An Abstract of a Letter from Petrus Van Muschenbroek, M. D. F. R. S. Professor of Mathematicks and Astronomy in the University of Utrecht, in Holland; To Dr. J. T. Desaguliers, F. R. S. concerning Experiments Made on the Indian Magnetick-Sand

Aliquando enim tardè & cum pessimorum Symptomatum Satellitio erupere variolæ; aliquando incassum cessit Incisio; illa tamen loca Inoculationis ulceribus cacoetheis repleta fuerunt. Nonnullis etiam lethalis fuit talis Operatio: Mumia enim fermentativa, e marcidis istis tuberculis, rancida ac veluti cadaverizata non solum experito scopo haud satisfacit, sed pravam sanguini poteat communicare putrilaginem. VII. An Abstract of a Letter from Petrus Van Muschenbroek, M. D. F. R. S. Professor of Mathematicks and Astronomy in the University of Utrecht, in Holland; to Dr. J. T. Desaguliers, F. R. S. concerning Experiments made on the Indian Magnetick-Sand. S I R, I don't know whether I dare take up your Time with my Trifles; yet I'll venture for once to acquaint you with some things concerning the Indian-Sand, which is attracted by the Loadstone. The Indian-Sand which is brought to Holland, is said to be chiefly gather'd upon the Sea-shore in Persia; then it is boiled in Water, to free it from its Saltnefs, and it is after this a black Powder, consisting of Grains of different Bignefs; some of which have a very rough Surface, and others have one part of their Surface something rough, and the other very shining: Their Figure is very irregular, like Grains of com- common Sand, only this Indian-Sand is smaller. These little Lumps have neither Taste nor Smell, and are friable, so as to be easily reduc'd to a very subtile Powder. It has some Parts, which are strongly attracted by the Loadstone; and others so very inactive, as scarcely to seem to be magnetical: the strongest are the blackest; but the inactive ones are more shining, and more inclining to the Colour of Lead; these are in the greatest Quantity, and from them the others are got out by a Loadstone. The Ingenious Moutenus has examin'd several ways such a kind of Sand which is brought from Virginia, and describ'd it in the Philos. Trans. N° 197. I have examin'd the Indian-Sand another way; of which, I have given an Account in my Physical Dissertations, pag. 127; but a great deal still remain'd to be considered, and as there is a great deal more of this Substance of the Lazy or Inactive, than of the Active or Magnetick sort, it was proper to try whether a Magnetick Virtue might not be excited or increased in all of it; and after a few Trials I found the Thing to succeed. I suspected that there might perhaps be too great a quantity of Sulphur adhering to the Sand, to suffer it to be turn'd into any Metalline Regulus by a long Continuance in the Fire; therefore, I toasted it in an open Crucible for two Hours with half the quantity of Pot-ash; afterwards I washed away the Salt with Water, and the Sand remain'd much blacker than before, of which I found more than a Quarter endued with a greater Magnetick Force. I do not scruple to attribute this Virtue to the Salt; because, tho' the Action of the Fire alone does encrease the Force of the Sand, yet yet it does not give it near so much attractive Force. Because common black Soap is made of Oil boiled with a Lixivium of Pot-ash, I had a mind to try whether Soap might not do more than Salt alone in raising the Virtue in the Sand; so I mix'd the Sand with an equal quantity of Soap, which I first expos'd to a gentle Fire in an open Crucible, to dry up the Soap which swells very much; then the Fire was encreas'd for three quarters of an Hour, all the oily Substance wholly consum'd, and the Matter in the Crucible was strongly fir'd; then afterwards boiling it in Water, and washing it well, I obtain'd a black Sand, which was all endued with a lively attracting Force. Very well pleas'd with this Success, I had a Mind to try whether I might raise a greater Force in it; wherefore I again roasted it with black Soap as before, and even a third Time; but no Addition was thereby made to its Virtue: I find that staying too long in the Fire is as prejudicial as staying too short a Time, between half an Hour and an Hour seem'd to me the most proper space of Time. After, I added to the black Soap half of Salt of Tartar, and mix'd thereto an equal quantity of Sand; which, when it had been exposed to a reverberatory Fire \( \frac{1}{4} \) of an Hour in a Crucible, I wash'd in Water; and then so great was