III. On the diurnal and annual inequalities of terrestrial magnetism, as deduced from observations made at the Royal Observatory, Greenwich, from 1858 to 1863; being a continuation of a communication on the diurnal inequalities from 1841 to 1857, printed in the Philosophical Transactions, 1863. With a note on the luno-diurnal and other lunar inequalities, as deduced from observations extending from

Abstract

The author states that the instruments employed are precisely the same which were used in the second part of the former investigation, from 1848 to 1857, mounted in the same place, and treated in the same manner. In describing the treatment of the photographic curves, he first gives the number of days which have been omitted in different years; because the character of the observations or curves was too disturbed to permit the usual treatment of the observations, or the drawing by hand of a pencil curve that would fairly represent the general course of the curve. The greatest numbers of omitted days occur in the years 1846, 1847, 1848; 1851, 1852, 1853, 1854; 1859, 1860. As the estimate of the amount of irregularity has been made throughout, by the same person, he considers that these years may be accepted as those in which the disturbances were the greatest. If they point to any cycle at all, it is one of 6 or 6½ years. These days being omitted, the ordinates of the pencilled curves on the other days were used as basis of all the following investigations. For the solar inequalities, they were treated by collecting the measures for every complete solar day, or for every solar hour bearing the same ordinal number, according as the annual or diurnal inequalities were the subject of inquiry; but in all cases these quantities were next grouped by months, and the monthly means were taken.