VARIATION of the compass is the deviation of the points of the mariner's compass from the corresponding points of the horizon, and is denominated east or west variation. East variation is when the north point of the compass is to the east of the true north point of the horizon. West variation is when the north point of the compass is to the west of the true north point of the horizon. The variation of the compass is found by various methods, which are given in the epitomes of navigation, by amplitude, azimuths, equal altitudes, &c. The true amplitude is reckoned from the east or west points. The true amplitude of any celestial object is an arch of the horizon contained between the north or south points thereof and the object's centre, at the time of its rising or setting.* The magnetic amplitude is an arch contained between the object's centre, when in the horizon, and the magnetic meridian; or it is the bearing of the object per compass, when in the horizon. The true azimuth of an object is the angle contained between the true meridian and the vertical passing through the object's centre. The sun is rising or setting when he appears to have the lower edge about two-thirds of his diameter above the horizon; because the dip being 4 minutes, the refraction about 33 minutes, which make 37 minutes, and the semidiameter being about 16 minutes, which, taken from 37, leaves 21. That being about two-thirds, and as the sun's centre must be in the horizon when he either rises or sets, this calculation will be near enough to find the sun's amplitude, although it may be liable to some small error; such as extraordinary refraction (which I have often noticed in high latitudes) and the lower limb of the sun being more refracted than the upper limb, &c., which error will not affect the magnetic amplitude. The magnetic azimuth is the angle contained between the magnetic meridian and the azimuth circle passing through the centre of the object. It is said that the attractive power of the magnet was known in Europe six hundred years before the Christian era; and by the Chinese records it is said that its directive property was known in that country at least a thousand years earlier. The invention of the compass is by some ascribed to Flavia Goya, of Amalphi, in Naples, about the year 1302. It, however, appears, from some French records, to have been known in that country previous to the year 1180. 7 Until the time of Columbus it was thought to be invariably the same, at the same place, in all ages. However, in the month of September, 1492, Columbus first discovered the variation of the needle. This discovery is also said to have been made by Sebestian Cabot, in the year 1497. Soon after, the variation was found to be different at different places. It was, however, affirmed to be constant at the same place; but, in 1635, Mr. Henry Gellibrand published his discovery of the change of variation, from a succession of observations, that the deviation was not a constant quantity, but that it gradually diminished. About the year 1657, the needle pointed due north at London, and ever since has been increasing westwardly; and at this time (1822) at London, is about 26 degrees. The magnetic needle is subject both to an annual and diurnal variation. In the years 1722 and 1723 Mr. Graham made a number of observations on the diurnal variation of the magnetic needle. In 1750 Mr. Wargenton took notice of the regular diurnal variation of the needle, and also of its being disturbed at the time of an aurora borealis, called by some northern lights.* About the latter part of the year 1756, Mr. Cantor began to make observations on the diurnal variation of the needle, and in nion There have been various opinions and conjectures respecting the cause and properties of these extraordinary phenomena. The most probable opi18, that they arise from exhalations, and are produced by a combustion of inflammable air, caused by electricity. This inflammable air is generated, particularly between the tropics, by many natural operations, such as the putrefaction of animal and vegetable substances, volcanoes, &c.; and, being lighter than any other air, ascends to the upper regions of the atmosphere, and by the motion of the earth is urged towards the poles; for it has been proved by experiments, that whatever is lighter, or swims on a fluid, which revolves on its axis, is urged towards the extreme points of that axis. Hence these inflammable particles continually accumulate at the poles, and, by meeting with heterogeneous matter, take fire, and cause those luminous appearances frequently seen towards the polar regions. In high latitudes, the aurora boreales appear with the greatest lustre, and extend over the greatest part of the hemisphere, varying their colours from all the tints of yellow, to the most obscure russet. the north-east parts of Siberia, Hudson's Bay, &c. they are attended by a continued hissing and cracking noise through the air, similar to that produced by fire-works. In 1759 he communicated several experiments to the Royal Society of London. The observations were made by him 603 days; 574 out of these the diurnal variation was regular. The absolute variation of the needle westwardly was increasing from about eight or nine o'clock in the morning, till about one or two in the afternoon, when the needle became stationary for some time. After that the variation westwardly was decreasing, and the needle returned back again to its former situation, in the night, or early next morning. The diurnal variation is irregular, when the needle moves slowly eastward in the latter part of the morning, or westward in the latter part of the afternoon; and also when it moves much either way, or suddenly both ways, in a short time. These irregularities seldom happen more than once or twice a month, and are always accompanied with an aurora borealis.