William Gilbert,[49] who wrote an able work on magnetic and electric forces in the year 1600, regarded terrestrial magnetism and electricity as two emanations of a single fundamental source pervading all matter, and he therefore treated of both at once. According to Gilbert’s idea, the earth itself is a magnet; whilst he considered that the inflections of the lines of equal declination and inclination depend upon the distribution of mass, the configuration of continents, or the form and extent of the deep intervening oceanic basins.
Till within the last eighty years, it appears to have been the received opinion that the intensity of terrestrial magnetism was the same at all parts of the earth’s surface. In the instructions drawn up by the French Academy for the expedition under La Pérouse, the first intimation is given of a contrary opinion. It is recommended that the time of vibration of a dipping-needle should be observed at stations widely remote, as a test of the equality or difference of the magnetic intensity; suggesting also that such observations should particularly be made at those parts of the earth where the dip was greatest and where it was least. The experiments, whatever their results may have been, which, in compliance with this recommendation, were made in the expedition of La Pérouse, perished in its general catastrophe; but the instructions survived.
In 1811, Hansteen took up the subject, and in 1819 published his celebrated work, clearly demonstrating the fluctuations which this element has undergone during the last two centuries; confirming in great detail the position of Halley, that “the whole magnetic system is in motion, that the moving force is very great as extending its effects from pole to pole, and that its motion is not per saltum, but a gradual and regular motion.”
THE NORTH AND SOUTH MAGNETIC POLES.
The knowledge of the geographical position of both Magnetic Poles is due to the scientific energy of the same navigator, Sir James Ross. His observations of the Northern Magnetic Pole were made during the second expedition of his uncle, Sir John Ross (1829–1833); and of the Southern during the Antarctic expedition under his own command (1839–1843). The Northern Magnetic Pole, in 70° 5′ lat., 96° 43′ W. long., is 5° of latitude farther from the ordinary pole of the earth than the Southern Magnetic Pole, 75° 35′ lat., 154° 10′ E. long.; whilst it is also situated farther west from Greenwich than the Northern Magnetic Pole. The latter belongs to the great island of Boothia Felix, which is situated very near the American continent, and is a portion of the district which Captain Parry had previously named North Somerset. It is not far distant from the western coast of Boothia Felix, near the promontory of Adelaide, which extends into King William’s Sound and Victoria Strait.
The Southern Magnetic Pole has been directly reached in the same manner as the Northern Pole. On 17th February 1841, the Erebus penetrated as far as 76° 12′ S. lat., and 164° E. long. As the inclination was here only 88° 40′, it was assumed that the Southern Magnetic Pole was about 160 nautical miles distant. Many accurate observations of declination, determining the intersection of the magnetic meridian, render it very probable that the South Magnetic Pole is situated in the interior of the great Antarctic region of South Victoria Land, west of the Prince Albert mountains, which approach the South Pole and are connected with the active volcano of Erebus, which is 12,400 feet in height.—Humboldt’s Cosmos, vol. v.
MAGNETIC STORMS.
The mysterious course of the magnetic needle is equally affected by time and space, by the sun’s course, and by changes of place on the earth’s surface. Between the tropics the hour of the day may be known by the direction of the needle as well as by the oscillations of the barometer. It is affected instantly, but transiently, by the northern light.
When the uniform horary motion of the needle is disturbed by a magnetic storm, the perturbation manifests itself simultaneously, in the strictest sense of the word, over hundreds and thousands of miles of sea and land, or propagates itself by degrees in short intervals every where over the earth’s surface.
Among numerous examples of perturbations occurring simultaneously and extending over wide portions of the earth’s surface, one of the most remarkable is that of September 25th, 1841, which was observed at Toronto in Canada, at the Cape of Good Hope, at Prague, and partially in Van Diemen’s Land. Sabine adds, “The English Sunday, on which it is deemed sinful, after midnight on Saturday, to register an observation, and to follow out the great phenomena of creation in their perfect development, interrupted the observation in Van Diemen’s Land, where, in consequence of the difference of the longitude, the magnetic storm fell on Sunday.”