Honour’d Sir

The very great advantages that would have accrued to the Sugar Islands from the Horizontal Windmill I lately constructed at St. Christopher had it’s power been greater, and the probability that the improvement Your Excellency so benevolently suggested may encrease that power sufficiently to answer the purpose of grinding Canes, both confirm me in the opinion of the expedience of some Magazine of mechanic Knowledge to which a beginner in the Science might resort and there see ready done to his hand, what otherwise he might employ years to bring to perfection. Nothing has put a greater stop to mechanic and indeed all Philosophical improvements than even the adepts being frequently oblig’d actum agere, to labour at experiments that have been made, but are lost for Want of having been recorded. Benefits will result from commemorating attempts, that have been unsuccessful; fruitless labour sav’d is no small advantage, and the time that would be thrown away might be employ’d in what might be of real use, and perhaps considerable emolument to the Community. Permit me therefore to suggest to that Philosophical Society of which you are President, their establishing and acquainting the World, when they have such a Magazine, open to the inspection of those, whose Genius leads them to attempt any (to them) new improvements. Your Excellency’s distinguish’d abilities may direct the Members to contribute, each in their several ways, plans necessary for the compleating so useful a Work; and, as bound in duty and gratitude for your genteel Attention, when the Experiments of the Horizontal Mill are compleated, I will remit a faithful Account of them with explanatory drawings to Your Philosophical Society. In the mean time as I find no farther Account of my Treatise on Magnetism has reach’d this place, than what the Critical Review of 1761 furnishes, I will endeavour to recollect what I can of it.

Some new Discoveries in that Branch of Natural Philosophy proving that there can be no such thing in Nature as an internal Central Loadstone: By the Revd Temple Henry Croker A.M. of Christ Church Oxon: Reader of the Temple Church and Chaplain to the Earl of Hillsborough, 1761.

“Quid quaeris, Carneades, Cur haec ita fiant? Non reperio Causam: latet fortasre obscuritate involuta Naturae; non enim me Deus istam scire, sed bis tantummodo uti, voluit.

Experimental Philosophy has shown the Variety of those Sciences, founded upon Data. System’s are exploded now, tho’ when their Authors had their Principia allow’d them, they boldly accounted for every appearance, but now when their first Position is denied, their Science is at an End. Neigher Des Cartes nor Dr. Halley have even approach’d to the knowledge of Magnetism, tho’ each have laid down a System for it; but they suppose a Cause and thence establish Effects, whereas Truth will sooner result from considering effects from them to find the Cause, or honestly confess it past finding out.

The Ancients knew nothing of the directive force of Magnetism, but amus’d themselves with its attractive quality. Some modern Philosophers have adopted that frivolous persuit and miscall’d it Science. The Chinese pretend to have employ’d it usefully to the purpose of Sailing about 400 Years before Christ, but Marco Paolo of Venice was more probably the real Discoverer of the Compass about the middle of the 13th Century after that blessed Ara. Flavio de Gioia of Naples claims the invention in 1,300, and the French make the same claim about a Century before him, but till the beginning of the 14th. Century the Compass was not known or at least not us’d in Navigation. Sebastian Cabot of Venice first observ’d the Variation of the Magnet, or it’s declination from the Pole, in 1,500 and Mr. Gallibrand, of England, observ’d the Variation of that Variation, or its different direction in different years, in about 1620. Mr. Canton of England shew’d the diurnal Variation and settled it with great Accuracy before 1750, so that all the Laws of the Horizontal Magnetism were known in about 300 Years after the first discovery of its use.

