COMPASS

, or Mariner's Compass, is an instrument used at sea by mariners to direct and ascertain the course of their ships. It consists of a circular brass box, which contains a paper card with the 32 points of the compass, or winds, fixed on a magnetic needle that always turns to the north, excepting a small deviation, which is variable at different places, and at the same place at different times. See Variation of the Compass.

The needle with the card turns on an upright pin fixed in the centre of the box. To the middle of the needle is fixed a brass conical socket or cap, by which the card hanging on the pin turns freely round the centre.

The top of the box is covered with a glass, to prevent the wind from disturbing the motion of the card. The whole is inclosed in another box of wood, where it is suspended by brass hoops or gimbals, to keep the card in a horizontal position during the motions of the ship. The whole is to be so placed in the ship, that the middle section of the box, parallel to its sides, may be parallel to the middle section of the ship along its keel.

The invention of the compass is usually ascribed to Flavio Gioia, or Flavio of Malphi, about the year 1302; and hence it is that the territory of Principato, the part of the kingdom of Naples where he was born, has a compass for its arms. He divided his compass only into 8 points. Others ascribe the invention to the Chinese; and Gilbert, in his book de Magnete, affirms that Marcus Paulus, a Venetian, making a journey to China, brought back the invention with him in 1260. What strengthens this conjecture is, that at first they used the compass, in the same manner as the Chinese still do, viz, letting it float on a small piece of cork, instead of suspending it on a pivot. It is added, that their emperor Chiningus, a celebrated astrologer, had a knowledge of it 1120 years before Christ. The Chinese divide their compass only into 24 points. But Ludi Vertomanus affirms, that when he was in the East-Indies, about the year 1500, he saw a pilot of a ship direct his course by a compass, fastened and framed as those now commonly used. And Barlow, in his book called The Navigator's Supply, anno 1597, says, that in a personal conference with two East-Indians, they affirmed, that instead of our compass, they use a magnetical needle of 6 inches, and longer, upon a pin in a dish of white China earth, filled with water; in the bottom of which they have two cross lines for the 4 principal winds, the rest of the divisions being left to the skill of their pilots. Also in the same book he says that the Portuguese, in their first discovery of the EastIndies, got a pilot of Mahinde, who brought them from thence in 33 days, within sight of Calicut.

But Fauchette relates some verses of Guoyot de Provence, who lived in France about the year 1200, which seem to make mention of the compass under the name of marinette, or mariner's stone; which shew it was used in France near 100 years before either the Malfite or Venetian one. The French even lay claim to the invention, from the fleur de lys with which most people distinguish the north point of the card. With as much reason Dr. Wallis ascribes it to the English, from its name compass, by which name most nations call it, and which he observes is used in many parts of England to signify a circle.

The mariner's compass was long very rude and imperfect, but at length received great improvement from the invention and experiments of Dr. Knight, who discovered the useful practice of making artificial magnets; and the farther emendations of Mr. Smeaton, and | Mr. M'Culloch, by which the needles are larger and stronger than formerly, and instead of swinging in gimbals, the compass is supported in its very centre upon a prop, and the centres of motion, gravity, and magnetism are brought almost all to the same point.

After the discovery of that most useful property of the magnet, or loadstone, viz, its giving a polarity to hardened iron or steel, the compass was many years in use before it was known in anywise to deviate from the poles of the world. About the middle of the 16th century, so confident were some persons that the needle invariably pointed due north, that they treated with contempt the notion of the variation, which about that time began to be suspected. However, careful observations soon discovered that in England, and its neighbourhood, the needle pointed to the eastward of the true north line; and the quantity of this deviation being known, mariners became as well satisfied as if the compass had none; because the true course could be obtained by making allowance for the true variation.

From succeeding observations it was afterwards found, that the deviation of the needle from the north was not a constant quantity, but that it gradually diminished, and at last, namely, about the year 1657, it was found that the needle pointed due north at London, and has ever since been going to the westward, till now the variation is upwards of two points of the compass: indeed it was 22° 41′ about the middle of the year 1781, as appears by the Philos. Trans. pa. 225, for that year, and is probably now somewhat more, which it would be of consequence to know; but why such useful observations and experiments, as those of the variation and dip of the magnetic needle, have been so long discontinued, to the prejudice of science, is best known to the learned President of that Society, and his Council. So that in any one place it may be suspected the variation has a kind of libratory motion, traversing through the north, to unknown limits eastward and westward. But the settling of this point must be left to time and suture experiments. See Variation, also Inclination, and Dip. Also for a farther description of different compasses and their uses, see that useful book, Robertson's Navigation, vol. 2. p. 231.

The Azimuth Compass differs from the common sea compass in this; that the circumference of the card or box is divided into degrees; and there is fitted to the box an index with two sights, which are upright pieces of brass, placed diametrically opposite to each other, having a slit down the middle of them, through which the sun or star is to be viewed at the time of observation.

The Use of the Azimuth Compass, is to take the bearing of any celestial object, when it is in, or above the horizon, that from the magnetical azimuth or amplitude, the variation of the needle may be known. See Azimuth, and Amplitude.

The figure of the compass card, with the names of the 32 points or winds, are as in fig. 5, plate vi; where other compasses are also exhibited.

As there are 32 whole points quite around the circle, which contains 360 degrees, therefore each point of the compass contains the 32d part of 360, that is 11 1/4 degrees, or 11° 15′; consequently the half point is 5° 37′ 30″, and the quarter point 2° 48′ 45″.

The points of the compass are otherwise called Rhumbs; and the numbers of degrees, minutes and seconds made by every quarter point with the meridian, are exhibited in the following table.

Compass Dials, are small dials, fitted in boxes, for the pocket, to shew the hour of the day by the direction of the needle that indicates how to place them right, by turning the dial about till the cock or style stand directly over the needle. But these can never be very exact, because of the variation of the needle itself; unless that variation be allowed for, in making and placing the instrument.