Abbildungen der Seite

it but four degrees north-east, conformably to my observations. Which did assure me that those declinations were not constant but had varied.

And that I might be convinced by myself, I made from time to time, experiments in divers places, and found still more and more diminution; so that, Apno 1660, in June, after I had very exactly traced a meridian by many azimuths, before and after noon, with a brass quadrant of six feet diameter, and applied good needles upon it, the one of seven, the other of ten inches long, I found that they declined but one degree or thereabout: and the last year, 1665, I found no more than ten minutes on the same meridian. Upon which having lately applied, since the receipt of the letter, the same two needles, it seems that the declination is yet less than the last year. But this I can assure you, that the declination is yet some minutes towards the east, at least at Paris. So that you may upon my word doubl* of the observation of your friend, whom perhaps the meridian or the needle, or the construction and division of bis compass may bave deceived to a degree and a half north west, which he at present assigns to the declination. But I doubt not but in twelve or fifteen years it will be found true what he affirms, as I have prognosticated by my hypothesis, which makes the declination to vary a degree every seven or eight years.

Of the Magnetical Variation, and the Tides, near Bristol.

By Captain SAM. STURMY, 1668. June 13, 1666, Captain Sturmy made the following magnetical observations in Rownham meadows, near Bristol, by the water side:

[ocr errors][merged small][merged small][merged small][merged small][merged small][merged small][merged small][merged small][merged small][merged small][merged small][merged small][merged small][merged small][merged small][merged small][ocr errors]

In this table he notes the greatest distance or difference to be 14 minutes; and so taking the mean for the true variation, he concludes it then and there to be just 1° 27', viz., June 13, 1666.

He observed again, in the same day of the next year, viz., June 13, 1667, and then found the variation increased about six minutes westerly.

• By the farour of the author, it is not conclusive, that because the declination is yet somewhat towards the east at Paris, it must therefore be so at London ; since it is known here that even the variation of Wbiteball differs from that of Limehouse ; which two places are but about four English miles distant from each other.

From many former observations Captain Sturmy assures, that the bighest spring and annual tides there are about the equinoxes, according as the moon is near the full or change, before or after that time.

The Variations of the Magnetic Needle predicted for many Years

following. By Mr. HENRY BOND, 1668. The doctrine of the magnet and magnetical motions is yet so obscure, that what hitherto has been discoursed and written upon that subject, proves very unsatisfactory. An intelligent mathematician and teacher of navigation in England, Mr. Henry Bond, having formed to himself an hypothesis of the variations of the needle, has thence calculated the following table ; showing how the variations of the magnetic needle will fall out for many years to come; which variation he conceives is now westward, and to have been so for some few years past; whereas they were formerly eastward. This philosophical prediction is bere made public, that inquisitive men may every where, from time to time, make observations accordingly, either to verify or invalidate the proposed theory.









Variation | Variation



16681 1°56' ||16783° 46' ||1688 5° 19' ||16986° 52' || 1708 8° 17' 1669 2 7 11679 3 50 1689 5 29 || 1699 7 1 ||1709 8 25 11670) 2 18 ||1680 4 0 | 16901 5 39 ||1700 7 10 1710) 8 33 11671| 2 28 ||16811 4 10 ||16911 5 48 ||17011 7 19 ||1711 8 41 1672 1682


7 28 11712 8 49 1673 2 49 1683 4 30 1693 6 7 || 1703 7 36 1713 8 56 1674 2 59 1684 4 40 1694 6 16 7 45 ||1714 9 4 11675 3 9 1685 4 50 | 1695 6 25 111705) 7 53 | 1715 9 11

1676 3 19 1686 5 0 ||1696 6 34 1706 8 1 1716 9 171 | 1677) 3 30 11687 5 10 1697| 6 43 1707| 8 9 ||

[ocr errors]


Magnetical Variations at Rome. By M. Auzout, 1670. The declination of the loadstone has for many years been observed not to continue always the same in the same places; and the variation to be such, that it can be no longer imputed to any defect in the observations, as it was believed at first, when it was not very great: it has been noted some years since, that the magnetic needle, which, almost every where, had declined eastward to eight, ten, and twelve degrees, after its diminishing, little by little, as far as to the meridian, began to decline westward.

M. Adrian Auzout has made here in Rome the following observation about the declination of the loadstone, on many meridian lines drawn as exactly as possibly he could with a needle, three quarters of a palm long;* and on all the lines it was seen to decline

* This is about six inches.

somewhat more than two degrees westward, and on some near two degrees and a half.

But by the observations here made formerly, it appears tbat the needle has declined eastward to eight degrees, and has afterwards been diminishing, till it is come to the other side, where we find it at present.

It seems not that this difference of ten degrees and more can be attributed to the change of the prole of the earth, as some esteemed, perhaps before they knew it was so great; nor, as others would have it, to the magnet, or to the iron, that are found in certain places, because there is but little loadstone; and M. Auzout affirms, that the mines which he has seen, make no impression at all on the needle. So that it is difficult to hit the true cause of such a variation; yet, however, if the direction of the magnet, and of the needle touched by it, depends on the flux of a certain matter, passing through the whole earth, or the exterior parts of it, straight along the axis, it may be said that it proceeds from cbanges made in the said flux, which, supposing the inequalities of the earth, and the alterations continually made therein, as well artificial, by excavations and such like other works, as natural, by corrosions caused by fire and water, or by the generation of metals and stones, cannot but in progress of time change its situation; as rivers cannot remain long without winding and changing their course, if it happen that the ground over which they run be unequal, or of a different nature.

