which the resultant of all the former acts, at the point where the observation is made. TERRESTRIAL MAGNETISM.-Every where, on the surface and in the interior of the globe, the magnetic needle takes a determinate position; this position varies in each country of the earth: as we advance, proceeding directly to the west, we see that it increases, and attains its maximum in the Atlantic Ocean. From this point, the western declination diminishes; and, at the east of the United States, the needle points exactly to the north pole, and consequently the declination is nothing; more westward, the declination becomes east. If we had gone toward the east, the west declination would have diminished; it would be nothing in the east of the Russian empire; and then east, if we had continued our course toward the east. In general, if, under any parallel, we take the circuit of the globe, we shall find a point where the needle is directed toward the north; afterwards the deviation becomes westerly, attains its maximum, and then diminishes, until it is nothing: thus the declination varies greatly. If we make our experiments under the equator, and repeat them every five degrees, we shall find that the difference between the maximum eastern and the maximum western declination increases as we approach the poles of the earth. Thus, in Greenland, the west declination is so great that the needle points to the west; and Parry found a point, in the west of Greenland, where the north pole of the needle was turned to the south. The dip presents similar differences; in our countries it is northern, that is to say, the north pole is directed downward, and forms an angle of 70° with the plane of the horizon. In proportion as we advance toward the south, the needle approaches the horizontal direction, and, in the neighbourhood of the equator, it is altogether parallel to the horizon the dip is, therefore, nothing. On passing into the southern hemisphere, we see the south pole of the needle dip, and the more so as we approach the south pole; going toward the north, the contrary would have been observed: thus, in one of the hemispheres, the dip is northern, and, in the other, southern. These two hemispheres are separated by a line of no dip, upon all points of which the needle is horizontal: this line, which cuts the equator in different points, and rises alternately into each hemisphere, is called the magnetic equator. MAGNETIC POLES OF THE EARTH.-In a magnet, each molecule attracts iron; but we may suppose that the effects of all these isolated elements concur in a single point, in like manner as, when considering the weight of a body, we always, in imagination, transfer it to the centre of gravity of the heavy body. These two points, in which are concentrated the resultant of all the forces distributed in the half of each magnetic bar, are called its poles. In like manner, the resultants of all the magnetic forces of the earth may be supposed applied to several points, that are called the magnetic poles of the earth. In our countries, as the magnetic needle is sensibly directed from south to north, it follows that the magnetic action of the earth is such, that the magnetic poles may be considere as situated in the neighbourhood of the poles of the earth: thus, formerly the action of the magnetic forces was placed at the poles themselves of the earth. More accurate observations have shewn that this is not the case. All the phenomena occur exactly as if there were two magnetic poles in each hemisphere. Thus, in the north of America, there exists a point, toward which the needle constantly turns. In proportion as we approach this point, the dipping needle increases in its tendency to become vertical: this point, which is situated to the west of Baffin's Bay, is, therefore, a magnetic pole. There exists another in the north of Siberia. Two similar points occur in the south of America and in New Holland. Although their position is not accurately determined, yet we cannot fail to remark their coincidence with the poles of cold.* Philosophers of the present day apply the term terrestrial magnetic poles to the points on the surface of our globe, where the magnetic needle, when suspended by its centre of gravity, remains vertical. In such points, the dip is equal to 90°, and the horizontal intensity is necessarily nothing. There exist only two such places on the terrestrial globe. The theory, recently published by M. GAUSS on terrestrial magnetism, assigns them, in the year 1825, the following positions: North pole, latitude 73° 30′, longitude 97° 30′ W. (of Paris). M. DUPERREY, from the results obtained in his late scientific expeditions to the two poles, has, on his part, found : North pole, latitude 70° 5′, longitude 99° 12′ W. The term magnetic poles has sometimes been applied to the points, where the total intensity attains its maximum value, superior to the total intensity of all the neighbouring points. It was long thought that these points of maximum intensity coincided with the magnetic poles; but we now know that they may be very distant apart. There exist two such points in the northern hemisphere, one situated in Siberia, the other in North America. According to M. GAUSS's theory, this last point would be situated in 54° INTENSITY OF TERRESTRIAL MAGNETISM. Confining ourselves in the study of terrestrial magnetism, in considering the direction of the needle, is only looking upon one part of the question, since we ought also to appreciate the intensity of the force by which it is directed. As, in any force, this intensity diminishes as we recede from its origin, it is therefore probable that it will attain its minimum at the magnetic equator, and increase as we approach the pole. Exact measurements have established this truth in the most positive manner. The intensity of the magnetic force is measured in the same manner as that of gravity. If we deviate a magnetic needle from its position of equilibrium in the magnetic meridian, it will return, making, like the pendulum, a series of oscillations, which are the more rapid as the terrestrial magnetism is more intense at the place of observation. To be convinced of this, we have merely to count the number of oscillations that a magnetic needle makes in a given time. Suppose that this number is ten now place on the plane of the magnetic meridian at the north of the needle the south pole of another magnet; the latter acts in the same direction as terrestrial magnetism; it tends