Vertical Magnetism or the Dipping Needle moving in a vertical plane was discover’d by Norman, Compass Maker in Wapping in 1576, which acts not as the Horizontal Needle, but to a point directly under it in the same Vertical, which point was then at 71°10' and is now at 74°30'. He thought its power greater than the horizontal and call’d 71°10' the Line respective, denying its being directed or attracted to any point, but supposing each Magnet a little Universe govern’d by its own laws and disregarding any impulse from the Earth or Heavens. Mr. Whitton found the Power of the Horizontal to be only One fourth Part of that Power which govern’d the Vertical or Dipping Needle, therefore concluded it the best instrument for Magnetic Observation, as it certainly would have been had it been influenc’d by pure Magnetism only unmix’d with any heterogeneous Cause. Dr. Halley and others made the same attempts but could never make the Dipping Needle answer any useful purpose. The reason will appear, when Experiments show it is influenc’d by two Powers and only one accounted for. The Publick were at the Expense of several Voyages, from which Dr. Halley plann’d a System, ingenious indeed, but neither satisfactory nor true. As to the last discovery of Artificial Magnets, they are the Cups and Balls of the Science; for, increasing the Attractive Power has no effect in settling the Directive: Every Magnet has both those Powers, but neither depend on, or are any Argument of the Strength of the other; A needle of double refin’d Steel, (Such as used by Watchmakers for their tools) lifting but one Ounce, will come to its point as soon and true, as a Needle of blister’d Steel that will lift seven Ounces; and has this advantage, that Electricity or lightning has no effect upon it, tho’ either will destroy the Virtue or invert the Poles, of the blister’d Steel needle.

After premising thus much I shall lay down some Magnetic Laws and Observations, and as many of the Experiments that prove them, as I can possibly recollect with the help of a few accidental minutes.

I. The Magnet is attracted to the Horizon, not to 75° below it, therefore there can be no internal Central Loadstone.

II. Needles when touch’d, lose gravity from the South, and, the North continuing as before therefore, as that destroys the balance they were possessed of before, Needles should be balanc’d (across the Magnetic Meridian) after touching not before it, as now erroneously done: for Attraction gives the Polarity and Gravity gives what is called the Dip. III. Seventy five Degrees is that place of rest, which the Needle (balanc’d before and not after touching) finds out; because that is the place where the horizontal Attraction and gravity meet in their powers. IV. The North Pole of a balanc’d Magnet will tend Northwardly to the Horizon from any Degree of elevation or depression from the South Horizon. V. No Needly, but the Norizontal, when at liberty to move in a vertical as well as horizontal plane, carries its points from N to S at equal heights. VI. A balanc’d Magnet, thrown into different vertical planes preserves an equality of motion in its approach to and recess from the horizon, which no other needle will in its approach to or recess from the point of it’s direction; therefore as Nature is ever regular in simple effects, the balanc’d Magnet seems to be the only one, actuated by a single power, that is, by attraction only, unmix’d with gravity, or any heterogeneous Cause. VII. Magnetism is no fluid, nor does it act upon Surfaces; the Magnet whether it is edgeways or flatways, attracting with equal power. These are the Effects. The Cause of Magnetism remains still unknown, and, at least till further discoveries (which may perhaps result from Experiments) can only be resolv’d into those general laws of Nature, which the Allmighty Being has stamp’d on the Creation; the Vis Magnetica, whatever that occult power may be, acting and directing Needles to the Horizon only.

Expt. I. Prepare a Magnet mathematically every straight and exact, twelve inches long: balance it after touching on its axis, so that it may rest at any Degree of elevation while it crosses the magnetic Meridian at right angles. Then set the N end of the Magnet 75°. below the Horizon; then turn the frame to the North, the Magnet will immediately forsake the place where it stood in the first position and so far from being inclin’d to 75° below the Horizon it will ride with some force towards the Zenith, make several vibrations and settle nearly horizontal at last. If replac’d and turn’d towards the S, when nearly on the Meridian, it will incline Nward, have no manner of stop at 75° the suppos’d place of greatest attraction, but rise to Horizon, pass it, vibrate and settle nearly horizontal. Can this proceed from attraction and has gravity no Share in it? The magnet when in such a situation, as not to feel or show the force of Attraction, would rest at any point, and whatever can destroy that state of rest, must be attraction and convince us that the Dip of the unbalanc’d needle to 75°, as not being attraction, must be gravity. There cannot therefore be any internal contral loadstone, for if the Needle was attracted so near the Center as 75° by such a loadstone, it would be almost as much affected when across as when on the Magnetic Meridian; but when across, it stands at any point and when on it, at the Horizon only.

