Chapter XIII: Part 13

Now it will not be superfluous to weigh well the arguments of those who say the Earth does not move; that we may be better able to satisfy the crowd of philosophizers who assert that this constancy and stability of the Earth is confirmed by the most convincing arguments. Aristotle does not allow that the Earth moves circularly, on the ground that each several part of it would be affected by this particular motion; that whereas now all the separate parts of the Earth are borne toward the middle in straight lines, that circular motion would be violent, and strange to nature, and not enduring. But it has been before proved that all actual portions of the Earth move in a circle, and that all magnetick bodies (fitly disposed) are borne around in an orbe. They are borne, however, toward the centre of the Earth in a {226} straight line (if the way be open) by a motion of aggregation as though to their own origin: they move by various motions agreeably to the conformation of the whole: a terrella is moved circularly by its innate forces. "Besides" (says he), "all things which are borne in an orbe, afterwards would seem to be abandoned by the first motion, and to be borne by several motions besides the first. The Earth must also be borne on by two sorts of motion, whether it be situate around a mid-point, or in the middle site of the universe: and if this were so, there must needs be at one time an advance, at another time a retrogression of the fixed stars: This, however, does not seem to be the case, but they rise and set always the same in the same places." But it by no means follows that a double motion must be assigned to the Earth. But if there be but one diurnal motion of the Earth around its poles, who does not see that the stars must always in the same manner rise and set at the same points of the horizon, even although there be another motion about which we are not disputing: since the mutations in the smaller orbit cause no variation of aspect in the fixed stars owing to their great distance, unless the axis of the Earth have varied its position, concerning which we raise a question when speaking of the cause of the præcession of the æquinoxes. In this argument are many flaws. For if the Earth revolve, that we asserted must needs occur not by reason of the first sphære, but of its innate forces. But if it were set in motion by the first sphære, there would be no successions of days and nights, for it would continue its course along with the _Primum Mobile_. But that the Earth is affected by a double movement at the time when it rotates around its own centre, because the rest of the stars move with a double motion, does not follow. Besides, he does not well consider the argument, nor do his interpreters understand the same. [Greek: toutou de sumbainontos, anankaion gignesthai parodous kai tropas tôn endedemenôn astrôn.] (Arist. _de Coelo_, ii. chap. 14.) That is, "If this be so, there must needs be changes, and retrogressions of the fixed stars." What some interpret as retrogressions or regressions, and changes of the fixed stars, others explain as diversions: which terms can in no way be understood of axial motion, unless he meant that the Earth moved by the _Primum Mobile_ is borne and turned over other poles diverse even from those which correspond to the first sphære, which is altogether absurd. Other later theorists suppose that the eastern ocean ought to be impelled so into western regions by that motion, that those parts of the Earth which are dry and free from water would be daily flooded by the eastern ocean. But the ocean is not acted upon by that movement, since nothing opposes it; and even the whole atmosphere is carried round: And for that reason in the Earth's course all the things in the air are not left behind by us nor do they seem to move toward the West: Wherefore also the clouds {227} are at rest in the air, unless the force of the winds drive them; and objects which are projected into the air fall again into their own place. But those foolish folk who think that towers, temples, and buildings must necessarily be shaken and overthrown by the Earth's motion, may fear lest men at the Antipodes should slip off into an opposite orbe, or that ships when sailing round the entire [249]globe should (as soon as they have dipped under the plane of our horizon) fall into the opposite region of the sky. But those follies are old wives' gossip, and the rubbish of certain philosophizers, men who, when they essay to treat of the highest truths and the fabrick of the universe, and hazard anything, can scarce understand aught _ultra crepidam_. They would have the Earth to be the centre of a circle; and therefore to rest motionless amid the rotation. But neither the stars nor the wandering globes move about the Earth's centre: the high heaven also does not move circularly round the Earth's centre; nor if the Earth were in the centre, is it a centre itself, but a body around a centre. Nor is it confident with reason that the heavenly bodies of the Peripateticks should attend on a centre so decadent and perishable as that of the Earth. They think that Nature seeks rest for the generation of things, and for promoting their increase while growing; and that accordingly the whole Earth is at rest. And yet all generation takes place from motion, without which the universal nature of things would become torpid. The motion of the Sun, the motion of the Moon, cause changes; the motion of the Earth