[DECLINATION OF THE MAGNETIC NEEDLE.] It is not improbable that Columbus was acquainted also with the diurnal v..riation, but nothing very accurate on this subject was known before the numerous and valuable observations made by Canton, in 1750. Ile showed that the needle vibrates, during the day and the night, tbrough an arc as great as 13k' in the midsummer, the minimum 7' occurring in the winter season ; he ascribed the diurnal variation to the action of solar heat affecting the intensity of the magnetism of the earth. The principles of thermoelectric currents were at that period unknown ; but Hooke, in 1684, showed that iron and steel rode could be magnetised by rapidly heating and cooling them in the magnetic meridian. Graham, inistrument-maker, of London, was the first who distinctly announced the diurnal variation, in 1772; the maximum declination being then 14* 35' west. The variation of the variation was first observed by Gunter. [GUNTER, in Bloc. Drv.] The dip was first observed by Robert Norman in 1576. His mode of adjusting the compass-needle led him to this discovery ; for he accu rately balanced the needle on its pivot previous to magnetising it. After it became a magnet it would no longer balance on the same point, without attaching a small weight at the south extremity. When freely suspended by the centre of gravity, the north extremity became depressed, the dip then being about 71* 50'. The dip undergoes diurnal variations, as well as the declination ' • but observations on the former are far the most The dip also changes by elevating the needle to considerable heights, on which subject Biot has made some very delicate expenments. A very simple law relative to the amount of the dip at different parts of the earth s surface was remarked by Professor Krafft, of St. Petersburg, In 1809 ; namely, if we measure the latitude from the magnetic equator, the tangent of the dip is double the tangent of such latitude. Mr. Barlow has illustrated this law by experiments on magnetised Iron balls acting on small needles at the surfaces ; and Biot has deduced the same law from theory.
The following is the inclination for London in the years stated :— The law of the magnetic forces was a long time undiscovered : Newton supposed it to follow the inverse cube of the distance, or some higher power ; for in his experiments, the variation of intensity and the effect of the mutual influences of the magnetic fluids in the bodies themselves being overlooked, an erroneous result was necessarily consequent. However, Mitchell, by a careful revision of the experi
ments of Dr. Brooke Taylor and of Haukebee ; Coulomb, by his elegant apparatus, the torsion balance ; Biot, by the method of observing the times of the oscillations of the small needles acted on ; and Hansteen of Christiania, by a series of refined experiments and calculations, have demonstrated the true law of magnetic action,— namely, directly as the intensity, and inversely as the square of the distance. But for the discussion of the whole question, we refer to Ilarris'e Rudimentary Magnetism,' part iii. [See also TERRESTRIAL MAGNETISM ] One essential property by which a magnet differs from soft iron under the magnetic influence, is this ; if we separate a magnetic bar into any number of minute parts, each such part will be endowed with polarity, similar to the whole : the position of those poles, or foci of greatest attraction, is permanent in a magnet of a given form ; but in soft iron it will change when the distance of the iron from the influencing magnet is altered.
Halley was sent out, under William and Mary, with the command of two ships, to make magnetic observations in different latitudes, both in the Atlantic and Pacific (in 1698-9), and was the first who con structed a magnetic chart, which possessed at the time great merit for accuracy : the most valued at the present day are those by Hansteen, constructed from observations subsequent to Halley, by various scientific travellers and nautical men, such as Humboldt, Ross, Parry, Scoresby, &c. [TERRESTRIAL MAGNETISM.] During a thunder-storm the poles of a magnet are frequently inverted, the explanation of which belongs to ELEcrno-MAosrsTista ; and the appearance of the aurora borealis is often attended with vibrations of the compass-needle, to the extent of several degrees. The actual mode in which the aurora is produced being still only matter of conjecture, it is impossible to decide whether the aurora is itself the cause of this magnetic phenomenon, or whether both are attributable to some unknown common cause.