VARIATION OF THE NEEDLE.
IN our article MAGNETISM, WC referred to this place for an account of the magnetical phenomena of our globe, and we shall now endeavour, so far as our narrow limits will permit, to give an account of the interesting discoveries which have been re cently made in this important branch of science.
The general principles of the variation and dip, or the declination and inclination of the needle, have been explained in the article above referred to, so that we shall at once proceed to consider how these two classes of phenomena are related to the equator and poles of the terrestrial globe. The measures of the variation and dip of the needle have been ob tained principally by navigators and travellers, at different times, and by means of instruments of va rious degrees of accuracy. From this cause, it is impossible to place great confidence in the accuracy of the results; and it is only from the general bear ing of the observations that we can deduce any thing like a general principle. The following table contains the best measures of the variation of the needle.
Such are the leading facts respecting the dip and variation of the needle. The first attempt, we believe, to project them, was made by one Church man, who traced lines through all the points of the globe where the variation was 0°, 5°, 10°, &c. and also lines through all those parts where the dip was 5°, and which he called lines of equal vari ation, and lines of equal dip. These lines, as pro jected by Churchman, are given in the Mercator's chart, in our article GEOGRAPHY, Plate CCLXIX. from the inspection of which it will be seen that the lines arc clearly related to four points or poles, Iwo in the northern, and two in the southern hemisphere.
About the year 1, professor Hansteen of Chris tiania, devoted his attention to this curious subject, and has pursued it with a diligence and a success which cannot he too highly praised. The results of his first investigations were published in his able and learned work, " On the Magnetism of the Earth," (Untcrsuchungen Ether Magnctismus der Erdc, 1819.) By means of all the observations which
he could collect,he was able to prove that there were four points of among the lines of variation, a weaker and a stronger point being in the vicinity of each pole. Each of the stronger poles lies nearly dia metrically opposite to the other, and the weaker poles have the same relative position. From a comparison of the observations made at different times, he found that these poles had a constant motion.
The motion of both the northern poles is from W. to E. obliquely, and of both the southern ones also obliquely from E. to W. With regard to the velocity of their motion, Prof. Ilansteen acknow ledges that the observations arc not sufficiently nu merous, but he has obtained the following results: N is the strongest pole in the north hemisphere, and its revolution round the north pole of the earth is performed in 1740 years.
S is the strongest pole in the south. hemisphere, and its revolution round the south pole of the earth is performed in 4609 years.
n is the weakest pole in the north hemisphere, and its revolution is performed in 860 years.
s is the weakest pole in the south hemisphere, and its revolution is performed in 1304 years.
Since this table was computed, Professor Elan steen has obtained many new sets of magnetical ob servations, and particularly those which have been made during the British Voyages of discovery to the Arctic Regions. These he has diligently com pared, and he has thus obtained new determinations of the position and times of revolution of the mag netic poles of the earth. The results of these we shall now lay before our readers in a very abbre viated, but, we trust, intelligible and useful form.
1. On the Position, ,S•e. of N. the strongest Magnetic Pole in North .lintriea.
By combining four observations on the declination of the needle, made on board his Majesty's sloop Bra zen in Hudson's Bay, in 1813, and which Prof. Han steen inspected in the :Marine Chart office at the Ad miralty in London, he obtained the following results: Distance of the Pole N from Lon. West of the Pole of the. Earth. Greenwich.