« ZurückWeiter »
On the Tendency of the Needle to a piece of Iron, held perpendicu
lar, in several Climates. By a Master of a Ship crossing the Equinoctial Line, Anno 1684 ; and communicated by Mr. ARTHUR BAYLY, F.R.S.
All the way from England to 10° north latitude, the north point of the needle tended to the upper end of the iron, and the south point to the lower end, very strongly. In lat. g° 42' north, and meridian distance from the Lizard, 9° 32' west, the south point of the needle strongly tended to the lower end of the iron, but the north point did not so strongly tend to the upper end as before Lat. 4° 33' north, and meridian distance 5° 18' west, from the Lizard, the north point of the needle began to decline from the upper end of the iron, and the south point to incline more strongly to the lower end. Lat. 52' south, and meridian distance 11° 52' west, from the Lizard, the north point of the needle neither tended to the upper end of the iron nor the lower end, but the south point still inclined to the lower end, though not so strongly. Lat. 5° 17' south, and meridian distance 15° 9' west, from the Lizard, the south point of the needle would turn to the lower end of the iron, about two points, but by removing the iron any further, it would fly away from it, and tend to the poles again : but it would not tend to the upper end at all, neither would the north point incline to either ; but when the iron lay horizontal, and the ends of the iron made to respect the poles of the world, the north point of the needle would turn to the south end of the iron, and contrarily the south point of the needle would turn to the north end of the iron, and alter its repect to the poles five or six points, and no further ; but holding the iron perpendicular, and its middle to the needle, it would still respect the poles. Lat. 8° 17' south, and meridian distance from the Lizard, 17° 35' west : the north point of the needle would not respect the upper end of the iron, but rather fly from it: but the south point would still somewhat respect the lower end, and alter its true position about two points; but laying the iron aslope over the compass, so that the upper end was towards the south pole, and the lower end to the north, then the north point would respect the lower end and follow it, but if you point the upper end to the north, and the lower end to the south, the north point will forsake it. But laying it horizontal, it would act as in the foregoing observations. Lat. 15° south, and 20° west, from the Lizard, the south point of the needle began to respect the upper end of the iron, and the north point the lower end, and followed it about one point; but laying the iron horizontal, the north point respected the south end of the iron, contrarywise, &c. Lat. 20° 20' south, and 19° 20' west, from the Lizard, the south point of the needle respected the upper end of the iron, and the north point the lower end pretty strongly, and followed it three or four points ; but laying it horizontal, it would act as before. Lat. 29° 25', and 13° 10' west, from the me
ridian of the Lizard, the south point of the needle respected the upper end of the iron, and the north point the lower end strongly.
Of the Invention and Improvement of the Mariner's Compass. By
Dr. Wallis, 1701. It is not agreed on where, or by whom, the mariner's compass was first invented. I have guessed it to be an English invention, not only because we have been long conversant in navigation, but even from the name compass, which is used in England ; I am sure it was wont to be so used in Kent when I was a youth, for what we otherwise call a circle. And I take it to be an old English word in that sense, though now, in imitation of the French, the word circle be more common. I know not whether a compass, or any word like it, be so used for a circle in any other language ; but rather cercle in French, circhio in Italian, circulo in Spanish, or some other word derived from the Latin circulus. And from hence the circulus nauticus may come to be called the mariner's compass, which name, being given it by the first inventors, might give occasion for like names in other languages ; as compas, compasso, zee-kompas, &c. Indeed the circinus, or instrument by which we describe a circle, called by us a pair of compasses, may have some like name in other languages ; but how anciently I do not know; nor that a circle absolutely considered, other than this circulus nauticus, is so called. How far this conjecture, from the name, may give us a title to the invention, till a better appears, I shall not determine, but only suggest to consideration.
