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The first were made at the School of Artillery and Engineers at Metz, the others at the Observatory of Marseilles.
Observations at Metz, (Nine Years.) South
H, minus 2, 4 (0-95) North. H. plus 2, 4 (0-95) South-west
2, 1 (0.83
+ 2, 1 (0.83) 0, 6 (0.23 East
+ 1,0 (0:39) North-west plus 0, 3 (0:11) South-east minus 0, 8 (0:31)
The difference between the extremes is sensibly less than in the observations at Paris. At the same time. it would be premature to draw general conclusions from this fact, which may perhaps be purely accidental. The following seems more decisive:
Observations at Marseilles, ( Five Years.)
+ 0, 7 (0.27) North-east West minus 0, 5 (0:19) East
plus 0, 2 (0.07) North-west. 0, 9 (0.35) South-east
+ 0, 5 (0:19) Although this table is incomplete, and founded on observations of only five years' continuance, and although the north and north-east winds are entirely omitted, there results from it no less important a consequence than this,-that if the direction of the winds exercises, at Marseilles, any influence on barometrical heights, that influence is very slight, and ought not always, in the case of winds of similar denominations, to have the same sign as in the north of France. Thus, while at Paris the southwest wind depresses the barometer considerably below the mean, its influence at Marseilles is positive; on the other hand, the north-west wind, which causes a considerable rise in the barometer at Paris, is that which produces the lowest depression at Marseilles. When observations such as these have been made at
different places, they will probably place meteorologists in a condition to explain a phenomenon which has hitherto baffled all their efforts.
OF THE DIURNAL VARIATIONS OF THE BAROMETER.-Numerous memoirs have been published on the diurnal variation of the barometer. This phenomenon has been studied from the equator to the regions in the vicinity of the pole,-at the level of the sea,
-on the immense plateaus of America, -on the insulated summits of the highest mountains, and the cause, notwithstanding, remains in obscurity.
It is still necessary, therefore, to multiply observations on the subject. In our climates, the vicinity of the sea appears to manifest itself by a sensible diminution in the extent of the diurnal oscillation; does the same thing take place between the tropics?
OBSERVATIONS ON Rain.--Navigators occasionally speak of rains which fall on their vessels while traversing the equinoctial regions, in terms which would lead us to suppose that it rains much more abundantly at sea than on land. But the truth still remains in the domain of mere conjecture; so seldom has the trouble been taken to procure exact measurements. These measurements, however, are by no means difficult. Captain Tuckey, for example, made many during his unfortunate expeVol. II.
dition to the river Zaïre, or Congo.' We know that the Bonite will be provided with a small udometer (rain-gauge.) It seems, therefore, expedient to recommend the commander to cause it to be placed on the stern of the vessel, in such a situation that it can neither receive the rain collected by the sails, nor that which falls from the cordage.
Navigators would add greatly to the interest of these observations, if they would observe at the same time the temperature of the rain, and the height from which it falls.
In order to obtain the temperature of rain with some degree of accuracy, it is necessary that the mass of the water should be considerable, relatively to the size of the vessel which contains it. A metal udometer will not answer for this purpose. It would be infinitely preferable to take a large funnel of some light stuff, very close in its texture, and to receive the water which runs from the bottom in a glass, whose sides are thin, and which contains a small thermometer. So much for the temperature. The elevation of the clouds in which the rain is formed cannot be determined but during the time of a storm; then, the number of seconds which elapse between the appearance of the flash and the arrival of the sound, multiplied by 1142—the velocity with which sound is propagated-gives the length of the hypothenuse of a right-angled triangle, whose vertical side is precisely the height required. This height may be calculated, if, by means of a reflecting instrument, we obtain the angle formed with the horizon by a line which, passing from the eye of the observer, terminates in that quarter of the cloud where the lightning first showed itself.
Let us suppose, for an instant, that there falls on the vessel rain whose temperature is below that which the clouds should possess, according to their height, and the known rate of the decrease of atmospheric heat; every one will understand the consequences which such a result would produce in meteorology.
Let us suppose, on the other hand, that during a day of hail (for it hails in the open sea), the same system of observations had proved that hail-stones were formed in a region where the atmospheric temperature was higher than the point at which water congeals,--science would thus be enriched with a valuable result, which every future theory of hail must necessarily account for.
We could adduce many other considerations to demonstrate the utility of the observations we have proposed; but the two preceding must suffice.
RAIN IN A PERFECTLY CLEAR SKY.–There are some extraordinary phenomena, concerning which science possesses but few observations; and for the reason, that those who have had the opportunity of witnessing them avoid describing them, from an apprehension that they might be regarded as undiscerning visionaries. In the number of these phenomena we may rank certain rains of the equinoctial regions.
Sometimes it rains between the tropics when the atmosphere is perfectly pure, and the sky of the most beautiful azure ! The drops are not very numerous, but they are larger than the greatest rain-drops in our climates. The fact is certain; we have the evidence of M. von Hum
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 eleinents. 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 Let us suppose that an observer, the problem, which observations made at starting from Paris, advances towards the the points mentioned would serve to solve. equator. So long as he continues in our In the northern hemisphere, the end of an hemisphere, the north end of his needle horizontal magnetic needle, which points will make a movement every morning totowards the north, moves from the east to wards the west ; in the other hemisphere, the west from 8 o'clock A.m. to 1 in the the north end of the same needle will afternoon, and from west to east from 14 move every morning towards the east. It P.M. to the following morning. Our he is impossible that this change from a misphere cannot be peculiar in this re- western movement to an eastern one can spect; the same effect produced on the take place in a sudden manner. There north end here, must be produced on the is, necessarily, between the zone where the south end to the south of the equator. first of these movements is observed and Thus, in the southern hemisphere, the end that where the second takes place, a line, of an horizontal magnetic needle which where, in the morning, the needle will points towards the south, will move from neither move to the east nor to the west, east to west from 84 A.M. till 14 in the —that is to say, it will remain stationafternoon, and from west to east from ary. 14 P.M. till the morning of the next day. Such a line must exist ; but where is Further, observation corroborates the sup- it to be found ? Is it a curve of equal inposition. Now let us compare the simul- tensity, or the magnetic, or the terrestrial taneous movements of the two needles, equator ? when comparing the same end, namely, Researches, continued during many that which points towards the NORTH. In months, in the places situated between the the southern hemisphere, the end pointing terrestrial and magnetic equators, such as towards the SOUTH moves from east to Pernambuco, Payta, Conception, the west from 8f A.m. to 14 P.M.,—therefore Pelew Islands, &c., would certainly lead the north end of the same needle makes a to the desired solution. But many months contrary movement. Thus, finally, in the of assiduous observation would be requi. souhern hemisphere, the end pointing to- site ; for, notwithstanding the skill of the wards the NORTH moves from west to east observer, the short stay of Captain Dufrom 84 A.M. till 1} P.M., which is pre- perrey at Conception and Payta, made at cisely opposite to the movement made by the request of the Academy, have left some the north end, at the same hours, in our doubts on the subject. hemisphere.
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 go 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.
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,