a voltaic circuit on the magnetic needle, and Seebeck's discovery of the influence of heat in inducing electric currents, that any well-founded theory of magnetic action and phenomena resulting from it has been formed. But we shall hereafter have occasion to refer to the origin of terrestrial magnetism; the diurnal variation in the direction is the present object of attention. If the unequal distribution of heat through metallic, and, in all probability, other bodies, can cause the development of electric currents, then the earth, containing metallic, as well as other substances, and subject to a constant variation of temperature, in consequence of the diurnal and annual revolution, must be in an unequal electric condition, and conduct through its mineral crust electric currents of varying intensity and character. The superficial crust of the earth can, in fact, only be considered as a vast thermo-electrical apparatus, and to its influence we may trace the diurnal variation of the needle. Mr. Christie, speaking of a course of experiments he made on this subject, says, "from these I drew the conclusion, that one part of the earth, with the atmosphere, being more heated than the other, two magnetic poles, or rather electric currents, producing effects referrible to such poles, would be formed on each side of the equator, poles of different names being opposed to each other on the contrary side of the equator; and that different points in the earth's equator becoming successively those of greatest heat, these poles would be carried round the axis of the earth, and would necessarily cause a deviation in the hori zontal needle. On comparing experimentally the effects that would result from the revolution of such poles with the diurnal deviation at London, as observed by Canton and Beaufoy, also with those observed by Lieut. Hood, at Fort Enterprise, and finally by the late Captain Foster, at Port Bowen, I found a close agreement in all cases, in the general character of the phenomena, and that the times of the maxima east and west did not differ greatly in the several cases. The double oscillation of the needle clearly resulted from this view of the subject. Some of the experiments to which I have referred showed, that when heat was applied to a globe, the electric currents excited were such, that, on the contrary sides of the equator, the deviations of the end of the needle of the same name as the latitude, were at the same time always in the same direction, either both towards east or

both towards west." The diurnal deviation is greatly influenced by local causes, such as the state of the weather, and especially the temperature, a circumstance which strongly tends to confirm the supposition that it is under the control of those electric currents, excited by the action of unequal currents of heat.

De Saussure made, some time since, a series of experiments on the Col du Géant at an elevation of about 11,300 feet above the level of the sea, with the object of determining the influence of elevation upon the diurnal change of variation. By comparing the result of these observations with those which he obtained at Chamouni and Geneva, he discovered that the change is but little influenced by the altitude of the place, the course of the diurnal variation being the same in the three places, the times of the least and greatest variations being later on the Col du Géant than at Chamouni or Geneva.

DIP.

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 is supposed to have been made in the year 1576. The same philosopher was aware of the fact that the dip changes with the situation of the 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 poles. If the poles of the earth's rotation were the magnetic poles, then this would be strictly true, and on the equator the 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 be very well illustrated by suspending a small magnetic needle over a large bar-magnet; when the middle

is situated directly over the centre of the magnet it will be horizontal, having no tendency to incline either 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 called the magnetic equator, on which the magnetic needle has no dip, and there are points in both hemispheres where it is vertical.

The magnetic equator is not an exact circle of the sphere, for although it does 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 magnetic equator several times during his voyage round the world, which was commenced in the year 1822 and completed in 1825. The results of his observations are given in the "Annales de Chimie et de Physique." The node of the magnetic equator, or that point where it crosses the 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 Hindostan, 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, in an irregular line, through the Atlantic towards the Island of St. Thomas. There is, however, a singular circumstance which has been observed concerning its passage through the Pacific Ocean. In longitude 1180 14' west, the magnetic equator crosses the equator of the earth, and in longitude 156° 30′ it has been traced at some distance to the south of the equator; but in the sea of China, in 113° east longitude, it is north of the equator, and, consequently, must have intersected it in some point between these two places. There are, then, at least three points

T

where the magnetic and terrestrial equators cross each other, and there are strong probabilities that there may be

even more.

But the dip is subject to change, as well as the variation. When Norman made his experiment, in 1576, the inclination was 71° 50'. In 1828 Captain Sabine found it to be 69° 47', and Captain Segelcke measured it at Woolwich in November, 1830, as 69° 38'. The magnetic equator is not, therefore, a fixed line, but varies with the position of the magnetic poles. According to the experiments of Captain Sabine, the dip has been decreasing about three minutes annually for the last fifty years.

There are many subjects of great interest connected with the magnetism of the earth, to which we cannot even allude in this outline sketch, but must close the chapter with an account of some of those opinions which have been entertained as to the origin of terrestrial magnetism.

ORIGIN OF TERRESTRIAL MAGNETISM.

The name of Gilbert will ever be associated with the science of magnetism, for we are equally indebted to him for the variety and accuracy of his experiments, and the soundness of his deductions, so far at least as the state of the science in his day admitted. This eminent philosopher, to whose memory posterity has not awarded justice, considered the earth to act upon a needle, as though it were itself a magnet, and that the direction of the magnet was due to the earth's magnetic power. That end of the needle which points to the north pole of the earth he called the south pole of the magnet, and that which points to the south he called the north pole. Although the names of the poles have been changed, they were accurately designated by Gilbert, and the names by which they are now known contradict the acknowledged principle that poles of the same name repel each other.

Halley accounted for the variation and dip by supposing the existence of four terrestrial magnetic poles, two in each hemisphere. One of the northern he conjectured to be about 70 from the pole of the earth, in the meridian of the Land's End, and the other about 15° from the pole, in the meridian of California. One of the south poles he supposed to be situated about 16° from the terrestrial pole of the southern

hemisphere, in a meridian about 20° to the westward of Magellan's Straits. The other, which Halley considered the most powerful of the four, he conceived to be about 20° from the pole of the earth, and in about 120° east longitude. One pole in each hemisphere is supposed, according to the terms of this theory, to have a revolution round the magnetic axis in a period of about seven hundred years.

Professor Hansteen has adopted Halley's hypothesis, so far at least as to acknowledge the existence of four magnetic poles, though he supposes them all to be in motion, the northern poles having a revolution from west to east, the southern from east to west. This theory accounts for many phenomena, and has received much attention from philosophers in this as well as in other countries.

There has been much speculation as to the cause of this rotary motion of the magnetic axis. Some writers have attributed it to the progressive oxydation of the metals, some to cold, and others to electricity. But the attention of all observers is now turned towards the application of the wonderful discoveries which have been made in the circumstances and effects of electro-magnetic action, hoping that the time may not be very far distant when the cause of magnetism shall be perfectly understood, and its phenomena accurately

traced.

CHAPTER VIII.

INTERIOR OF THE EARTH.

WHEN the semi-diameter of the earth is compared with the depths to which geological researches have extended, it is almost literally true, that we are ignorant of the constitution and condition of its interior. But it is singular that, with so few facilities for investigation, so much should have been determined, which in all probability is nearly allied to truth, if it be not truth itself. A few strong-minded individuals commenced the investigation, and although practical inquiries were for some time retarded by the visionary theories