boldt that he has observed the occurrence in the interior of continents, and Captain Beechey states that he has witnessed it in the open sea. With regard to the circumstances on which such a singular precipitation of water depends we are entirely ignorant. In Europe we sometimes see during the day, in cold and perfectly clear weather, small crystals of ice falling slowly from the air, their size increasing with every particle of humidity they congeal in their passage. Does not this approximation put us in the way of obtaining the desired explanation? Have not the large rain-drops been at first, in the higher regions of the atmosphere, small particles of ice excessively cold; then have they not become, as they descended, large ice-flakes by means of accumulation; and when lower still, have they not melted into drops of water? It will be readily understood that the only object with which these conjectures are brought forward in this place is, to show in what point of view the phenomenon may be studied, and to stimulate our young travellers, in particular, to observe carefully if, during these singular rains, the region of the sky from which they fall presents any traces of halo. If such traces are perceived, however slight they may be, the existence of crystals of ice in the higher regions of the air would be demonstrated. In the present day there is scarcely any country where meteorologists are not to be found, but it must be confessed that their observations are usually made at hours selected without proper discernment, and with instruments either inaccurate in themselves, or improperly placed. It does not now appear difficult to deduce the mean temperature of the day from observations made at any hour; thus a meteorological table, whatever may be the hours of observation in it, may be possessed of value, by the mere condition that the instruments employed will admit of comparison with a standard barometer and thermometer. We think it proper to recommend these comparisons to the officers of the Bonite. Wherever they can be effected, local meteorological observations will be of value. A collection from the newspapers of countries will often supply what would otherwise be obtained with difficulty. MAGNETISM. DIURNAL VARIATIONS OF THE DECLINATION (VARIATION)*.—Of late years science has been enriched with a considerable number of observations on the diurnal variations of the magnetic needle; but the greater part of these observations have been made either in islands, or on the western sides of continents. Corresponding observations made on the eastern sides would at present be very useful. They would serve, in fact, to submit to an almost decisive test the greater part of the explanations of this mysterious phenomenon which have been promulgated. The route prescribed for the expedition does not allow us to suppose that the Bonite can harbour or even remain some time at points situated between the terrestrial and the magnetic equators, such as Pernambuco, Payta, Cape Comorin, and the Pelew Islands. Had it been otherwise, we should have particularly recommended the erection of M. *The declination of the magnetic needle is popularly, but improperly, called the variation, in Great Britain. Gambey's beautiful instrument, in a firm position, at a distance from every ferruginous mass, and that the oscillations of the needle should have been attended to with the most scrupulous care*. INCLINATION (or Dip).-In general it will be attended with little advantage to bestow much care on observing the diurnal variations of the horizontal magnetic needle in places where the expedition is not stationary for a whole week. It is different, however, with the other magnetic elements. Wherever the Bonite stops, though it be only for a few hours, it would be desirable to measure, if possible, the declination, the inclination, and the intensity. In the attempts to reconcile observations on the inclination made at remote periods, in different regions of the earth, in the neighbourhood of the magnetic equator, it was ascertained, some years ago, that this equator is advancing progressively and entirely from the east to the west. At present it is supposed that this movement is accompanied with a change of form. The study of lines of equal inclination, regarded under the same point of view, will not be less interesting; when all these lines shall have been traced upon the charts, it will be curious to follow them with the eye, in all their displacements and changes of curvature; important truths may emanate from such an examination. It will now be understood why we require as many measurements of inclination as can be collected. * At all events we shall here present the problem, which observations made at the points mentioned would serve to solve. In the northern hemisphere, the end of an horizontal magnetic needle, which points towards the north, moves from the east to the west from 8 o'clock A. M. to 14 in the afternoon, and from west to east from 14 P.M. to the following morning. Our hemisphere cannot be peculiar in this respect; the same effect produced on the north end here, must be produced on the south end to the south of the equator. Thus, in the southern hemisphere, the end of an horizontal magnetic needle which points towards the south, will move from east to west from 8 A.M. till 14 in the afternoon, and from west to east from 1 P.M. till the morning of the next day. Further, observation corroborates the supposition. Now let us compare the simultaneous movements of the two needles, when comparing the same end, namely, that which points towards the NORTH. In the southern hemisphere, the end