I think it is now agreed on all hands, that what we called the variation of the variation is an English discovery of Mr. Gellibrand, if I mistake not, one of Sir Thomas Gresham's professors at Gresham College, about the year 1625. That is, that the magnetic needle, in its horizontal position, does not retain the same declination or variation from the true north, in the same place, at all times, but successively varies that declination from time to time. Which, though it were about that time a new discovery, is now admitted as an undoubted truth.
And what is called the dipping needle, is admitted also to be an English discovery, somewhat prior to the former; I cannot say at present whether by Mr. Blagrave, or some other Greshamite. That is, that the magnetic needle, besides its direction towards the north, in its horizontal position, has also a direction of altitude above the horizon; and, if duly poised about a horizontal axis, will point to a determinate degree of altitude or elevation above the horizon, in this or that place respectively. Of which discovery, though made so long ago, I do not find that much use has hitherto been made, that of its horizontal declination being more serviceable.
It is also an English observation, that not only a magnetic needle, but any piece of iron, if kept long in the same position, will of itself contract a polarity. As for instance, an erect bar in a window, after long continuance in that position will, if duly poised, be found with its upper end to point towards the north, and southward with the other end; and if afterwards it be continued long in a contrary position, it will attain a contrary polarity.
And Mr. Gilbert's notion of the earth's whole body being but one great magnet, and lesser magnets being so many terrellas sympathising with the whole, is English also. It has been observed also, that a magnetic needle, if heated red hot, will lose its polarity; and if then cooled in a contrary position, will acquire a contrary polarity. It has also been observed by our English mariners, that upon a great flash of lightning at sea, their magnetic needle has lost its former polarity, and contracted the contrary, pointing the wrong way, and directing the mariner to a wrong course.
And in general, the doctrine of magnetism has been more improved by our English naturalists than by any other nation. And if some of the Gresham gentlemen would take the pains to give us a true history of these and the like improvements, it would be an acceptable service for the honour of the nation, and of that college in particular, as well as of the Royal Society.
Abstract of a Letter from Dr. Wallis to Captain EDMUND HAL
LEY; concerning the Captain's Map of Magnetic Variations, and other things relating to the Magnet. May 23, 1702.
Your magnetical chart fixes the business of magnetic variation, in these seas, for the present time. If similar observations had been made in former ages, and transmitted to us, it would have been of great use. And if such be made in future, from time to time, and recorded, by which it may appear at what rate the variation varies, it will afford a great insight into the magnetic doctrine, about which we are now so much in the dark.
The doctrine of the magnet has been mostly improved at Gresham college, or by those related to it, and there conversant, for an age or two last past ; as Blagrave, Gunter, Gellibrand, Gilbert, Norwood, Wright, Brigs, Forster, &c., and of late by yourself.
I believe it was about the beginning of the reign of King Charles the First, that Mr. Gellibrand caused the great concave dial to be erected in the Privy Garden, at Whitehall, which I think is still remaining, with great care to fix a true meridian line ; and with a large magnetic needle, showing its variation from that meridian, from time to time. I think it not amiss if exact observations were now made, whether the meridian be now just the same as it was then : for it is very possible, that the pole of the earth may in time suffer some little variation, though it may not readily be discerned, which may cause an alteration of the meridian line. And this, if so, will be more discernible nearer the pole, than farther off. And
though such provisions were made at Whitehall for observing the needle's variation from the true north ; and though no doubt notice has been given many times what the variation has been at such times ; yet I doubt no register has been kept of such observations, whence we might form a scheme to know how such variations proceed from time to time.
It has been observed also of what is called the dipping needle, that, besides the horizontal direction towards the north, it has also a direction of altitude above and below the horizon, if balanced on a horizontal axis ; pointing as it were, with its northern end, in our climate, to some point within the body of the earth. Whether or no this direction varies from time to time, like that of its horizontal position northward, I cannot tell ; nor do I know whether it has yet been observed ; nor whether the southern end in other parts of the world dips, as the northern end does with us. All which things deserve serious consideration.
