Xvi FOURTH REPORT — 1834. 



between electricity aild magnetism, now so fully established,^-^ 

 the connexion between metalliferous veins and lines of elevation, 

 and between the latter and the isodynamical lines of terrestrial 

 magnetic intensity, as suggested by Professor Necker of Geneva, 

 — point out a bond of union between this subject and that of ter- 

 restrial magnetism, on which we have a report by Mr. Christie, 

 where the very interesting direct observations of Mr. Fox of Fal- 

 mouth, on the electro-magnetic action of mineral veins, are par- 

 ticularly noticed. Mr. Christie's theory of the diurnal variation 

 of the needle, which he is desirous should be submitted to the 

 test of a laboratory experiment, is likewise intimately connected 

 with the actual constitution of our globe*. The whole subject 

 of Terrestrial Magnetism is one of the most interesting and 

 progressive of the experimental sciences. The determination of 

 the direction of the magnetic energy by means of two spheri- 

 cal coordinates, termed the variation and the dip, and the mea- 

 sure of the intensity of that force, are the great objects of imme- 

 diate research, as forming a basis of theory. The existence of 

 four points on the earth's surface, to which the needle tends, has 

 long been known ; and the position of two of these (in Northern 

 Asia and America,) has recently been elucidated by the perse- 

 vering efforts of Professor Hansteen and Commander Ross. The 

 precise numerical determination of the elements just alluded to 

 acquires a deep and peculiar interest from the multiplied varia- 

 tions which they undergo. Not only are these elements subject 

 to abrupt and capricious changes, which Baron Humboldt has 

 termed magnetic storms ; but gradual and progressive variations 

 are undergone at different hours of the day, at different seasons 

 of the year, and throughout longer periods, which may even per- 

 haps bear a comparison with the sublime cycles of Astronomy. 

 *' Natural History forms a more prominent subject in this 

 volume than in the last, though the reports of Professor Lindley 

 * on the principal questions at present debated in the Philosophy 

 of Botany'; and of Dr. Charles Henry ' on the Philosophy of 

 the Nervous System', refer only to particular departments of 

 widely extended subjects, which are again to be resumed in more 

 general reports, undertaken for the present meeting,^that by 

 Mr. Bentham, on Systematic Botany, and by Dr. Clarke of 

 Cambridge, on Physiology in general. We cannot but remark 

 with pleasure, that one of the points for inquiry particularly in- 

 sisted on by Professor Lindley, that of the influence of the che- 

 mical nature of soils, and of the excretions of plants, was taken 

 up at an early period of the existence of the Association, by one 



» Report, p. 122-3. 



