The intensity of that part of the magnetic force which acts in a horizontal direction is therefore, at St. Petersburgh, In order to deduce from these results the ratio of the entire force at the two places, the dip at Berlin was taken at 68° 39′ 30′′, as determined in the same manner, and by the same instrument with which the preceding result for St. Petersburgh was obtained. With these data, and using the mean of the results deduced from the two needles, the following expression is found for the intensity of the magnetic force at St. Petersburgh; that of Berlin being considered as unity: 0.932. secant (71° 12' 25") secant (68° 39' 30'') = 1.0526 *. 11. VARIATION OF THE NEEDLE. (From a Letter of Professor Hansteen to M. Kupffer.) Orenburg, Jan. 1, 1830. The magnetical observations in the eastern parts of Siberia prove that there is a considerable western variation in places eastward of the line of no variation which passes near Irkutsk. Professor Hansteen has likewise discovered the line of no variation which had already been established by the observations of M. Schubert. The observations of Captain Wrangel had led M. Kupffer to believe that the variations on both sides of this line of no variation had equal signs; but the very accurate observations of M. Hansteen now prove, that they change sign from one side to the other of that line; and that, consequently, this line of no variation which traverses Siberia has the same property as those which pass near Kasan, and through the United States of America †. 12. ON THE FIGURE OF THE MAGNETIC EQUATOR. The following are deductions from observations made during the voyages in the Coquille, by L. J. Duperrey, commander of the expedition. It had been previously known that the magnetic equator, or that line surrounding the globe where the needle arranges itself in a horizontal position, was not a regular great circle of the sphere, but an irregular line cutting the equator in two points. From the observations made by M. Duperrey, combined with those of Captain Sabine and others, its true figure may now be considered as ascertained with a considerable degree of accuracy. M. Duperrey crossed the magnetic equator six times; but it was not so much from observations made on that line, as from those made about thirty degrees * Petersburgh Transactions, 1830, + Idem, 1831. on each side of it, that its true figure has been deduced. If we could determine the inclination of the dipping needle at any place, then we could, by a trigonometrical formula, ascertain the latitude of that point where the dip would be nothing; or, in other words, the corresponding point in the magnetic equator. The formula which we employ for the purpose is the following:-The tangent of the magnetic latitude is equal to half the tangent of the inclination of the dipping needle. From these principles it is found that the node of the magentic equator, or that point where it crosses the equator, is near the island of St. Thomas, about 3° 20′ to the east of the meridian of Paris. It then advances rapidly towards the north-east, across the continent of Africa, and the Red Sea at the Straits of Babelmandel. It then stretches almost parallel to the equator for a short distance, and gradually declines, passing through the south of Hindostan, and touching the northern extremity of the island of Ceylon. It then stretches in an irregular line through Malacca, the northern point of the island of Borneo, stretching onwards to the south of the Carolinas, and again crossing the equator at about 175 degrees east of Paris. It then passes on, making a small angle with the equator, till it reaches about 100 degrees west from Paris, when it begins to deviate rapidly from the equator, and sweeps through South America, at its greatest distance from the equator, being about sixteen degrees. It then rises, in a very irregular line, through the Atlantic Ocean, till it reaches the island of St. Thomas. By tracing this line on a map it will be seen, that the two points at which it crosses the equator are almost diametrically opposite; that in the ocean it declines very little from the equator, but where it approaches islands it feel their influence, and its deviation increases; and when it reaches the massy continents of Africa and America, their influence seems to be powerfully felt, and its deviation from the equator becomes greatest. If the magnetic equator be the resultant of electric currents, circulating perhaps at no very great depth below the surface of the earth, then it is obvious that such a country as South America, abounding with metalliferous veins, ought to have a decided influence on the needle, and here we observe that its deviation is the greatest*. 