Nautae etiam mare legentes, cum beneficium claritatis solis in tempore nubilo non sentiunt, aut etiam cum caligne nocturnarum tenebrarum mundus obvolvitur, et ignorant in quem mundi cardinem prora tendat, acum super mangentem ponunt, quae circulariter circumvolvitur usque dum, ejus motu cessante.
Mariners at sea, when, through cloudy weather in the day which hides the sun, or through the darkness of night, they lose knowlege of the quarter of the world to which they are sailing, touch a needle with a magnet, which will turn round till, on its motion ceasing, its point will be directed towards the north.

Qui ergo munitam vult habere navem habet etiam acum jaculo suppositam. Rotabitur enim et circumvolvetur acus, donec cuspis acus respiciat orientem sicque comprehendunt quo tendere debeant nautaw cum Cynosura latet in aeris turbatione; quamvis ad occasum numquam tendat, propter circuli brevitatem.
If then one wishes a ship well provided with all things, then one must have also a needle mounted on a dart. The needle will be oscillated and turn until the point of the needle directs itself to the East* [North], thus making known to sailors the route which they should hold while the Little Bear is concealed from them by the vicissitudes of the atmosphere; for it never disappears under the horizon because of the smallness of the circle it describes.

Gilbert shall live, till Load-stones cease to draw,
Or British Fleets the boundless Ocean awe.

I have been driven to assume for some time, especially in relation to the gases, a sort of conducting power for magnetism. Mere space is Zero. One substance being made to occupy a given portion of space will cause more lines of force to pass through that space than before, and another substance will cause less to pass. The former I now call Paramagnetic & the latter are the diamagnetic. The former need not of necessity assume a polarity of particles such as iron has with magnetic, and the latter do not assume any such polarity either direct or reverse. I do not say more to you just now because my own thoughts are only in the act of formation, but this I may say: that the atmosphere has an extraordinary magnetic constitution, & I hope & expect to find in it the cause of the annual & diurnal variations, but keep this to yourself until I have time to see what harvest will spring from my growing ideas.

I must reject fluids and ethers of all kinds, magnetical, electrical, and universal, to whatever quintessential thinness they may be treble distilled, and (as it were) super-substantiated.

In like manner, the loadstone has from nature its two poles, a northern and a southern; fixed, definite points in the stone, which are the primary termini of the movements and effects, and the limits and regulators of the several actions and properties. It is to be understood, however, that not from a mathematical point does the force of the stone emanate, but from the parts themselves; and all these parts in the whole—while they belong to the whole—the nearer they are to the poles of the stone the stronger virtues do they acquire and pour out on other bodies. These poles look toward the poles of the earth, and move toward them, and are subject to them. The magnetic poles may be found in very loadstone, whether strong and powerful (male, as the term was in antiquity) or faint, weak, and female; whether its shape is due to design or to chance, and whether it be long, or flat, or four-square, or three-cornered or polished; whether it be rough, broken-off, or unpolished: the loadstone ever has and ever shows its poles.

Magnetic lines of force convey a far better and purer idea than the phrase magnetic current or magnetic flood: it avoids the assumption of a current or of two currents and also of fluids or a fluid, yet conveys a full and useful pictorial idea to the mind.

Since the discovery of secret things and in the investigation of hidden causes, stronger reasons are obtained from sure experiments and demonstrated arguments than from probable conjectures and the opinions of philosophical speculators of the common sort; therefore to the end that the noble substance of that great loadstone, our common mother (the earth), still quite unknown, and also the forces extraordinary and exalted of this globe may the better be understood, we have decided first to begin with the common stony and ferruginous matter, and magnetic bodies, and the parts of the earth that we may handle and may perceive with the senses; then to proceed with plain magnetic experiments, and to penetrate to the inner parts of the earth.

The transition from sea-floor spreading to plate tectonics is largely a change of emphasis. Sea-floor spreading is a view about the method of production of new oceans floor on the ridge axis. The magnetic lineations give the history of this production back into the late Mesozoic and illuminate the history of the new aseismic parts of the ocean floor. This naturally directed attention to the relation of the sea-floor to the continents. There are two approaches: in the first, one looks back in time to earlier arrangements of the continents; in the second, one considers the current problem of the disposal of the rapidly growing sea floor.