XIII. On the diurnal inequalities of terrestrial magnetism, as deduced from observa­tions made at the Royal Observatory, Greenwich, from 1841 to 1857

It has been usual for the Royal Society to receive among their communications and to publish in their ‘Transactions’ 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 those subjects. The paper which I have the honour now to submit to the Society consists principally of results of this class. 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 the seventeen years; during the last ten years of which the magnetic indications have been automa­tically recorded by photographic self-registration, on a system which has been continued to the present time (1863) and is still to be continued. I offer these results to the Royal Society in the hope that they will prove no unimportant contribution to a record of the state of Terrestrial Magnetism at Greenwich, through a period which is likely to be esteemed a very important one in the general history of the science. The magnets of the three magnetometers (Declination, Horizontal Force, Vertical Force), from which these indications are obtained, are 2-foot magnets, such as were introduced by Gauss about the time of commencing this series of observations; two of them were prepared at Gottingen. If I had now to establish a magnetical apparatus, I should probably adopt magnets of smaller dimensions. Yet there are advantages in the use of large magnets, as the power of carrying large mirrors, &c., which I would not lightly forego. And, judging from the completeness and delicacy of the registers of magnetic storms made by all three instruments, I have reason to believe that the general accuracy of the records is almost as great as it will be possible to obtain with any instru­ments. I have therefore not thought it necessary to make any change in the instru­mental system.