awakens the internal breath of the globe; animals themselves do not live without motion, and the ceaseless activity of the heart and arteries. For of no moment are the arguments for a simple straight motion toward the centre, that this is the only kind in the Earth, and that in a simple body there is one motion only and that a simple one. For that straight motion is only a tendency toward their own origin, not of the parts of the Earth only, but of those of the Sun also, of the Moon, and of the rest of the sphæres which also move in an orbit. Joannes Costæus, who raises doubts concerning the cause of the Earth's motion, looking for it externally and internally, understands magnetick vigour to be internal, active, and disponent; also that the Sun is an external promotive cause, and that the Earth is not so vile and abject a body as it is generally considered. Accordingly there is a diurnal movement on the part of the Earth for its own sake and for its advantage. Those who make out that that terrestrial motion (if such there be) takes place not only in longitude, but also in latitude, talk nonsense. For Nature has set in the Earth determinate poles, and definite unconfused revolutions. Thus the Moon revolves with respect to the Sun in a monthly course; yet having her own definite poles, facing determinate parts of the heaven. To suppose that the air moves the Earth would be {228} ridiculous. For air is only exhalation, and is an enveloping effluvium from the Earth itself; the winds also are only a rush of the exhalations in some part near the Earth's surface; the height of its motion is slight, and in all regions there are various winds unlike and contrary. Some writers, not finding in the matter of the Earth the cause (for they say that they find nothing except solidity and consistency), deny it to be in its form; and they only admit as qualities of the Earth cold and dryness, which are unable to move the Earth. The Stoicks attribute a soul to the Earth, whence they pronounce (amid the laughter of the learned) the Earth to be an animal. This magnetick form, whether vigour or soul, is astral. Let the learned lament and bewail the fact that none of those old Peripateticks, nor even those common philosophizers heretofore, nor Joannes Costæus, who mocks at such things, were able to apprehend this grand and important natural fact. But as to the notion that surface inequality of mountains and valleys would prevent the Earth's diurnal revolution, there is nothing in it: for they do not mar the Earth's roundness, being but slight excrescences compared with the whole Earth; nor does the Earth revolve alone without its emanations. Beyond the emanations, there is no renitency. There is no more labour exerted in the Earth's motion than in the march of the rest of the Stars: nor is it excelled in dignity by some stars. To say that it is frivolous to suppose that the Earth rather seeks a view of the Sun, than the Sun of the Earth, is a mark of great obstinacy and unwisdom. Of the theory of the rotation we have often spoken. If anyone seek the cause of the revolution, or of other tendency of the Earth, from the sea surrounding it, or from the motion of the air, or from the Earth's gravity, he would be no less silly as a theorist than those who stubbornly ground their opinions on the sentiments of the ancients. Ptolemy's reasonings are of no weight; for when our true principles are laid down, the truth comes to light, and it is superfluous to refute them. Let Costæus recognize and philosophers see how unfruitful and vain a thing it becomes then to take one's stand on the principles and unproved opinions of certain ancients. Some raise a doubt how it can be that, if the Earth move round its own axis, a globe of iron or of lead dropped from the highest point of a tower falls exactly perpendicularly to a spot of the Earth below itself. Also how it is that cannon balls from a large culverin, fired with the same quantity and strength of powder, in the same direction and at a like elevation through the same air, would be cast at a like distance from a given spot both Eastward and Westward, supposing the Earth to move Eastward. But those who bring forward this kind of argument are being misled: not attending to the nature of primary globes, and the combination of parts with their globes, even though they be not adjoined by solid parts. Whereas the motion of the Earth in the diurnal revolution does not involve the separation of her more {229} solid circumference from the surrounding bodies; but all her effluvia surround her, and in them heavy bodies projected in any way by force, move on uniformly along with the Earth in general coherence. And this also takes place in all primary bodies, the Sun, the Moon, the Earth, the parts betaking themselves to their first origins and sources, with which they connect themselves with the same appetence as terrene things, which we call heavy, with the Earth. So lunar things tend to the Moon, solar things to the Sun, within the orbes of their own effluvia. The emanations hold together by continuity of substance, and heavy bodies are also united with the Earth by their own gravity, and move on together in the general motion: especially when there is no renitency of bodies in the way. And for this cause, on account of the Earth's diurnal revolution, bodies are neither set in motion, nor retarded; they do not overtake it, nor do they fall short behind it when violently projected toward East or West.

