Diurnal Variation of the Magnet at Kew. 



141 



days will affect the form of the curve of diurnal range unless 

 the disturbances themselves occur indifferently at all hours 

 of the day and night. 



Mr. Whipple, the Superintendent of the Kew Observatory, 

 made a comparison of the methods of Wild and Sabine, and 

 that in use at Greenwich for the years 1870-71-72 (Rep. 

 Brit. Assoc. 1886, p. 71). 



His general conclusion was that the difference between the 

 Kew results, as treated by the two methods of Sabine and 

 Wild, was less than the difference between the Greenwich 

 and Kew results in either case. 



The range of the differences is given below : — 



Mean of three years 1870-72. 





Range of Differences. 



Kew (Sabine and Wild) 



-0'-3 to +0H 

 -0'6 to +0''6 

 -0'-9 to +0'-7 



Greenwich and Kew (Sabine)... 

 Greenwich and Kew (Wild) ... 



It would have been interesting if similar comparisons could 

 have been carried out for more recent years ; but the fact 

 that the Kew magnetic year has not been identical with that 

 used at Greenwich has made such a comparison impossible. 

 The Kew magnetic year has hitherto been from October to 

 October, to admit of the observations being included in the 

 Annual Report made by the Committee to the Royal Society. 

 We understand that in future the civil year will be adopted. 



Besides this, some of the Kew results have been obtained 

 not by measuring the hourly ordinates, but by drawing free- 

 hand a mean curve among the superposed traces of " quiet 

 day" curves. 



It was therefore determined to make some further com- 

 parison of the different methods of reduction, and at the 

 request of Professor Riicker we have determined the diurnal 

 variation of the declination at Kew for three recent civil 

 years. 



The years selected were 1883, a year of maximum sun- 

 spots, 1886, and 1887. The mean diurnal range for each 

 year was found by Wild's method — the hourly ordinates for 

 about five quiet days in each month being measured. 



The results we obtained have been compared with the 



