Paris, that the compass-needle had been for some time stationary, and that it was then returning to the true north. If that be a fact, I have no doubt its confirmation will be made known in due time.

Thomas Yeates, in England, has, with great labour and ingenuity, constructed a variation chart of the navigable globe, from latitude 60 degrees north to 60 degrees south, chiefly from actual observations made by European navigators and astronomers, as recorded in manuscript journals at the hydrographical offices of the admiralty in the East-India house, compared with Spanish surveys in the Pacific Ocean, and collated with tables of variation. This chart was nearly ready in 1816.

In north latitude the north polar star is very convenient for finding the variation of the compass, especially in high latitudes, where, in the day time, the sun is mostly obscured by haze or clouds.

The north polar star is about 1 degree 42 minutes to the south of the north pole, which causes it to make a small circle round the pole, from east to west, or rather the globe revolves from west to east; but which motion is not perceptible, like that of the other stars, in consequence of its being so near the pole. Those who choose to observe its bearings on the meridian, will find that its utmost deviation takes place when it is about six hours from the meridian, on either side of the pole. When it is at that distance from the meridian, above the pole, it is also at the same distance from the meridian below the pole. It bears due north on the meridian of either pole, one degree and fortytwo miles of the compass, its greatest distance, agreeing to a little more than one-seventh of a point of the compass. Thus, after it has passed the meridian above the pole, the deviation increases until it is six hours, or nearly from that meridian, which is one-quarter of a circle, or one-quarter part of its revolution, or thereabout: then it begins to approach the meridian below the pole; as will be better understood by referring to the following figure.

I do not consider it necessary to add any of my own observations on the variation of the compass, as it is continually changing, and is generally different in different ships; and, as opportunities so frequently occur, in high latitudes, to ascertain it by observations of the polar star. In low latitudes, opportunities by the sun are frequent.

How to retain the Magnetical Power of a Compass.

WHEN ships are on long voyages, or lay long in port, and their compasses not in use, the compass-cards should be taken off the spindle-point, and wrapped together in pairs, in dry brown paper, with the north point of one to the south point of the other, and kept at a distance from other pairs. This will improve their magnetic virtue.

Cautions on using Compasses.

WHILE the compasses are in use, be careful that the lubber's point be placed perpendicular to the fore side of the binnacle. When they are not set perpendicular to each other in the binnacle, the compasses are frequently thought to differ.

Wooden compass-bowls are very apt to be turned on one side, by accident; in consequence of which the compasses are thought frequently to be out of the way.

All binnacles should be nailed with copper nails.

Causes which frequently affect Compasses.

CARE should be taken to examine the compasses, in order to ascertain whether they are not affected by something on board of the ship. Iron, when hot, has much greater influence on the

needle, than when cold. It may, therefore, be much affected by a hot stove in the cabin. In the year 1797, a schooner was lost on Chincoteague shoals, in consequence of a hot stove in her cabin attracting the compass-needle out of its proper direction. From a like cause, in coming round from New-York, in the brig Eliza, of Philadelphia, in the same year, my compass, in the binnacle, was attracted four points out of the way; and, if I had not discovered this in time, I should have run the vessel on Barnegat shoals.

It may also be observed, that if a bar of iron stand in a vertical position for a long period, it will become magnetic; and if it remain in that position fourteen years, it will become a complete magnet.

From the above considerations, even brass stoves in the cabin, with copper pipes, may affect the needle; for the pipe being heated, and coming up through the deck, may communicate heat to the iron near it; and also tin is frequently nailed on the bulkhead to prevent the pipe from burning it; which iron or tin, becoming thus heated, attracts the needle; and perhaps, also, the tin may be rendered magnetic, by being a considerable time on the bulk-head, in a vertical position; in which case its power of attraction is further increased.

CAUSES OF THE ERRORS

OF

THE COMPASS.

I will here insert Captain Sabin's observations on the Isabella's compasses, on her late voyage of discovery; from the Philosophical Transactions, 1819.

"It is proposed to show in what respects the effects of local attraction, in the above-mentioned ships, were conformable to the observations which had been made in preceding voyages; and how far the errors, which were found to take place on different courses, and under different dips of the magnetic needle, corresponded with the rules for calculating corrections, which Captain Flinders had found useful in his own experience, and which he had recommended for a more extensive trial.

may

It may be desirable to premise, that the irregularities here alluded to, are not those accidental disturbances which be caused by iron placed inadvertently too near the compasses, but the permanent and constant effect of the mass of iron contained in a ship, affecting its compasses at all times, and in a greater or a less degree, according as its influence is more or less powerful, in comparison to the directive force of the earth's magnetism.

That errors have always existed from this cause, may be inferred from the uncertainty which experience has attached to the results of azimuths observed in ships. The cause however appears to have been very long unsuspected, whilst its effects have produced a general impression, that the azimuth compass was in itself an imperfect instrument and only to be relied on within certain undefined and variable limits.

It was reserved to the accurate observation, and the habit of recording and comparing apparently trivial and accidental dif

ferences in results, which distinguished the late Mr. Wales, (astronomer in the second voyage of Captain Cook,) to enable him to lead the way to a knowledge of the nature and causes of these errors. He remarks," that in the passage of the Resolution and Adventure to the Cape of Good Hope, and subsequently, the greatest west variations had happened when the ship's head was north and easterly, and the least when it was south and westerly, differing very materially from one another with the ship's head in different positions, and still more when observed in different ships." Thus manifesting that they were something more than accidental.

This voyage was the last in which Mr. Wales embarked, and the investigation does not appear to have been pursued in England until the voyage of discovery to Terra Australis, in the first years of the present century. The survey of the coast of New Holland being carried on in a considerable measure by intersection of compass bearings, taken from the deck of the Investigator, so much embarrassment and perplexity were found to arise from the effects of local attraction, that much of Captain Flinders' attention and thoughts were necessarily devoted to a consideration of some means of remedying the inconvenience.

On his return to England, he obtained permission from the Lords Commissioners of the Admiralty, to make a course of experiments in ships under their direction, at the principal sea ports, with a view to ascertain if compasses were similarly affected in other ships, and to try the general applicability of rules which he had found useful in correcting the errors in the Investigator. These rules, with the observations and reasonings on which they were founded, were published in a short paper in the Philosophical Transactions, and in a more detailed form in Appendix No. 2, in the voyage to Terra Australis. There are three points in these statements chiefly worthy of attention, from their practical importance, and on which it seems desirable therefore, to notice how far his observations have been confirmed by those made in the Isabella and Alexander. First, he found that in every ship a compass would differ very materially from itself on being removed from one part of the ship to another. Experience of this source of irregularity had induced him early in his voyage to confine the use of the compass, with which his survey was carried on, to one particular spot. The place he selected was determined by convenience; in other respects it was on the binnacle, and exactly a-midships.

The Isabella and Alexander had not completed half their voyage across the Atlantic, before it was found that the binnacle compasses of the one ship differed very materially, in indiating the course steered, from those of the other; namely, onc