A Letter of Mr. de la Hire of the Royal Academy of the Sciences at Paris, concerning a New Sort of Magnetical Compass, with Several Curious Magnetical Experiments

A Letter of Mr. De la Hire of the Royal Academy of the Sciences at Paris, concerning a new sort of Magnetical Compass, with several curious Magnetical Experiments. YOU know Sr. that there is nothing which creates so much trouble in long Voyages on the Sea, as the Variation of the Magnetical 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 Earth, and at a considerable interval of time, one might discover some Period of this motion, and establish a System which might be of great use in Navigation. But seeing our oldest Observations were made but about a hundred Years since, and in some particular places only, they only serve to let us know, 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 upon the Observations of Pilotes, by reason of the gross Errors which it is not easy for them to prevent. For it often happens that near the place where the Compass is, there is much Iron, which draws the Needle, and causes it to shew a point on the Horizon much different from what it would, were it farther from the Iron; which makes it be thought that there is a considerable Variation where perhaps there is none at all. And it may so happen that in the same place where the Year before an Observation was made, if in the next, the Iron Instruments be found otherwise placed than they were the time before, either in the same Ship or another, the Needle will shew a Variation much differing from that found the first time. And this sort of Caution not being Observed at Sea may be cause of very great Errors in the Observations of the Needles Variation tho' not affecting the Course of the Vessel. For the Needle being drawn after a certain manner will constantly observe the same situation in respect of the North, provided the Iron round about it be not stirred: And you shall not fail to steer true upon any point of the Compass, if this false Variation be observed after the usual Manner by the Amplitudes of the Sun. We cannot therefore hope to be secure of any thing 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 from 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 propositions had had no effect; I judged that after all that they had done by means of the Loadstone, it was not to be hoped to draw any farther 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 Enquiry, when there accidentally fell into my hands a Terrella or Spherical Loadstone, of three Inches Diameter; with which being minded to make some Experiments, with a little Needle whose foot might easily be placed upon the Stone, I soon Observed that which hath been already noted by several, viz. that this Globe of Magnet caused the Needle to have the same changes which are found in the Compass in differ- rent 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 upon tryal 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 shewed me the Pole of the Stone) but that the Needle however placed always directed itself some one way. I determined by this means, as well as I could, the Point called the South Pole; but I was much surprized to find it 18 Degrees distant from a Cross deep engraven on the Stone, which according to all appearance had heretofore been the Pole of this Stone, as it had been Observed by him that 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 Vertue changed in the Loadstone, as we see they change in the Earth, one might derive great advantages therefrom as to the Variations of the Magnetical Needle. For if this change of these Poles in the Load-stone were certain, and that it was Analogous to the change of the Poles of the Magnetique Vertue in the Earth, it is not to be doubted but a Terrella, being suspended at liberty, would remain immoveable, and that one point thereof would regard the Pole of the World, which might be called the true Pole of the stone, whilst the Poles of its Vertue would pass successively from one part to another, after the same manner as they change in the Earth. After having well considered this Hypothesis, and having 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, thirteen Degrees 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 five Degrees Westward. Upon this Hypothesis, which I know not that any one else has yet thought upon, I have invented a new sort of Needle for the Compass, which may have the same alterations as a Spherical Load-stone, and at the same time the same conveniencies as the ordinary Needle hath. I caused a Ring of three Inches diameter to be made of Steel Wire; from which there went three Radii of very fine Brass-wire meeting at the Center in a Cap perfectly like that of an ordinary Compass, that so this Circle might rest on a Pin in its Center, and be at full liberty to turn round, its Center being fixt. This done I gave the Magnetical touch to this Steel Ring, by applying indifferently to a Point thereof, one of the Poles of a strong Load-stone, and the other Pole of the Stone to the opposite Point, to give the greater Vertue to the Ring. Then I observed that the Ring was strongly Magnetical, and that the Point called the South Pole did readily turn itself towards the North, and after several Vibrations stopped there; and that 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 Magnetick Vertue change in the Load-stone after the same manner as they do on the Earth; it seems likely that the same thing should happen to this Ring, and that one Point thereof should always exactly respect the North. But to informe my self if a Steel Ring had the same effects as a Terrella, I made the following Experiment. Having touched a Steel Ring, and having 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 ordinary Hypothesis of the Magnet, the Magnetic Vertue 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 Load-stone, so as the Point of the Iron reached beyond the Stone; and having afterwards presented this point to the Magnetic Ring, I observed that different Points of this Ring did apply to