pointing towards the SOUTH moves from east to west from 8 A.M. to 14 P.M.,-therefore the north end of the same needle makes a contrary movement. Thus, finally, in the souhern hemisphere, the end pointing towards the NORTH moves from west to east from 8 A.M. till 14 P.M., which is precisely opposite to the movement made by the north end, at the same hours, in our hemisphere. Let us suppose that an observer, starting from Paris, advances towards the equator. So long as he continues in our hemisphere, the north end of his needle will make a movement every morning towards the west; in the other hemisphere, the north end of the same needle will move every morning towards the east. It is impossible that this change from a western movement to an eastern one can There take place in a sudden_manner. Such a line must exist; but where is it to be found? Is it a curve of equal intensity, or the magnetic, or the terrestrial equator? Researches, continued during many months, in the places situated between the terrestrial and magnetic equators, such as Pernambuco, Payta, Conception, the Pelew Islands, &c., would certainly lead to the desired solution. But many months of assiduous observation would be requisite; for, notwithstanding the skill of the observer, the short stay of Captain Duperrey at Conception and Payta, made at the request of the Academy, have left some doubts on the subject. The question has been often agitated, whether, in a determinate place, the dipping-needle would mark exactly the same degree at the surface of the ground, at a great height in the air, and at a great depth in a mine. The absence of uniformity in the chemical composition of the earth, renders the solution of this problem very difficult. Observations of the measurements made in a balloon are not sufficiently exact. When the physicien takes his station on a mountain, he is exposed to local attraction; ferruginous masses may there greatly alter the position of the needle, and nothing be present to point out the effect. The same uncertainty attends observations made in the galleries of mines. Not that it is absolutely impossible to determine the influence of accidental circumstances in each place; but then for this purpose it is necessary to have instruments of the most perfect kind;—to be able to from the station chosen, in all directions, and to great distances; and, finally,—to repeat the experiments a greater number of times than a traveller has generally an opportunity of doing. But, however this may be, observations of this kind are worthy of attention. Their mass will, perhaps, one day lead to some general result. go With regard to the declination, its immense utility is so well known to navigators, that any recommendation on the subject would be superfluous. OBSERVATIONS ON INTENSITY.-Observations on the intensity are not of earlier date than the travels of Entrecasteaux and M. von Humboldt, and yet they have already thrown a strong light on the complicated, and at the same time highly interesting, subject of terrestrial magnetism. Observations of this nature ought, in the highest degree, to attract the attention of the officers of the Bonite, for at present the theorist is arrested at every step by the want of exact measurements. The aërial excursions of MM. Biot and Gay Lussac, undertaken some time since under the auspices of the Academy, were in a great measure designed for the examination of the following important question: Has the magnetic force, which, on the surface of the earth, directs the magnetic needle towards the north, exactly the same intensity at every height to which it may be elevated? The observations of our two fellow-members, those of M. von Humboldt in mountainous countries, and the still older observations of Saussure, all seem to concur in showing, that at the greatest heights which man has yet reached there is no appreciable decrease in magnetic force. This conclusion has recently been disputed. It has been remarked, that, in the ascent of M. Gay Lussac, for example, the thermometer which indicated 87.8° Fahr. on the ground at the time of departure, sunk as low as 15.8° Fahr. in the region of the atmosphere where the needle was made to oscillate a second time. And as it is now satisfactorily proved that the same needle, occupying the same place, and under the influence of the same force, will oscillate so much the more quickly, as the temperature is diminished, it becomes necessary, on account of the state of the thermometer, that a certain reduction be made in the intensity indicated by the higher observations, in order that those made in the balloon and on the earth may be comparable. Without this correction, the needle would appear equally attracted above and below; therefore, in spite of appearances, there was a real decrease. This diminution of the magnetic force with the elevation seems likewise to result from the observations made in 1829, on the summit of Mount Elbrouz (in the Caucasus), by M. Kupffer. In this case an exact account was taken of the effects of temperature, and yet various irregularities in the series of inclinations threw some doubt on the result. We conceive, therefore, that comparisons of the magnetic intensity, at bases and at summits of mountains, ought to be particularly recommended to the officers of the Bonite. Mowna-Roa, in the Sandwich Islands, seems to be well adapted for the purpose. Observations may likewise be made on the Tacora, if the expedition stop for a few days at Arica. LUMINOUS METEORS, ON LIGHTNING.