Let E F G be the Earth's globe, A its centre, L E the ascending effluvia: Just as the orbe of the effluvia progresses with the Earth, so also does the unmoved part of the circle at the straight line L E progress along with the general revolution. At L and E, a heavy body, M, falls perpendicularly toward E, taking the shortest way to the centre, nor is that right movement of weight, or of aggregation compounded with a circular movement, but is a simple right motion, never leaving the line L E. But when thrown with an equal force from E toward F, and from E toward G, it completes an equal distance on either side, even though the daily rotation of the Earth is in process: just as twenty paces of a man mark an equal space whether toward East or West: so the Earth's diurnal motion {230} is by no means refuted by the illustrious Tycho Brahe, through arguments such as these.

The tendency toward its origin (which, in the case of the Earth, is called by Philosophers weight) causes no resistance to the diurnal revolution, nor does it direct the Earth, nor does it retain the parts of the Earth in place, for in regard to the Earth's solidity they are imponderous, nor do they incline further, but are at rest in the mass. If there be a flaw in the mass, such as a deep cavity (say 1000 fathoms), a homogenic portion of the Earth, or compacted terrestrial matter, descends through that space (whether filled with water or air) toward an origin more assured than air or water, seeking a solid globe. But the centre of the Earth, as also the Earth as a whole, is imponderous; the separated parts tend toward their own origin, but that tendency we call weight; the parts united are at rest; and even if they were ponderable, they would introduce no hindrance to the diurnal revolution. For if around the axis A B, there be a weight at C, it is balanced from E; if at F, from G; if at H, from I. So internally at L, they are balanced from M: the whole globe, then, having a natural axis, is balanced in æquilibrio, and is easily set in motion by the slighted cause, but especially because the Earth in her own place is nowise heavy nor lacking in balance. Therefore weight neither hinders the diurnal revolution, nor influences either the direction or continuance in position. Wherefore it is manifest that no sufficiently strong reason has yet been found out by Philosophers against the motion of the Earth.

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{231} CHAP. VI.

