a considerable variation, where perhaps there is none at all. We cannot therefore hope to be secure of anything from the observations we have at present, and especially from those made at sea, which are the most considerable. This put me upon finding out some means, independent of observations, to discover the variation at sea; but having considered that several learned men of this age had proposed divers ways of making magnetical needles, which should not be subject to variation, and that all these projects had failed of effect, I judged that after all that they had done by means of the loadstone, it was not to be hoped to draw any further advantage from it, since the stone itself, as far as might be guessed from the experiments hitherto made, was subject to the same variation.

I had quite given over this inquiry, when there accidentally came to my hands a terrella, or spherical loadstone, of three inches diameter, with which trying some experiments, with a small needle, the foot of which might easily be placed upon the stone, I soon observed what has been already noted by several, viz., that this globe of magnet caused the needle to have the same changes as are found in the compass in different parts of the world, as well in respect of the direction towards the two poles, as of the inclination towards that which is next it; and on trial I satisfied myself that it was not possible to find the point where the needle would stand indifferently in any position, which point would have exactly showed me the pole of the stone; but that the needle, however placed, always directed itself one certain way. I determined by this means, as well as I could, the point called the south pole; but I was much surprised to find it 18° distant from a cross deep engraven on the stone, which probably had heretofore been the pole of this stone, as it had been observed by the person who cut it. This change of the poles of this stone having revived my former thoughts concerning the variation of the needle, I believed that if it were true that the poles of the magnetical virtue changed in the loadstone, as we see they change in the earth, we might hence derive great advantages, as to the variations of the magnetical needle. For if this change of the poles in the loadstone were certain, and that it was analogous to the change of the poles of the magnetic virtue of the earth, it is not to be doubted but a terrella, being suspended at liberty, would reremain immoveable, and that one point thereof would regard the pole of the world, which might be called the true pole of the stone, while the poles of its virtue would pass successively from one part to another, after the same manner as they change in the earth.

Having well considered this hypothesis, and cleared up some doubts which I had concerning the position of the stone at the time when its pole had formerly been determined; I concluded that this former pole was distant from the point I call the true pole 13° towards the east, in the place where it had been marked, and which is unknown to me, since that at this time, in this country, the needle varies about 5° westward.

On this hypothesis I have invented a new sort of needle for the compass, which may have the same alterations as a spherical loadstone, and at the same time the same conveniences as the ordinary needle. I caused a ring of three inches diameter to be made of steel wire, from which issued three radii of very fine brass wire, meeting at the centre in a cap perfectly like that of an ordinary compass, that so this circle might rest on a pin in its centre, and be at full liberty to turn round, its centre being fixed. This done. I gave the magnetical touch to this steel ring, by applying indifferently to a point of it one of the poles of a strong loadstone, and the other pole of the stone to the opposite point, to give the greater virtue to the ring. The ring then became strongly magnetical, and the point called the south pole readily turned itself towards the north, and after several vibrations stopped there; and it had also the same inclination towards the pole which is found in needles after they have been touched. Lastly, I fixed upon the ring a small fleur-de-lis of brass, in the point which exactly respected the north, the ring being first well settled.

If the poles of the magnetic virtue in the loadstone change after the same manner as they do on the earth, it seems likely that the same thing would happen to this ring, and that one point thereof should always exactly respect the north. But to inform myself if a steel ring had the same effects as a terrella, I made the following experiment: having touched a steel ring and laid it on a paper, I strewed the filings of steel upon it, and then gently shaking the paper, I saw that the direction of the magnetical matter passed directly cross the ring from one pole to the other, and that there were two vortices on the sides, as it is observed in the spherical magnet, which seems very surprising; for according to the common hypothesis of the magnet, the magnetical virtue passing more easily in the steel than in the air, should run on both sides of the pole round the ring, and only form a pole opposite to the first. But I was further confirmed in this opinion by applying a flat and pointed piece of iron, like the blade of a knife, to a loadstone, so as the point of the iron reached beyond the stone, and having afterwards presented this point to the magnetical ring, I observed that different points of this ring applied to the point of the iron, according as the several parts of it had been applied to the stone; which happens not in the magnetical needle, for that always presents one of its ends to the point of the iron, being not disposed, by reason of its length, to receive the magnetical matter in all the parts thereof analogous to those of the stone. It must only be noted, that in an irregular stone the magnetical virtue appears stronger towards the angles than in the other parts, which may cause some irregularity in this experiment, if it be tried with a stone that is very uneven.

These experiments gave me the curiosity of making another, by touching two semicircles of steel. Having joined the two ends touched by the same poles, I observed by the steel dust the same

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effect as in the ring. But having joined the ends differently touched, I found that immediately the two half rings run together and adhered to each other; and by the steel dust strewed on paper, I observed that there were four vortices, one in the middle of each semicircle, and one at each of the places where they joined, and that the two latter were less than the others, and much stronger. I saw likewise that there were four poles, each of which was within a vortex, and that each retained in its semicircle the virtue of the ends of the half rings.

Having touched a steel wire that was straight, I tried to make a ring of it; but I found that it had quite lost its virtue; which cannot be attributed to the junction of the poles, since they ought to adhere together, according to the other experiments which have been made; but only to what has been already noted, that when a magnetical virgula is a little bent, it loses its virtue, which cannot happen but from the alteration in the pores of the steel.

I further remarked, that a ring of steel touched, for a long time retains its virtue, though it be put into a position contrary to its poles. And this experiment is confirmed by another, much more considerable, which is, that a ring of steel, touched with a strong load-stone, cannot without difficulty receive a contrary touch from a magnet less strong than the first; but that in time by degrees it resumes its former virtue, much as we see magnets do, which being applied to another stone, by the poles of the same denomination, lose their first virtue, and take the contrary; which they afterwards lose by degrees, to re-assume the first.