the Point of the Iron, according as the several parts thereof had been applied to the Stone: which happens not in the Magnetic 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 Magnetic matter in all the parts thereof analogous to those of the Stone. It must only be noted that in an irregular Stone the Magnetic Vertue 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 Semi-circles of Steel. Having joined the two ends touched by the same Poles, I observed by the Steel-dust the same effect as in the Ring. But having joined the ends differently touched; I found that immediately the two half-rings run together and stuck to each other; and by the Steel-dust strowed on Paper I observed, that there were four Vortices, one in the middle of each semi-circle, and one at each of the places where they were 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 semi-circle the Vertue of the ends of the half Rings. I would trie, after having touched a Steel-Wire that was straight, to make a Ring thereof; but I found that it had quite quite lost its Vertue: which cannot be attributed to the junction of the Poles, since they ought to stick together, according to the other Experiments which have been made; but only to this that hath been already noted, that when a Magnetical Virgula is a little bent, it looses its Virtue, which cannot happen but from the alteration of the Pores of the Steel. I farther remarked that a Ring of Steel having been touched does for a long time retain its Vertue although it be put in a position contrary to its Poles. And this Experiment is confirmed by another much more considerable: Which is, that a Ring of Steel having been 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 little and little it resumes its former Vertue, much as we see Magnets do, which being applyed to another Stone, by the Poles of the same denomination, loose their first Vertue and take a contrary; which they afterwards loose by degrees, to reassume their first. After I had presented this new Systeme of the Magnet to the Academy, there were made some Experiments upon a Terrella of Much the same diameter with mine, but whose Poles were not diametrically opposite; and upon a half-Globe very much bigger than the Terrella. We could find in them no considerable difference or alteration of Poles: Yet because of some circumstances, the Company thought fit that some Experiments should be made with this sort of Compass. If some of these Compasses were carried into very remote parts, where it is known that the Magnetical Needle has a great Variation; one might be certain in little time whether this Hypothesis holds or no, and whether we may expect from it those advantages, which I have concluded from the supposed immobility of a Terrella hung at liberty. It remains only to explain after what manner these circular Needles may be touched anew, when it is perceived that they have lost their first Vigour. According to this Hypothesis, it is evident that if the Circle be not touched in the point that answers to that of the Stone, with regard to its Variation, the little Fleur de Lis which marked the true North, may decline a little from it; and the difficulty of finding the corresponding points on the Ring and the Stone, would cause that the touch of the Circle could not be refreshed, without taking great care and first observing the Meridian line. But to avoid all these difficulties, you need only apply the Poles of the Stone to the Ring; and the Ring, which is suspended upon its pivot, will turn so as the Point answering to the Pole of the Vertue of the Stone which is applied to it, will come as near to it as possible: In so much that without touching the one or the other, the Ring will not fail to receive very much force. The same may be done at the opposite Pole. I doubt not but you are curious enough to see if the Poles do change in the Terrella, when you shall meet with one fit for this Experiment. There might several other things be noted upon this Subject, and it were to be wished that some other particular observations might be made as opportunity shall offer; but unless curiosities of this nature fall into the hands of such as have a great love for the advancement of the Sciences, it is not to be hoped that we shall have any certain information in a matter so nice, &c. Paris April. 26. 1687. This Letter having been produced and Read before the Royal Society; it was Ordered that the Terrella, which has been in their Repository these 25 Years, the gift of their Royal founder King Charles the Second, should be examined, to see if there be any sensible alteration in the Poles thereof: And upon tryal it was found that the Points which are marked thereon with crosses, were as near as could be discerned the true Poles of the Stone; notwithstanding that the Variation has changed at London full 4 Degrees since this Torella has been in the Society's Custody; and perhaps many more since it was marked: and had there been a change in the Poles of the Loadstone analogous thereto, it must needs have been perceived in this, whose Diameter is about $4\frac{1}{2}$ Inches. However to put this matter past dispute, care was taken to find out exactly and mark the Poles of the Society's great Load-stone, the Sphere of whose activity is above 9 Foot Radius, and whose Poles are 13 Inches asunder, whereby 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 hath received the Magnetical Vertue may change place, after the same manner as the Poles of the Earth's Magnetism are observed to do; tho' it was lookt upon as an ingenious hint and worth prosecution, yet some of the Company, well skill'd in Magneticks were of opinion, rather that such a Circular Needle would librate on its Center, so as to respect the Magnetical Meridian with the Points that had at first received the touch, than that the Ring remaining immoveable, the dereactive Vertue should be transferred therein from place to place, either by length of time, or by transporting this Compais into those parts where the Variation of the Needle is considerably different. A R.