-M. Fusinieri has been lately studying the effects of lightning under an entirely new point of view. According to this physicien, the electrical sparks issuing from ordinary machines, which we see as they traverse the air, contain brass in a state of fusion, and incandescent molecules of zinc, when they emanate from a brass conductor; if the sparks issue from a ball of silver they contain impalpable particles of that metal. In the same way, a globe of gold gives rise to sparks, which contain, during their passage through the air, melted gold, &c. &c. The molecules in the centre only of all these sparks are melted; on their exterior surfaces the metallic particles undergo a greater or less degree of combustion, in consequence of their contact with the oxygen of the atmosphere. When a spark issuing from a globe of gold passes through a silver plate, even of considerable thickness, there is seen on the two surfaces of the plate, at the points where the electric spark entered and emerged, a circular stratum of gold, the thickness of which must be very inconsiderable, since, if left to itself, it volatilizes, and disappears entirely in a short time. According to M. Fusinieri, these two metallic spots are formed by the fused gold which the electric spark contained. The deposit on the first face is nothing extraordinary; but, by adopting the explanation of the Italian physicien for the spot on the opposite surface, we are obliged to admit, that the gold disseminated through the spark has passed, at least in part, along with it through the whole thickness of the silver plate! It is unnecessary to add, that a spark issuing from a ball of copper gives rise to similar phenomena. A spark emanating from one metal, and passing through another, does not only lose a portion of the molecules with which it was at first charged, but it acquires new molecules at the expense of the metal traversed. M. Fusinieri even asserts, that, at each passage of the spark, reciprocal changes are produced between the two metals present; that when the spark, for example, leaves silver to pass to copper, it not only transports a portion of the first metal to the second, but that it likewise transports copper to the silver! I will insist no longer, however, on these phenomena; I have cited them here only with a view to show that the sparks of our ordinary machines contain ponderable matter. M. Fusinieri affirms that similar matter exists in lightning, and that in this case also it is in states of minute division, of ignition, and of combustion. According to him, this transported matter is the true cause of the transient smells which thunder occasions when it falls, and also of the pulverulent deposits which remain round fractures through which the electrical matter has forced a passage. In these deposits, which have been too much neglected hitherto by observers, M. Fusinieri has detected metallic iron, iron in different degrees of oxidation, and sulphur. The ferruginous spots left on the walls of houses may be found, when strictly examined, to arise from the iron with which the lightning was charged, derived from that which occurs in every building; but what is to be said regarding the sulphureous spots on these same walls, and especially the ferruginous marks which are found on trees struck with lightning in the open fields? M. Fusinieri conceives himself authorized to infer from these experiments, that the atmosphere contains, at every height, or at least as far as the region of stormy clouds, iron, sulphur, and other substances, on the nature of which chemical analysis has been hitherto silent; that the electrical spark is charged with them, and that it carries them to the surface of the earth, where they form attenuated deposits round the points struck with the lightning. This new method of regarding electrical phenomena, assuredly deserves to be followed up with that accuracy which is suited to the present state of science. Every one who witnesses a stroke of lightning, would perform a very useful service by carefully collecting the black or coloured matter which the electrical fluid seems to deposit, at all those stages of its progress where it undergoes sudden changes of velocity. A careful chemical analysis of these deposits may lead to unexpected discoveries of high importance. FALLING STARS.-Ever since observations were made with accuracy on falling stars, it has been evident how greatly these long-despised phenomena, these pretended atmospheric meteors,-these so-called trains. of inflamed hydrogen gas, are deserving of attentive examination. Their parallax has already placed them much higher than the sensible limits given to our atmosphere by received theories*. During the inquiry concerning the apparent direction in which these falling stars ordinarily move, it was ascertained that even though they are inflamed in our atmosphere, they do not originate in it, but that they enter it from without. The direction most common to them, seems diametrically to be opposed to that of the earth in its orbit! It is desirable that this result should be established by the investiComparative observations made in motion round the sun. Even although 1823 at Breslau, Dresden, Leipsic, Brieg, we were inclined to regard half of this apand Gleiwitz, by Professor Brandes and parent velocity as an illusion arising from many of his pupils, have assigned no less the effect of the earth's movement in its than 500 English miles as the height of orbit, there would still remain 18 miles per certain falling stars. second as the real velocity of the star, a degree of rapidity which exceeds that of all the superior planets, except the earth. The apparent speed of these meteors is found sometimes to be 36 miles per second. This is nearly double that of the earth's |