834 Royal Society : — 



April 23. — Major-General Sabine, President, in the Chair. 



The following communication was read : — ■ 



" On the Diurnal Inequalities of Terrestrial Magnetism, as deduced 

 from observations made at the Royal Observatory, Greenwich, from 

 184 1 to 1857." By George Biddell Airy, F.R.S., Astronomer Royal, 



The author describes this paper as one of the class which gives 

 the epitomized results of long series of voluminous observations and 

 laborious calculations, of which the fundamental details have been 

 printed in works specially devoted to these subjects. It exhibits in 

 curves the diurnal inequalities of terrestrial magnetism, as obtained 

 by the use of instruments essentially the same, through the whole 

 period of seventeen years, during the last ten years of which the 

 magnetic indications have been automatically recorded by photogra- 

 phic self-registration, on a system which has been continued to the 

 present time, and is still to be continued. 



From the last months of 1840 to the end of 1847, the observations 

 were made by eye, every two hours. From the beginning of 1848, 

 for the declination and horizontal force magnetometers, and from 

 the beginning of 1849, for the vertical force magnetometer, the mag- 

 netic indications are recorded by Mr. Brooke's photographic appa- 

 ratus. 



In preparing the reductions of the magnetic records from 1848 to 

 1857 (which are printed in the " Results of Magnetical and Meteoro- 

 logical Observations for 1859," bound in the volume of * Greenwich 

 Observations/ 1859, and also issued separately), the days of unusual 

 magnetic disturbance had been separated from the rest, and the re- 

 ductions applied to the mass so diminished. For unity of plan, it 

 appeared expedient to follow the same course for the reductions from 

 1841 to 1847. In consequence of this, the numbers which are used 

 here differ in some cases by small quantities from those printed in 

 the 'Greenwich Magnetical Observations from 1841 to 1847.' The 

 numbers in the reductions from 1848 to 1857 are adopted without 

 change. 



The author remarks that, taking the number of omitted days as a 

 rough measure of the amount of magnetic disturbance, there is no 

 appearance of decennial cycle in their recurrence, and no distinct re- 

 lation to the magnitude of diurnal changes. 



The author then proceeds to the description of the curves. The 

 first four sheets contain the curves in which the horizontal abscissa 

 represents the declination at each hour as compared with the mean 

 for the twenty-four hours, and the vertical ordinate represents the 

 horizontal force at each hour as compared with the mean for the 

 twenty-four hours. On the different sheets the days are differently 

 grouped, thus : — On sheet I. all the observations at each nominal 

 hour throughout the year are combined ; this sheet contains the 

 separate curves for 1841, 1842, 1843, 1844, 1895, 1846, 1847. On 

 sheet II. similar curves are formed for 1848, 1849, 1850, 1851, 

 1852, 1853, 1854, 1855, 1856, 1857. On sheet III. all the obser- 

 vations at each nominal hour through all the months January from 

 1841 to 1847 are combined to form the January curve; all those 