On the cause of the definite time, of an entire _rotation of the Earth._

Diurnal motion is due to causes which have now to be sought, arising from magnetick vigour and from the confederated bodies; that is to say, why the diurnal rotation of the Earth is completed in the space of twenty-four hours. For no curious art, whether of Clepsydras or of sand-clocks, or those contrivances of little toothed wheels which are set in motion by weights, or by the force of a bent steel band, can discover any degree of difference in the time. But as soon as the diurnal rotation has been gone through, it at once begins over again. But we would take as the day the absolute turning of a meridian of the Earth, from sun to sun. This is somewhat greater than one whole revolution of it; in this way the yearly course is completed in 365 and nearly ¼ turnings with respect to the sun. From this sure and regular motion of the Earth, the number and time of 365 days, 5 hours, 55 minutes, in solar tropical years is always certain and definite, except that there are some slight differences due to other causes. The Earth therefore revolves not fortuitously, or by chance, or precipitately; but with a rather high intelligence, equably, and with a wondrous regularity, in no other way than all the rest of the movable stars, which have definite periods belonging to their motions. For the Sun himself being the agent and incitor of the universe in motion, other wandering globes set within the range of his forces, when acted on and stirred, also regulate each its own proper courses by its own forces; and they are turned about in periods corresponding to the extent of their greater rotation, and the differences of their effused forces, and their intelligence for higher good. And for that cause Saturn, having a wider orbit, is borne round it in a longer time, Jupiter a shorter, and Mars still less; while Venus takes nine months, Mercury 80 days, on the hypotheses of Copernicus; the Moon going round the Earth with respect to the Sun in 29 days, 12 hours, 44 minutes. We have asserted that the Earth moves circularly about its centre, completing a day by an entire revolution with respect to the Sun. The Moon revolves in a monthly course around the Earth, and, repeating a conjunction with the Sun after a former synodic conjunction, constitutes the month or Lunar day. The Moon's mean concentrick orbit, according to numerous observations of Copernicus and later astronomers, is found to be distant 29 and about 5/6 diameters of the Earth from the Earth's centre. The Moon's revolution with respect to the Sun takes place in 29½ days and 44 minutes of time. We reckon the motion with respect to the sun, not the periodic motion, {232} just as a day is one entire revolution of the Earth with respect to the Sun, not one periodick revolution; because the Sun is the cause of lunar as of terrestrial motion: also, because (on the hypotheses of later observers) the synodical month is truly periodic, on account of the Earth's motion in a great orbit. The proportion of diameters to circumferences is the same. And the concentrick orbit of the Moon contains twice over 29 and ½ great circles of the Earth & a little more. The Moon & the Earth, then, agree together in a double proportion of motion; & the Earth moves in the space of twenty-four hours, in its diurnal motion; because the Moon has a motion proportional to the Earth, but the Earth a motion agreeing with the lunar motion in a nearly double proportion. There is some difference in details, because the distances of the stars in details have not been examined sufficiently exactly, nor are mathematicians as yet agreed about them. The Earth therefore revolves in a space of 24 hours, as the Moon in her monthly course, by a magnetick confederation of both stars, the globes being forwarded in their movement by the Sun, according to the proportion of their orbits, as Aristotle allows, _de Coelo_, bk. ii., chap. 10. "It happens" (he says) "that the motions are performed through a proportion existing between them severally, namely, at the same intervals in which some are swifter, others slower," But it is more agreeable to the relation between the Moon and the Earth, that that harmony of motion should be due to the fact that they are bodies rather near together, and very like each other in nature and substance, and that the Moon has more evident effects upon the Earth than the rest of the stars, the Sun excepted; also because the Moon alone of all the planets conducts her revolutions, directly (however diverse even), with reference to the Earth's centre, and is especially akin to the Earth, and bound to it as with chains. This, then, is the true symmetry and harmony between the motions of the Earth and the Moon; not that old oft-besung harmony of coelestial motions, which assumes that the nearer any sphære is to the _Primum Mobile_ and that fictitious and pretended rapidest Prime Motion, the less does it offer resistance thereto, and the slower it is borne by its own motion from west to east: but that the more remote it is, the greater is its velocity, and the more freely does it complete its own movement; and therefore that the Moon (being at the greatest distance from the _Primum Mobile_) revolves the most swiftly. Those vain tales have been conceded in order that the _Primum Mobile_ may be accepted, and be thought to have certain effects in retarding the motions of the lower heavens; as though the motion of the stars arose from retardation, and were not inherent and natural; and as though a furious force were perpetually driving the rest of the heaven (except only the _Primum Mobile_) with frenzied incitations. Much more likely is it that the stars are borne around symmetrically by their own forces, with a certain mutual concert and harmony.

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{233} CHAP. VII.

On the primary magnetick nature of the Earth, whereby its poles are parted from the poles _of the Ecliptick._

Primarily having shown the manner and causes of the diurnal revolution of the Earth, which is partly brought about from the vigour of the magnetick virtue, partly effected by the præ-eminence and light of the Sun; there now follows an account of the distance of its poles from the poles of the Ecliptick--a supremely necessary fact. For if the poles of the universe or of the Earth remained fast at the poles of the Zodiack, then the Æquator of the Earth would lie exactly beneath the line of the Ecliptick, and there would be no variation in the seasons of the year, no Winter, no Summer, nor Spring, nor Autumn: but one and the same invariable aspect of things would continue. The direction of the axis of the Earth has receded therefore from the pole of the Zodiack (for lasting good) just so far as is sufficient for the generation and variety of things. Accordingly the declination of the tropicks and the inclination of the Earth's pole remain perpetually in the twenty-fourth degree; though now only 23 degrees 28 minutes are counted; or, as others make out, 29 minutes: But once it was 23 degrees 52 minutes, which are the extreme limits of the declinations hitherto observed. And that has been prudently ordained by nature, and is arranged by the primary excellence of the Earth. For if those poles (of the Earth and the Ecliptick) were to be parted by a much greater distance, then when the Sun approached the tropick, all things in the other deserted part of the globe, in some higher latitude, would be desolate and (by reason of the too prolonged absence of the Sun) brought to destruction. As it is, however, all is so proportioned that the whole terrestrial globe has its own varying seasons in succession, and alternations of condition, appropriate and needful: either from the more direct and vertical radiation of light, or from its increased tarriance above the horizon.