I could wish that you yourself would take some pains, for I know not who can do it better; or whom else you shall think fit to associate, to collect, and give us a brief history of what has been done in this kind, how, when, and by whom; and by what steps the doctrine of the magnet has been gradually promoted; for it is a pity the memory of it should be lost. And perhaps it may be the last request I may live to beg of you, being now at the age of eighty-six ; and it is for the public, and not for myself. Or if it be too great a task for you to undertake at present, having your hands full of other weighty business, I wish the Royal Society would seriously recommend it to the care of some other fit person of their members, who may be able and willing to undertake it: as a thing that would be welcome to the inquisitive world, would be an honour to the nation, to Gresham college in particular, and to the Royal Society.
I have, in the letter abovementioned, given my conjecture, that the mariner's compass was originally an English invention. Not only because England was at that time as famous for navigation, as any nation that I know ; for the Holland sea-trade was not then in being, nor for a long time after ; but even from the name of mariner's compass, for what in Latin is called circulus nauticus For the word compass is an ancient English word, for what we otherwise call by a French name, a circle. And I am sure that within my memory, in the place where I was born and bred, it was wont to be commonly so called, though in latter times the word circle is more in use. And if we consult Minshew's dictionary, we shall find that he takes circle and compass indifferently to signify the same with circulus. And hence it is that circinus, in English is called a compass, or a pair of compasses, being the instrument by which we describe a compass or circle. Now I do not know that the word compass then was, or now is, in any other language, so used for a circle indefinitely, or for any other circle than the circulus nauticus. But now in all languages, French, Italian, German, &c., the circulus nauticus has the name of compass, or somewhat analogous, compass, compasso, zee-kompas, &c. Which name I guess, together with the art, they borrowed from England.
I might urge the same from another name, bossola, bossolo, &c. For, as circulus nauticus is the mariner's compass, so pyxis nautica, is the mariner's box, for the English box is from the Latin pyxis ; and pyxidula (as a diminutive from pyxis) must be boxel, or some word like it, which easily passes into the French buxole, boussole ; and the Italian bossola, boussola ; all which seem to be from the English boxel (pyxidula) a little box; softening the sound of the letter x into ss; as in Alessandro, for Alexandro.
All which, though it be not a direct demonstration, is at least a probable conjecture, and a plausible pretence to the invention, till a better claim appears. For, in the case of new inventions, when they come abroad, they commonly take their names from whence the invention itself is taken. And where inventions creep in by degrees, it must not be thought strange if it be not easy to say who is the first inventor. In the present case; he who first observed that the magnet has a polarity, or inclination northward, made the first step towards this invention. This I think was at first wont to be showed, by putting a magnet into a little boat, swimming on water, when it was observed, that this magnet would of itself so steer this little boat, as that a certain point in the magnet would turn towards the north : which point was thence called the magnet's north pole. He that afterwards observed that this verticity or polarity was communicable to a piece of iron or steel rubbed on a magnet, added a further step towards the business in hand. And he who contrived a way to set a needle, or piece of steel, so touched, on a sharp pin, so as in the air to move horizontally on it, and thus of itself to find out the north, and point towards it, had now discovered a new experiment in natural philosophy very surprising.
But this could not yet be called circulus nauticus, or the mariner's compass, till they had further contrived a way how to put a needle, thus poised, into a box, with a compass or circle round it; so divided as to denote the azimuthal points of the horizon, or the points of the compass; and so commodiously to fix this box to the ship, as thereby to instruct the mariner towards what point of the compass the ship moved; that he might put it into such a course as was proper for his voyage. And it was now indeed pyxis nautica, or circulus nauticus, the mariner's box or compass, but not till then. And he who first contrived this application completed this invention of circulus nauticus. But all those antecedent discoveries were steps towards it, and parts of the invention.
Now it is not likely that all these discoveries were made at once, by the same man, and at the same time, but successively, by the joint advice of several inquisitive men, and in a considerable tract of time ; yet all perhaps of the same nation, and probably the English. But whoever gave the first hint of this invention, certain