the Virtue of the Sand, that if it did not exceed the former, at least it was equal to it. Because I had observ'd the Oilinefs of the Soap to conduce much to excite the Vertue in the Sand; I mix'd Beef-Tallow with an equal quantity of Sand, and having very well clos'd the Crucible, I expos'd the whole Mass to a reverberatory Fire for two Hours, whereby the Sand became much blacker, and receiv'd a great deal of attractive Virtue: but that Sand became much more active which was burn'd two Hours with an equal quantity of Pitch, as likewise very black, subtile, and very little shining: but when it was expos'd a longer Time in the same Crucible, I observ'd it to be weaker; as also, when it was in the Crucible with the Pitch but $\frac{1}{8}$ of an Hour, it scarce acquir'd any Virtue; so that there must be a determin'd Action of Fire to raise the Vertue in the Sand. Yet I cou'd not raise a greater Virtue in the Sand than by the means following, viz. mixing the Sand in the Crucible with equal parts of Rosin, Pitch, Frankinsence, and Rape Oil, and exposing it to a reverberatory Fire for an Hour, having first well clos'd up the Crucible. Between the black Coals of the oily Matter, theresticks a very black Sand, which leaps up swiftly to the Loadstone, as soon as it is brought near it. Then I consider'd whether the Sand did not acquire the greatest Force as it came nearer to the Nature of Steel, by burning it with the Bodies abovemention'd; and suspecting this, in order to try it, I put it among such Bodies as turn Iron into Steel, according to the Operations describ'd by that great Experimenter Monsr. Reaumur, in that excellent Book, entitluled, The Art of turning Iron into Steel. I took therefore three parts of Sand, two parts of Chimney-Soot; and of Sea-Salt, Powder'd Charcoal, and Ashes, one Part each. Having accurately curately mix'd all these Bodies together, they were expos'd for six Hours in a close Crucible to a strong Fire; and then the whole Mass was boil'd and wash'd in Water, then dried, and so received a great deal of attracting Force; but it was not near so active as that which was prepar'd with Soap, or in the manner last describ'd. And now, what can this Sand be? Is it an imperfect Magnet, or subtile Powder of it, which when it is grown up into a greater Lump, makes the vulgar Loadstones? So I conjectur'd at first; but when I found by Experience that common Loadstones expos'd to the Fire, according to some of the Methods above-mention'd, did rather lose of their Force than gain, I alter'd my Opinion; and now confess that I have not yet penetrated into the Knowledge of the Nature of this Matter. Whatever it be, it is certain that there are several kinds of this Sand, brought from different Countries of the Earth: For it is brought from Persia; some is brought from Virginia; there is another sort in Italy, which is common enough at Leghorn, and this last is naturally very attractive; there are two sorts found in the Eber, a River of Hassia; of which, one is like the Italian, and the other consists of large Grains, almost as big as Hemp-Seed, but scarce having any Virtue. I have besides a very strong Sort, which I am told was got near old Ragusa in Dalmatia. Nobody knows how many kinds of this Sand there are: that Time, and the diligent Observations of Philosophers must hereafter shew. I herewith send you a little Box, containing one Paper with the natural Sand; another, the Sand after having burn'd it with Soap in the manner describ'd. It is no Treasure; but if you have none of it, it may be worth your Acceptance. I am, Sir, Utrecht, Jan. Your, &c. &c. 15th, 1733. O.S. Petrus Van Musschenbroek VIII. An Account of some Observations made in London, by Mr. George Graham, F.R.S. and at Black-River in Jamaica, by Colin Campbell, Esq., F.R.S. concerning the Going of a Clock; in order to determine the Difference between the Lengths of Isochronal Pendulums in those Places. Communicated by J. Bradley, M.A. Astr. Prof. Savill. Oxon. F.R.S. Altho' it is now above Sixty Years since Mr. Richer first discovered, that Pendulums of the same Length, do not perform their Vibrations in equal Times in different Latitudes; and tho' several Experiments made since in different parts of the Earth concur to prove, that Pendulums swinging Seconds are in general shorter as we approach the Equator; yet what the real Difference is between their Lengths in different Latitudes, does not seem to have been determined with sufficient Exactness,