13. NEW DIPping Needle. In dipping needles formed for ascertaining the dip in different latitudes, the axis must be made cylindrical. But in the one made by M. Gambey, at Paris, to be used at St. Petersburgh, the axis is a knife-edge, as in a fine hydrostatic balance. The edge is placed exactly in the centre of gravity of the whole compound needle, and so fixed, that when the needle dips 71° the edge rests perpendicularly on two agate plates. It can, therefore, only be employed at those places where the dip is but a little more or less than 71°, and is intended to ascertain minute variations of the inclination at the same place t. Annales de Chimie et de Physique. + Idem. 14. POWERFUL ELECTRO-MAGNETS. Professor Henry, of the Albany Academy, and Dr. Ten Eyck, have extended the ingenious experiments of Mr. Sturgeon, by adopting the principle of Professor Schweiger's galvanometer, and produced magnetic effects on soft iron, which we could scarcely have expected from the feeble voltaic current which they employed. The circumstances on which the increase of electro-magnetic power depends, are, first-an increase of the mass of soft iron; and secondly-an increase in the number of coils without increasing the length of the wire. A cylindrical bar of soft iron, ten inches long and half an inch diameter, was bent into the form of a horseshoe, and wound with thirty feet of copper wire, covered with silk thread. With a pair of plates two inches and a half square, dipped into dilute acid, the soft iron became a magnet capable of raising fourteen pounds. A wire of the same length as the first was wound over it, and the ends soldered to the copper and zinc plates: the effect was now doubled, and the temporary magnet actually supported twenty-eight pounds. With plates of four inches by six, it supported more than fifty times its own weight. But the greatest effect which has ever been produced on soft iron by voltaic electricity was, by using a bar of iron two inches square and twenty inches long, having the edges rounded and being bent into the form of a horseshoe magnet. Around the horseshoe 540 feet of copper bell-wire was wound in nine coils of sixty feet each. These coils were not continued from one end of the magnet to the other; but each of them was wound round a space of the horseshoe about one inch long, leaving the ends of the lines projecting and properly numbered. By soldering the alternate ends to a copper cylinder, and the others to a smaller cylinder of zinc, containing only twofifths of a square foot, and placing the one within the other in dilute acid, the following extraordinary effect was produced. When the armature of soft iron was placed across the ends of the horseshoe, and weights added till the temporary magnet could support no more, it was found that the total weight amounted to 650 pounds, an astonishing effect for such a small battery, and requiring only half a pint of dilute acid for its submersion. With a larger battery the weight raised was 750 pounds, which seemed to be the maximum of magnetic power which could be developed in that bar by voltaic electricity. This appears to be the strongest magnet ever constructed, either by the ordinary modes of magnetizing steel bars, or by the voltaic current. Mr. Peal's magnet weighs fifty-three pounds, and lifts 310 pounds, or about six times its own weight; whereas this temporary one weighs only twenty-one pounds, and raises more than thirty-five times its own weight. When the ends of the wires were united so as to form a continuous wire of 540 feet, the weight raised was only 145 pounds. When a battery containing twenty-five double plates, and presenting the same surface with the cylindrical battery, was employed, it was found that a greater electro-magnetic effect was produced with a long copper wire than with a short one, for when the ends of the battery were connected with the coil surrounding a small horseshoe magnet, it raised only seven ounces; but when one-fifth of a mile of copper-wire was interposed, it raised eight ounces. The author gives his opinion of the cause of this remarkable effect in the form of a query. 'On a little consideration, however, the above result does not appear so extraordinary as at first sight, since a current from a trough possesses more projectile force, to use Professor Hare's expression, and approximates somewhat in intensity to the electricity from the common machine. May it not also be a fact, that the galvanic fluid, in order to produce the greatest magnetic effect, should move with a small velocity, and that, in passing through one-fifth of a mile, its velocity is so retarded as to produce a greater magnetic action?' Dr. Ten Eyck varied these experiments, so as to get a small temporary magnet which should raise the greatest number of times its own weight. With a small horseshoe of round iron slightly flattened, one inch in length, and inch in diameter, and wound with three feet of brass wire, it raised, by means of the cylindrical battery, 420 times its own weight. The author remarks, the strongest magnet we can find described, is one worn by Sir Isaac Newton in a ring, weighing three grains; it is said to have raised 746 grains, or 250 times its own weight.' Hence it is evident that a much greater degree of magnetism can be developed in soft iron by a galvanic current, than in steel, by the ordinary method of touching*. 