

 with those of Mr. Whipple. 



It is well known that the average daily oscillation of the magnet is 

 affected by the magnetic weather. Sabine showed that magnetic 

 storms do not merely buffet the needle now in this direction and now 

 in that — they affect its average behavior, so that the mean swing east 

 and west is different according as we deduce it only from days of mag- 

 netic calm or include those of storm. 



Mr. Whipple reduced the Kew observations by two methods,' one of 

 which depended on the calmest days only, while the other included 

 those which were moderately disturbed. Neither agreed exactly with 

 the method in use at Greenwich, but the difference between the 

 results deduced from them was so small when compared with the differ- 

 ence between either and that obtained at Greenwich, that it seemed 

 possible that the diurnal variations, even at these closely neighboring 

 places, might differ appreciably. The question whether this is so has 

 now been answered. In 1890, at the request of the Kew committee, 

 the astronomer royal undertook to select early in each year five quiet 

 days in each of the preceding twelve months. It was also agreed that, 

 whether they adopted other methods or not, the chief English magnetic 

 observatories should determine the diurnal variations from these days 

 alone. The Greenwich ^ and Kew observations for 1890 have therefore 

 been worked up in exactly the same way, with the result that the dis- 

 crepancy, which had persisted for twenty years, has entirely disap- 

 peared, and that the two diurnal ranges at the two observatories are 

 in as close accord as could be expected. 



If, therefore, we may judge from a single year, the cause of the dif- 

 ference lay in the choice of days. Greenwich will in future give us two 

 diurnal variations, one obtained from the most quiet days only, the 

 other from all days except those of violent storm, and in these we shall 

 have most valuable data for studying the mean effect of disturbances 

 on the diurnal variation. 



To this satisfactory conclusion I have only one suggestion to add. 

 The astronomer royal and M. Mascart now publish for the same stormy 



' Sabiue's and Wild's. 



* The Greenwich observations for subseij[uent years have not yet been published. 