If this should be the case, there would be no hopes of finding a regular hypothesis for that change, forasmuch as it would depend on causes that have no regularity at all in them, as most of the mutations of nature are.

From this observation mathematicians are invited from time to time to make the like in their countries, to see whether in this change there be any regularity. If it had been observed every year we should already know the progress thereof, and see whether there were an uniformity, and in what time the needle did exactly respect the pole. Wherefore it were very desirable that, for the future, ihey would use greater care and diligence in making most exact meridians, as well for their own observations as for the conveniency of those who in their travels shall have the curiosity of observing with the needle itself, as M. Auzout designed to do in the cities where he passed, if he had found meridians there, or such as had been unsuspected of the proximity of iron.

It were well to observe whether the declination which almost through all Europe has been eastward, be now every where westward; as also, whether in America, where the declination was almost every where westward, it be increased or no proportionably; and so of other parts of the world.

So far this relation : in pursuance of which, order has been given by the Royal Society, that precise meridians be made in several places of England, for observing the present declination of the needle from them here in London,and other cities of this kingdom; and

that even those meridians that were made very exactly many years ago be examined by a careful describing of new ones, to see whether they still hold true, in regard of the suspected alterations in nature.

Extract of a Letter, written by M. Hevelius, from Dantzick,

July 5, 1670; containing chiefly a late Observation on the Variation of the Magnetic Needle, with an Account of some other Curiosities in those Parts.

In the year 1642 I observed the declination of the magnet here, as about the same time at Konigsberg did M. Linnemann, the then professor of mathematics there, and we both found the magnetic needle at that time to decline from the north 3° 5' westward. But now (Anno 1670) it is far otherwise; for it declines at present, as I bave very carefully observed, 7° 20' to the same quarter; so that in the space of twenty-eight years that declination is increased 4° 15'. In the year 1628, if I remember right, I found it near 1° westward; which declination was affirmed by the learned Peter Cruger (once my worthy preceptor) to have been about the beginning of this age, or the end of the next foregoing, 8° 30' eastward ; the same Cruger also, making use of that eastern declination in describing all his dials, as may be seen in the tract he has written on that subject, though it be not certainly known, by whom, and in what year that observation was made.

Further, it appears by our more recent observations, that this declination of the loadstone does here, at Dantzic, almost every seventh year, or, to speak more precisely, every sixth year and seven months, increase to one whole degree, and so each year to 9' 6". Which is sufficiently confirmed by the observations made at Limehouse, near London, by those three famous Englishmen, Burrow, Gunter, and Gellibrand : of whom the first found the declination, An. 1580, to be 11° 16'; the second, 5° 0' 30", An. 1622; the third, 4° 3' 30", An. 1634.

Lastly, it being now certain that the needle's declination varies in one and the same place; the accurate observations of the subsequent years will show how far this deflection will proceed, and where, and in what distance from the true meridian, the very bounds of this declination really are; especially, whether this libration and variation will be the same, and regular at all times and in all places; or whether, and how long, it will remain stationary. All which particulars, that they may be accurately discovered, is a thing very much desired. Possibly considerable speculations and researches may arise from such observations. As for me, I am almost of the opinion, that this magnetical diversity comes from the motion of the earth. Doubtless, as there is a certain libration in the moon, so it is not absurd to me, to hold a kind of libration in the earth, from the annual and diurnal motion of the same. For that the cause of this declination and variation of the loadstone is inherent in the stone itself, or lo be ascribed to ethereal corpuscles, is not imaginable by me; nor

can I vet devise any cause of those appearances, except we impute them to the globe of the earth, and the variation of the meridian.

On the Magnetical Variation at Nuremberg, in the Year 1685.

By M. G. C. EIMART. In the beginning of August, 1685, having taken all possible care to be certain of our meridians, we tried several magnetic needles, both those old ones we had employed in observing about five years before, and many new ones, of a middling length, the longest not above six inches, but which were more slender and active; and, what is strange, we found the declination of the needle not to vary one minute, but exactly agreeing with the former in every meridian, being again just 5° 5' io the west. Whether it has gone further in the meantime, or its deviation be now retrograde, which might by chance be the case, is not certain ; for I am not willing to assert that, being rather inclined to think it has been stationary at that point, its motion doubtl: ss being circular.*

On a new kind of Magnetical Compass, with several curious Magne

tical Experiments. By M. De La Hire, of the Royal Academy of Sciences at Paris, 1687.

There is nothing which creates so much trouble in long voyages on the sea, as the variation of the magnetic needle, both because this variation is different in differing places, and because in the same place it changes considerably in process of time. It seems that if we had exact observations of the irregularities of this variation made all over the carth, and at considerable intervals of time, we might discover some period of this motion, and establish a system which might be of great use in navigation. But since our oldest observations were made but about 100 years since, and in some particular places only, they only serve to show, that if there be a regular motion, it must needs be very slow ; so that we can conclude nothing certain for the time to come from all that has been hitherto observed. This is not because of any difficulty that there is in ascertaining this variation by observation, since it is found to change but few minutes in a year; but too much reliance must not be laid upon the observations of pilots, by reason of the gross errors which it is not easy for them to avoid. For it often happens that near the place where the compass is, there is much iron, which draws the needle, and causes it to show a point on the horizon much different from what it would were it farther from the iron; which makes it appear as if there is

* This doubt has probably been owing to some inaccuracy in the observations, as the variation has certainly been gradually increasing westward in Europe, since about the year 1666, and is now near 23° west ; but whether at present stationary seems to be uncertain, owing to the want of observations on this curious pheno


« ZurückWeiter »