15. ON THE INTENSITY OF THE EARTH'S MAGNETISM.-(Kupffer.) i. During a scientific journey in the neighbourhood of Mount Elbrouz, undertaken by the Academy of Sciences at St. Petersburgh, M. Kupffer and M. Lenz made a series of important magnetical observations, of which we shall give a short abstract. Observations had frequently been made at different elevations above the surface of the sea, and nearly at the same place, on the intensity of the earth's magnetism; but, contrary to what might be expected, no diminution of intensity had been observed in rising above the level of the sea. MM. Gay-Lussac and Biot, during their aërial ascent, had made the same needle oscillate at the surface of the earth, and at the height of 6000 mètres, without observing any diminution in the time of an oscillation. Hence it was concluded that no sensible diminution of the intensity of the earth's magnetism had taken place at that elevation. In this conclusion, an element, which has considerable effect on the magnetism of the needle itself, namely, temperature, was entirely overlooked. The effect of temperature on the magnetism of needles has been carefully observed by M. Kupffer, and entered as an element in the data from which the intensity of the earth's magnetism was deduced. *Silliman's Journal. See our account of Professor Moll's experiments, at page 379 of this volume. When a magnetic needle is heated, the coercitive force of the steel is diminished, and the strength of the magnet considerably impaired. Besides, the magnetism of the earth acts by induction on a needle, and, when the temper is not very hard, is a quantity which increases or diminishes the magnetism of the needle, according to the position in which it is placed. It is also necessary to pay attention to the hour of the day at which the observations are made. At eleven o'clock A. M., the earth's intensity is greatest; at five o'clock P. M., its intensity is least. Another element, which M. Kupffer has made to enter into the calculation, is the difference of intensity depending on the dif ference of latitude and longitude of the places of observation. The observations were made in the valley of Malka and on Kharbis, at an elevation of 4500 feet. After making the proper corrections due to the causes which we have mentioned, it was found that the duration in one oscillation of an excellent needle, made by Gambey, was .063 of a second, which, for every 1000 feet of perpendicular ascent (supposing the diminution uniform), is .014 of a second. In comparing these observations with those of MM. GayLussac and Biot, M. Kupffer has arrived at the following remarkable law :- That the increase of intensity which a needle, of the usual degree of hardness, acquires by a diminution of temperature, according to the height, is almost entirely compensated by the diminution of the intensity in the magnetic forces of the earth, due to that elevation.' Since the magnetic intensity of the earth is thus found to diminish at a small height above the surface, it appears obvious the cause must reside nearer the surface than was formerly supposed, and affords an additional proof that the declination and dip of the needle depend on electric currents circulating about the earth at no very considerable depth. ii. It had been long known that, in our hemisphere, the north pole of a magnetic needle marches towards the west from eight o'clock in the morning till two o'clock in the afternoon, and returns towards the east during the rest of the day; but its oscillations during the night are so irregular, that it has been impossible to ascertain whether or not there be a corresponding period during the night. These irregularities have been carefully attended to by M. Arago, and he has generally found, that during those periods when the oscillations of the needle were very irregular, the aurora borealis has been visible in the north. Hence it has, I think, been too hastily concluded, that the aurora borealis has a decided effect on the needle. It appears more probable that the aurora borealis, and the disturbing force on the needle, are both effects of the same cause the unknown cause of all terrestrial magnetic phenomena. M. Kupffer has observed at Kasan, that the irregular march of the needle took place on the same day, and almost at the same instant, with those observed at Paris. Could a trifling electrical atmospheric phenomenon in the north of Scotland produce the same effect on a needle in Paris, almost directly south of it, and on |