to bring the needle back to its natural position, and it will execute its ten oscillations in a much shorter time than before. Thus, then, in order to know the intensity of the magnetic force on different points of the globe, we have only to make the same needle oscillate at these different points, and see in how long a time it makes a determinate number of oscillations. REGULAR VARIATIONS OF TERRESTRIAL MAG NETISM. The elements of terrestrial magnetism that we have to study are not so constant as might be imagined from what we have just said. Let us choose a magnetic needle of sufficient length to enable us to read minutes on the arc it describes, and suspend it to a filament of silk: this needle will never be at rest. Its movements do not depend on accidental shakings of the ground; this is proved by the certain regularity with which it moves. If we note its position hourly for a month, and take the mean of the hourly observations, we shall find that in Germany and France it is in its most easterly position at 8 A.M.; it then goes toward the west, and between 1 and 2 P.M. it has devi latitude N., and 101° 20' longitude W.; and the Siberian point is 71° 30′ latitude N., and 117° 40' longitude W. With regard to the maxima of intensity of the southern hemisphere, the observations are still too few in the Antarctic seas to enable us to say whether there are one or two, and to determine their situation, even approximately.-B. ated several degrees (minutes ?) toward the west; it then returns to the east, and about midnight it affects nearly the same direction as in the morning; it then remains stationary for several hours, and moves slowly toward the east. The amplitude of these regular oscillations is greater in summer than in winter, and during a fine than during a cloudy day. In our countries they embrace about 15'; they increase as we approach the pole, and diminish in amplitude toward the equator. But it has been proved that the north pole, in all places, is directed more toward the east in the morning than in the afternoon. The dip and the intensity shew analogous variations; but hitherto they have not been so much studied; they are always connected with the unequal temperatures of the countries situated east and west of the place of observation. Before noon, the countries situated on the east being hotter than those on the west, the north pole of the needle recedes; in the afternoon it is the reverse: the north pole recedes from the hotter countries situated on the west, and it again turns toward the east. The greater the difference of temperature is during the day, the more marked are these oscillations and thus their amplitude is greater in summer than in winter. The dip appears to vary with equal regularity throughout the year; however, this element has not yet been determined by a sufficient number of observations. IRREGULAR VARIATIONS OF TERRESTRIAL MAGNETISM.-If we choose very mobile needles, and are able to determine the position to a few seconds nearly, as can easily be done with M. Gauss's apparatus, we then see that, in our countries, the needle does not execute its movements in a regular manner; it marches, for a certain time, in a very uniform manner, it then stops and returns back, following its accustomed course. Even with less perfect apparatus, we may convince ourselves that the needle moves for several days toward the east or toward the west, and then returns to its mean direction in the plane of the magnetic meridian. These disturbances, on one of which we are about specially to fix our attention, exist in the dip and in the intensity, and are probably due to an anormal distribution of temperature at the surface of the globe. This supposition is the more probable, as the dip depends, like the height of the barometer, on the direction of the wind and on temperature. But these disturbances are manifested over a more extensive surface than those of the barometer; for powerful deviations have been observed, at one and the same time, in western Europe and in the interior of Asia. These are the principal facts presented by terrestrial magnetism. In my Treatise on Meteorology, I have pointed out the laws and the methods of observation; here I have simply offered the facts that are essential towards enabling us to comprehend the principal phenomena of aurora boreales, that appertain to Meteorology, because they probably occur at the limits of our atmosphere. AURORÆ BOREALES (nordlichter, nordscheine, polarlichter).-Under these names are comprised the luminous phenomena that are seen in the north by the inhabitants of Europe; however, travellers have seen aurora in the neighbourhood of the south pole: they are called auroræ australes (südlichter). DARK SEGMENT.-According to the unanimous testimony of observers in the north of Europe, who have seen many aurora boreales, their course is as follows,* if we may believe M. Argelander: a dirty appearance of the sky, in the neighbourhood of the horizon, in the direction of the north, precedes the aurora borealis; the colours soon become deeper, and a circular segment, of a greater or less size, is seen surrounded by a luminous arc: this segment has the appearance of a thick cloud. Bergmann and M. Hansteen say that at Upsal and Christiania this segment is sometimes black, or of a deep grey passing to violet. The more we advance towards the north the blacker is this segment; and, in high latitudes, it can hardly be distinguished. The segment is also perceived in lower latitudes; all the observers in Germany noticed it in the aurora borealis of January 7, 1831. With the existence of this segment is connected Gissler's observation, who said that, in Sweden, on the high mountains, the traveller is sometimes suddenly enveloped in a very transparent fog of a whitish grey colour, passing slightly to green, which rises from the ground, and is transformed into the aurora borealis. Ancient observers have spoken of this analogy between the aurora borealis and light clouds ; some travellers in the polar regions have again mentioned these appearances. Wrangel says positively that, as soon Vide also an excellent description of the aurora borealis by M. LOTTIN, in M. POUILLET's Eléments de Physique, t. ii. p. 663,23 and the succinct resumé of all the observations made on this phenomenon during the wintering at Bosekop, in a letter to M. ARAGO. (Comptes rendus de l'Acad. des Sciences. t. x. p. 289.) 23 Vide Note w, Appendix II; and plates vii. and viii. |