After having presented this new system of the magnet to the academy, there were made some experiments on a terrella, of much the same diameter with mine, but whose poles were not diametrically opposite; and on a half-globe, much larger than the terrella. But no considerable difference or alteration of poles were found in Paris, April 26, 1687.

This letter having been read before the Royal Society, it was ordered that the terrella, which has been in their repository these twenty-five years, the gift of their royal founder King Charles II, I should be examined, to see if there be any sensible alteration in its poles and on trial it was found, that the points which are marked on it with crosses, were, as near as could be discerned, the true poles of the stone; notwithstanding that the variation has changed at London full four degrees since this terrella has been in the society's custody; and perhaps many more since it was marked; and had there been a similar change in the poles of the loadstone, it must needs have been perceived in this, whose diameter is about four-and-a-half -inches. However, to put this matter past dispute, care was taken to find out exactly, and mark the poles of the society's great loadstone, the sphere of whose activity is above nine feet radius, and whose poles are thirteen inches asunder, by which, if this translation of the poles be real, it cannot fail of being made very sensible in future

times. As to the supposition that the points in which the iron has received the magnetical virtue may change place, after the same manner as the poles of the earth's magnetism are observed to do; though it was considered as an ingenious hint, and worth prosecution, yet some of the company, well skilled in magnetics, were of opinion, rather that such a circular needle would librate on its centre, so as to respect the magnetical meridian with the points that had at first received the touch, than that the ring remaining immoveable, the directive virtue in it should be transferred from place to place, either by length of time, or by transporting this compass into those parts where the variation of the needle is considerably different.

Observations on the Thermometer and Magnetic Needle, during a Voyage to the Cape of Good Hope. By Mr. JAMES CUNNINGHAM, April 6, 1700.

The greatest height of the spirit in the thermometer was two divisions below extreme heat, when we were near the equinoctial. Two degrees to the northward of the line, the north point of the needle inclined eight degrees downward, but as we went to the southward it was inclined above forty-eight degrees upward.

The Undertakings of Mr. HENRY BOND, Senior, a famous Teacher of the Art of Navigation, in London, concerning the Variation of the Magnetical Compass, and the Inclination of the Inclinatory Needle; as the result and conclusion of thirty-eight years Magnetical Study, 1673.

Mr. Bond can show the cause of the variation of the magnetical needle, or compass, by the motion of two magnetical poles; how these poles are found; and what their distance is from the poles of the earth; what their annual motion is, and from whence it proceeds. By calculation he finds all the variations that have been observed at or near London, for above ninety years past; and so by consequence it may be found at London to the end of the world. He has calculated a table to every five minutes of the inclination of the inclinatory needle; so that by the needle's inclination, and that table, and the latitude of the place, he can find the longitude of any place in the world. By that table also he finds Mr. Robert Norman's inclination, which he found An. 1576; and can show, what will be the greatest and least inclination of the inclinatory needle, in any latitude in the word. He has four examples of finding the longitude by the help of the inclinatory needle; one at Balsora, in the East Indies, in the year 1657. Another at Cape Charles, on the coast of Virginia, before that time. The third at the Cape of Good Hope. The fourth at the Straits of Magellan.

Extract of a Letter of Mr. HEATHCOT to Mr. FLAMSTEED, from Cabo Corso Castle, on the Coast of Guinea, of Dec. 14, 1683.

The sea runs here along the shore continually to the eastward, at a very great rate, except at full and change, for then it runs to westward, or at least slackens very much. November 24th, I took the time of the high water at the castle, as near as I could, at 3h. 30m. p.m., it flowed about six feet. The magnetical variation here is 3° 49', from the north to the westward.

Magnetical Experiments. By Mr. SELLERS, 1667.

Mr. Sellers states, that he had often made trial with many needles touching them on each hemisphere of the stone, in all variety of ways he could imagine, to find if it were possible, by that means, to cause any of these needles to vary in its direction: but that he always found the contrary, all of them conforming to the magnetical meridian, and standing north and south, as other needles that were touched on the very pole of the stone. He adds that some of these experiments he tried in London, when there was no variation known.

That on frequent trials of touching needles with different loadstones of several magnitudes, as also of different virtue, the needles touched gave all of them the same directions. This he thinks is confirmed by all the needles and sea compasses made in several parts of the world, and consequently touched on several stones of different countries, yet all agreeing in this magnetical harmony, that they all give the same directions. That having sometimes drawn a needle only over the pole of the stone, within the sphere of its virtue, without touching the stone, it has received the same directive quality from the stone as if it had been really touched upon the stone itself, though not altogether as strong as if it had touched the stone. Again, that having touched needles on the stone with faint strokes, and other needles with stronger, all these needles received the same effect from the stone, both for strength and direction; he conceiving that it is not the fainter or stronger touches on the stone, nor the multiplicity of strokes, that varies the needle's strength or direction, but that the nature of the steel whereof the needle is made, and the temper that is given thereunto, cause different effects as to the strength it receives from the stone; himself having tried all sorts of steel that he could possibly procure, and all the different tempers he could imagine, for the most powerful receiving and retaining the virtue from the loadstone; he also affirms that he has fully satisfied himself that he can infuse such virtue into a piece of steel, that it shall take up a piece of iron of two ounces weight or more; and give also to a needle the virtue of conforming to the magnetical meridian, without the help of a loadstone or any thing else that bas received virtue therefrom.