then this would be strictly true
then this would be strictly true
The needle is subject to a perpendicular as well as a vertical motion. Norman discovered that a needle, accurately balanced, and perfectly horizontal before it was touched by a magnet, always lost its position after the magnetic principle was communicated to it, the north pole declining below the horizon in those countries situated in the northern hemisphere. This ingenious philosopher invented an instrument by which to measure the inclination or dip, and determined it to be, in London, about 71° 50′. This experiment ia supposed to hsve been made in the year 1576. The same philosopher was aware of the fact that the dip changes with the situation of tbe place .in which the needle is suspended, though he was not aware of the circumstances which influence this change. It may be stated, as a general law, that the dip increases from the equator to the polos. If the poles of the earth’s rotation were the magnetic poles, then this would be strictly true, and on tho equator tne magnet would be horizontal, while at the poles it would be vertical. But, as the terrestrial and magnetic poles do not coincide, neither can the terrestrial and magnetic equator. The dip may bo very well illustrated by suspending a small magnetic needle over a large bar magnet; when tne middle is situated directly over the centre of the msgnet it will be horizontal, having no tendency to incline cither to one pole or the other. But, as it is removed from this point to either end, the inclination is observed; and when situated over the pole, it would, if uninfluenced by other forces, be quite vertical. The same appearances are observed on the surface of the earth, for it acts as though it were a magnet; and there is a line callcd the magnotic equator, on which the magnetic needle has no dip, aud there arc points in both hemispheres where it is vertical. The magnetic equator is not an exact circle of the sphere, for although it docs not recede from the terrestrial equator, north or south, more than 15° or 16°, yet it cuts the equator in three or more places. Captain Duperry crossed the pleted in 1825. The results of his observations are given in the ” Annalcs dc Chimic et de Physique.” The node of the magnetic equator, or that point wncre it crosses tho equator of the earth, is near the Island of St. Thomas, about 3° 20′ to the east of the meridian of Paris. From this point it advances rapidly to the northeast, and crosses the continent of Africa. It then stretches onward, for a short distance, in a line almost parallel to the’equator, but, gradually declining, passes through the south of Hindoetan, touching the northern extremity of the Island of Ceylon, and forms an irregular line passing through Malacca, the north of the Island of Borneo, and to the south of the Carolines. At about 175° east of Paris it again crosses the equator, and makes but a small angle until it reaches about 100° west from Paris, when it takes an eccentric course through South America, having, in some places, a distance from it of 16°. It then passes, m an irregular line, through the Atlantic towards tne Island of St. Thomas.