of 'Terrestrial Magnetism. 117 



tions in the two curves are in the same direction at nearly the 

 same hour; but the deflections, represented are at Hobarton 

 those of the south end of the magnet, and at Kew those of the 

 north end of the magnet. No decennial influence is perceptible 

 in the lunar-diurnal variation. 



The peculiar feature of the lunar-diurnal variation, which is 

 consistently manifested in all the elements and at all the stations, 

 is that of a double and symmetrical progression in each lunar 

 day. This has been regarded by some physicists as indicating 

 that the magnetism of the moon, of which it is the effect, is an 

 induced magnetism. There are, however, difficulties in this sup- 

 position which appear to stand in the way of its immediate and 

 general reception. Meanwhile every additional locality in which 

 the phenomena are made the subject of satisfactory investigation 

 enlarges the basis of induction on which the physical explanation 

 of the magnetic relations undoubtedly subsisting between the 

 earth and her satellite must eventually rest*. 



V. I have thus brought before you some of the most notable 

 instances in which a cosmical connexion has been traced between 

 the sun and moon and magnetic variations observed on the earth. 

 They are all furnished by phenomena which, in the Report of 

 the Royal Society, were classified as being either periodical or 

 irregular, but which may now be comprised under the single 

 name of " periodical variations/' inasmuch as those which were 

 supposed to be " irregular " are now known to be subject in their 

 occurrence to laws of regular periodical action. But there still 

 remains a distinct class of magnetic variations which there is 

 also much reason to believe are, on a more extended scale, pe- 

 riodical, and probably governed by cosmical relations, to the dis- 

 covery of which, however, we as yet possess no clue. Many of 

 my hearers are familiar with, and others may have heard of, the 

 so-called " isogonic," " isoclinal," and " isodynamic " magnetic 

 lines, which, traced on maps of the globe or of any particular 

 portion of it, represent the results of " magnetic surveys/'' and 

 show the distribution of the magnetic influence on the surface of 

 our planet. When any such map, based on observations of a 

 given year for example, is compared with one based on observa- 

 tions of an earlier or later epoch, changes are seen to have taken 

 place which we believe to be in accordance with laws of regular 

 periodicity extending over a cycle of several centuries, though 



* The second volume of the 'St. Helena Magnetic Observations,' published 

 in 1860, contains the lunar-diurnal variations of the declination, inclina- 

 tion, and total force at Toronto, St. Helena, and the Cape of Good Hope; 

 of the declination and inclination at Hobarton; and of the declination at 

 Kew and Pekin. The lunar-diurnal variation of the declination at Nert- 

 ehmsk, and of the total force at Hobarton, are prepared for publication. 



