direction of the magnetic needle there." Arago and Others, Phil. Mag. vol. ii. p. 334.

Exp. This is in accordance with the preceding phenomenon, and since the Aurora is active in elevated parts of the atmosphere, its influence must extend to great distances.

PH. 176. Various natural causes act upon the magnetic needle, so as to cause a sudden change in its position, or at least to disturb the regularity of its diurnal variations, and of all these causes the Aurora Borealis appears to be the most energetic and infallible. Edin. Phil. Journ. for Sept. 1828.

Exp. The two last, and ph. 170, will account for the effects here stated.

PH. 177. The declination of the needle is considerably affected by heat.

Exp. In consequence of an increased temperature, the ethereal current will enter the earth in the polar regions, especially at the general magnetic poles, in a space more extended in length and breadth, and will be more diffused, and hence the needle must be subject from this cause to an alteration in its position.

PH. 178. Many iron mines are found to be magnetic, and they generally contain natural magnets.

Exp. Since iron long remaining in the same position acquires permanent magnetism, it follows that the iron in mines is susceptible of acquiring the same properties.

PH. 179. The magnetic needle in mountainous districts is subject to extraordinary motions, and great aberrations on being moved to small distances, and to different parts of the same mountain.

Exp. Many mountains contain much iron, and this is in general magnetic, and hence it affects the needle by

its attractions and repulsions, which have been already explained.

PH. 180. There are found, by observations, certain lines on the earth where the needle has no declination, that is, in which its direction is north and south. These lines

are in many parts very oblique to the geographical meridians, they contain many inflections, and constitute very irregular curves, which are continually undergoing variations in place and form. These curves are rather to be considered as irregular zones than as single lines.

Exp. The different general magnetic poles in the frigid zones and their continual but unequal oscillations, and, upon the whole, their progress westward, will not only shew that there ought to be some such lines where the compass has no declination, but also that these lines should be very changeable in form and position.

PH. 181. In Dr. Halley's chart, as laid down for 1700 by Musschenbroek in his Dissertation on the Magnet, one of the lines of no declination coming from the southern regions of the Atlantic ocean crosses the meridian of London in 58° S., and extending northward, a little inclining to the west, passes somewhat east of Ascension island, and crosses the equator at about 15° W. long. and rising northward it more rapidly inclines towards the west, till passing through the Bermudas, it proceeds nearly west to Carolina in the United States. Observations made in several succeeding years shewed that the northern part of this line moved westward, and the southern part eastward.

Exp. The westward motion of the nearest general magnetic pole of the earth in the north frigid zone will account for the westward motion of the northern part of this line. Also if we allow that there are two principal magnetic poles in the southern frigid zone, which, from

what we have advanced on this subject, is rendered almost certain, then if one of these poles was at that time in a little more than 90° west longitude, and the other in less than 90° east longitude, the motion eastward of the southern branch would be the natural consequence. But if there be one general pole in the south frigid zone, its position evidently might at that period be such as to produce the observed effect.

PH. 182. In the same chart we find another line marked in which the needle has no declination; it passes through New Holland, the islands of Timor, Celebes, Mindora, and the eastern part of China, cutting the equator in long. 119° E.

Exp. If we admit of a general magnetic pole within the antarctic circle, somewhere between 50° and 70° of east longitude, as stated in the last to be probable, this line ought to be found in or near the regions where it is delineated. However the nature of the surface of the earth, as consisting of land, water, and mountainous districts, &c. ought to have a considerable effect, as well as the general magnetic poles, in regulating the positions, and motions of the lines of no declination, which is the case in fact.

PH. 183. At present the line of no declination is returned nearly to the situation it had in 1700, at its southern part in about 60° south latitude, where it cuts the meridian of London, but from thence it now proceeds, northwest to the coast of Paraguay, where it takes its direction nearly in the meridian along the coasts of Brazil, till reaching the latitude of Cayenne, it turns suddenly to the north-west, where it is directed to the United States, and other parts of North America. The general motion of this line is therefore westward, but the motions of its different parts are unequal, and sometimes in op

posite directions, and doubtless its motions are subject to very great variations and irregularities.

Exp. All this is exactly conformable to, and in accordance with, what we have already advanced on this subject.

Obs. There are found at present three, or four lines of no declination, the phenomena of which answer to the explanations given of the corresponding appearances, and it will be unnecessary to swell this work with an account of these, since the way has been opened sufficiently for pursuing such researches.

PH. 184. The lines, in which there is any particular declination of the needle east or west, are also irregular curves, their general motions, and those of their different parts are also very irregular, and are farther modified by the positions and variations of the lines of no declination.

Exp. These facts necessarily follow from the reasons assigned, in the four last phenomena, respecting the line where there is no declination.

Inclination of the Needle.

PH. 185. The magnetic needle is also subject to another kind of deviation. Let it be placed in equilibrium on its pivot, before it is rendered magnetic, then, after it is magnetised, it will be found, that, at most places of the earth, its equilibrium will be destroyed, the north pole being depressed on the north side of the magnetic equator, and the south pole on the south side. This is called the dip, or inclination of the needle.

Exp. Suppose AB, fig. 81, a north magnet, of which

A is the south, and B the north pole, and which had been previously balanced on an axis passing through its center of gravity; when magnetised, and placed in the plane of the magnetic meridian, let a person, on the east side, look westward on this needle with the end B northward, and it will be seen, and understood from ph. 41, and others, that the ethereal matter, revolving round the magnet, proceeds downward on the side facing him, and towards the end B; also the ethereal current, which is going round the earth, is moving downward and towards the west, hence the combined effect will be an action on the east side of the needle on those faces of the spiral channels in the magnet, which are towards B, and this will cause a depression of the end B, more or less as the terrestrial current is more or less directed downward. Next, let a south magnet, AB, fig. 82, be similarly circumstanced with the end A northward, this end being in this needle its north pole. Here it will be seen, that the ethereal current still descends on the side facing the spectator, but now it proceeds southward, viz. from A towards B, and the terrestrial current as before flows northward, still descending, hence it now falls on the south sides of the spiral channels in AB, that is, on the side nearest B; but these spirals likewise proceed the opposite way to those in fig. 81, hence evidently here, as well as in the north needle, the combined effect of the two currents is to deflect the north end downward. The explanation would be similar when the needle is in southern latitudes.

PH. 186. In the tropical regions a course of points is found, in which the needle has no inclination, that is, in which it always rests horizontally; if these points be connected, so as to form a curve round the globe, this line is called the magnetic equator, or rather, the magnetic equator is an irregular belt round the earth, extending