RECENT VALUES OF THE MAGNETIC ELEMENTS. 433 



values for the month or year. For this purpose use may be 

 made of the curves of every day in the year, or of every day 

 free from serious disturbance, or of any smaller number of days 

 that may be deemed sufficient. At some observatories, as 

 at Greenwich, measurements are taken for every hour, at 

 others for every sixth, at others for every eighth hour, and 

 various other plans are elsewhere in vogue. 



The operation of taking measurements for every hour 

 of every day unfortunately entails a very large amount of 

 labour and may also distort the average diurnal oscillation 

 with the effects of magnetic storms which are not uniformly 

 distributed over the twenty-four hours. This led Professor 

 Wild of St. Petersburg, many years ago, to suggest the 

 restriction of the hourly measurement to the curves for a 

 comparatively small number of quiet days, or days of no ap- 

 preciable disturbance, selected by actual inspection. The 

 adequacy of five quiet days a month for most purposes 

 seems indicated by independent researches of Professor 

 Riicker 1 and the late Mr. Whipple. 1 This number 

 has met with the approval of the Magnetic Com- 

 mittee of the British Association, and to secure uniformity 

 the Astronomer Royal has undertaken to choose from inspec- 

 tion of the Greenwich curves the most suitable quiet days 

 and to communicate these to other observatories. At pre- 

 sent the observatories at Falmouth and Kew employ these 

 days alone for their monthly and annual means, whilst at 

 Greenwich special tables for these days are constructed in 

 addition to the more complete tables. 



Mean annual values deduced from the magnetic curves 

 are the most satisfactory when obtainable, but unfortunately 

 a good many observatories furnished with magnetographs 

 do not state them explicitly in their publications. In some 

 of these cases data are o-iven from which mean annual 

 values may be calculated, but in other instances this does 

 not appear to be the case. Not unfrequently results are 

 stated in terms of arbitrary scale divisions, whose values are 

 not given in the publication of the year under consideration. 

 Research into the publications of previous years may lead 



1 See B. A. Report for 1890, p. 172. 