* The diurnal variation in the months of June and July is almost double that in January and December. Mr. Cantor supposes that the diurnal heat of the sun has this influence upon the magnetic parts of the earth, or rather upon the magnet included in the earth: but Mr. Epinus has shown that this supposition is inadmissible; because, agreeably to the hypothesis, the magnetic nucleus must be very profound, and it will be known that the solar heat does not penetrate to very great depths. There are caves at so great a distance from the surface of the earth, that the thermometer remains always at the same height. The diurnal heat not penetrating to that depth, it is not probable that its effects extend still further. A writer, of considerable authority, says, in his Theory of Longitude, that the needle is subject to both annual and diurnal vibratory motions. In the first of these, the motion of the north end of the needle is in general towards the east, from the time of the vernal equinox to the summer solstice; and during the other nine months its motion is in general towards the west. In the second, the needle is stationary from noon till about three P. M., and from thence till about eight in the evening, it slowly approaches the east. It again continues stationary about eight in the morning; and from that time till noon it gradually approaches the west. The mean quantity of the diurnal variaation, at the observatory of Paris, in each month of the year 1791, according to the observations of M. J. D. Cassini, is as follows: We have few accounts of the aurora australis, or southern lights. Captain Cook observed this phenomenon, in 1773, at 58 degrees south latitude. It consisted of long columns of clear white light, shooting up from the heavens to the eastward, almost to the zenith, and gradually spreading over the whole southern part of the sky Though these columns were, in many respects, similar to the aurora borealis, yet they seemed to differ from them in being always of a whitish colour. This diurnal variation is too small to affect the ship's course. I have inserted it here merely to give the navigator an understanding of it. There are many circumstances, besides those already men tioned, which may put the needle out. Some of which are, The compasses attracting each other, when they both are in the binnacle at one time, The quick motion of the ship, Care should be taken that the binnacle be placed perpendicular to the ship's keel, as well as that the lubber's point in the compass-dish be properly placed; both of which are often neglected; and the want of attention to these particulars is often the cause of the disagreement of the compass with its true point. Upon the Polarity of the Sewing Needle. THERE are few needles that do not possess polarity. With little exception, therefore, all needles, when floated on water, will tend to the magnetic meridian. Their points will turn to the south. This it is necessary for seamen to be acquainted with: for, should they be so unfortunate as to quit their vessel without a compass, by the use of a common sewing-needle, which seamen are seldom without, they are supplied with a tolerable substitute. If a needle should be found without sufficient polarity, hold the point towards the south, and rub it strongly from the middle towards the point, with the back of a knife. This will excite a very strong polarity. A dry needle, gently dropped on the surface of water, will swim, unless its upper part become wet; in which case it will immediately sink. If several needles be rubbed as above directed, and stuck together, through a small piece of cork, or other light wood, with their points one way, they will move with a much quicker motion than either of them singly. To Touch the Compass. HAVING two strong magnetic bars, lay the compass-needle as nearly north and south as possible, with the intended north to the northward; join the two magnets in a line considerably above the needle, with the north end of each being northward; then the south end of each will be consequently southward; bring them down upon the needle, so that the place of junction may be over the centre of the needle; then draw them asunder, along each half of the needle; continue their motion until they are eight inches clear of the ends of the needle, and, by a circular motion, bring them again to the centre, and join them as before. Repeat this operation six or seven times, taking care not to put the magnets out of their parallelism, and the needle will be sufficiently magnetical.† The proper persons to sell magnets are mathematical instrument makers, who will inform purchasers of magnets how to apply them to the needle, as magnets are differently marked. In France I have seen magnetic bars marked as they were to be applied. The horse-shoe magnet is now generally used. The method of touching the compass-needle with it is, to bring it from a considerable distance above, perpendicularly down upon the centre of the needle; then draw the north magnet from the centre to the south point of the needle, and, by a circular motion, bring it again to the centre. Continue that motion until clear of the needle eight inches; repeat this motion six or seven times. Then the south magnet must be applied to the north half of the needle, as has been done on the south; and the needle will be perfectly magnetic. If the magnet should be applied wrong on the needle, it will be discovered by the north point of the compass-needle turning to the south; which may be ascertained by comparing it with other compasses on board the vessel. In all cases, when touching the needle, it must be kept as nearly north and south as possible, with the north end to the northward. When I was in Marseilles, in 1817, I saw a statement in the gazette, that a discovery had been made in the observatory at The reason for drawing the north end of the magnet towards the south point of the compass-needle is, that the north end of the magnet gives a south polarity, and the south end a north polarity, which is treated of under the title Magnetism," which see. The magnet must be held flat, when drawn upon the needle; in order to do which, the needle ought to be raised on a block, or other convenient thing. |