Expt. II. Prepare a Dipping Needle imbalanc’d, hang it on it’s Axis: In England it points to 75°, below the Horizon: Add weight to the S end till it is horizontal: Take away the weight and weigh the Needle in a nice pair of Scales: then take it out and destroy the verticity of the needle, letting the Weights still remain in the aoopsite scale: then put the needle into the same scale as before and you will find the weight so much increas’d by the Verticity being thus destroy’d, that in order to balance the Scales you will be oblig’d to put the former counterpoise (us’d at the S end when it had it’s verticity perfect) into the opposite scale against the Needle. Mr. Whitton, who was surprizingly accurate, observes that a needle after touching is lighter than before. This Experiment proves that the weight is lost from the S end. Magnetism in needles, twelve inches long is but as one to 300, and the Weight lost but a 4,000 part. The difficulties of friction and resistance may obscure the truth, but the Proportion between absolute gravity and the tendency to 75°. is sufficient to account for that gravity. This tendency has been ascrib’d to attraction, because it accompanies it and as it seems the most powerful, horiaontal attraction has been deem’d onlya part of the other; but they are distinct properties, tho’ belonging to the same Agent, and they produce different effects. Attraction gives the Polarity, Gravity gives the Dip and they who reason upon these two Properties as if they were but One, can no more attain to the real true nature of the, than if they were to account for a piece of Metal as pure Gold in the Hydrostatic balance, which has a considerable allow of Silver.

Expt. III. Balance a Magnet across the Magnetic Meridian. Turn the N end to the S elevating it 75° above the Horizon. Being then in direct opposition to the hithertosuppos’d point of greatest attraction, One would expect that it would be equally dispos’d to return to that point by turning to the South and would go that way as easily as by the North; but the effect is quite the reverse. The Experiment was suggested by the common Mariner’s Compass, which when turn’d opposite to it’s due tendency will return either way indifferently, the Attraction having equal force and being the same, whether by East or West: It therefore returns the way most favour’d by it’s situation; but it is not so in the supposed Dip. For, bring the N end of the Magnet down from 75°. above the S Norizon to only 60° above it, still it will rather rise, than descend. Bring it down to 45, to 30, to 20, and even to 10, still it will rise instead of descending: Then bring it Horizontal or opposite the place where it settled itself in the N. Here the same effect will appear, as in the horizontal magnet: it will remain there, as being the place exactly opposite to that which it naturally inclines to. Why? Not thro’ choice, but because in that position the power of attraction cannot influence it, as drawing it like the reins of a bridle equally on each side, and not being able to overcome the friction or it’s Centers, but depress it a little lower, it will be attracted, pass the Nadir and after some vibrations settle itself horizontal to the N. These Experiments are sufficient to show the Errors of Dr. Halley’s System; and that of Des Cartes Magnetic Effluvia continaully issuing out of the S, moving round the Earth in a Magnetic Atmosphere, and reentering at the N pole, is equally frivolous, for then Magnets must be affected in proportion to their surfaces, not their Contents, but the Magnet is attracted with exactly the same power edgeways as flatways. Many more Experiments are in the treatise, which equally confirm the fifth and Sixth Laws, but of them I have no Minutes and they are too nice to be recollected. The Machine by which the horizontal tendency was discover’d is as impossible to be describ’d without the book, for which I have written to England and if I procure one, will remit it to the Phil: Society: Let the Members persue Magnetic Enquiries with these lights and womthing curious may result: The Discovery of the horizontal tendency of the Magnetic power seems to open a new and large field for their enquiries, by it’s acting thus at right Angles to the Action of the Elements. Earth and Water tend to the Nadir, Fire to the Zenith, Air in all directions and Magnetism to the Horizon. The most active Powers in Nature are unimpeded by each other in their progress, tho’ crossing at right Angles and at all others with respect to Air and Magnetism. All the Facts laid down may be depended upon with the greatest Security, and this Conclusion must follow.

Magnetism is no Fluid, as not acting upon Surfaces, but substance. It’s direction is not to 75° below the Horizon, but to it. Consequently there can be no internal Central Loadstone.

The Cause therefore of Magnetism remains still unknown, notwithstanding the confident assertions of Philosophy and at least till further discoveries (which must result, not from fancy, but from Facts, not from artfully devised Systems, but from real Experiments) can only be resolv’d into those general laws of Nature, which the Allmighty Creator has stamped upon his Creation, tho’ He as yet withholds them from our weak sight, to battle human pride, and humble the Arrogance of presumptuous Philosophy. I have the Honour to remain Your Excellency’s most oblig’d most humble Servant

His Excellency Benjamin Franklin President of Pensylvania and the Philosophical Society.