Around these poles of the Ecliptick the direction of the poles of the Earth is borne: and by this motion the præcession of the æquinoxes is apparent to us.

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{234} CHAP. VIII.

On the Præcession of the Æquinoxes, from the magnetick motion of the poles of the Earth, in the Arctick _and Antarctick circle of the Zodiack._

Primitive mathematicians, since they did not pay attention to the inequælities of the years, made no distinction between the æquinoctial, or solstitial revolving year, and that which is taken from some one of the fixed stars. Even the Olympick years, which they used to reckon from the rising of the dogstar, they thought to be the same as those counted from the solstice. Hipparchus of Rhodes was the first to call attention to the fact that these differ from each other, and discovered that the year was longer when measured by the fixed stars than by the æquinox or solstice: whence he supposed that there was in the fixed stars also some motion in a common sequence; but very slow, and not at once perceptible. After him Menelaus, a Roman geometer, then Ptolemy, and long afterward Mahometes Aractensis, and several more, in all their literary memoirs, perceived that the fixed stars and the whole firmament proceeded in an orderly sequence, regarding as they did the heaven, not the earth, and not understanding the magnetical inclinations. But we shall demomstrate that it proceeds rather from a certain rotatory motion of the Earth's axis, than that that eighth sphære (so called) the firmament, or non-moving empyrean, revolves studded with innumerable globes and stars, whose distances from the Earth have never been proved by anyone, nor can be proved (the whole universe gliding, as it were). And surely it should seem much more likely that the appearances in the heavens should be clearly accounted for by a certain inflection and inclination of the comparatively small body of the Earth, than by the setting in motion of the whole system of the universe; especially if this motion is to be regarded as ordained solely for the Earth's advantage: While for the fixed stars, or for the planets, it is of no use at all. For this motion the rising and settings of stars in every Horizon, as well as their culminations at the height of the heavens, are shifted so much that the stars which once were vertical are now some degrees distant from the zenith. For nature has taken care, through the Earth's soul or magnetick vigour, that, just as it was needful in tempering, receiving, and warding off the sun's rays and light, by suitable seasons, that the points toward which the Earth's pole is directed should be 23 degrees and more {235} from the poles of the Ecliptick[250]: so now for moderating and for receiving the luminous rays of the fixed stars in due turn and succession, the Earth's poles should revolve at the same distance from the Ecliptick at the Ecliptick's arctick circle; or rather that they should creep at a gentle pace, that the actions of the stars should not always remain at the same parallel circles, but should have a rather slow mutation. For the influences of the stars are not so forceful as that a swifter course should be desired. Slowly, then, is the Earth's axis inflected; and the stars' rays, falling upon the face of the Earth, shift only in so long a time as a diameter of the arctick or polar circle is extended: whence the star at the extremity of the tail of the Cynosure, which once was 12 degrees 24 minutes (namely, in the time of Hipparchus) distant from the pole of the universe, or from that point which the pole of the Earth used to face, is now only 2 degrees and 52 minutes distant from the same point; whence from its nearness it is called by the moderns _Polaris._ Some time it will be only ½ degree away from the pole: afterward it will begin to recede from the pole until it will be 48 degrees distant; and this, according to the Prutenical tables, will be in Anno Domini 15000. Thus _Lucida Lyræ_ (which to us southern Britons now almost culminates) will some time approach to the pole of the world, to about the fifth degree. So all the stars shift their rays of light at the surface of the Earth, through this wonderful magnetical inflection of the Earth's axis. Hence come new varieties of the seasons of the year, and lands become more fruitful or more barren; hence the characters and manners of nations are changed; kingdoms and laws are altered, in accordance with the virtue of the fixed stars as they culminate, and the strength thence received or lost in accordance with the singular and specifick nature of each; or on account of new configurations with the planets in other places of the Zodiack; on account also of risings and settings, and of new concurrences at the meridian. The Præcession of the æquinoxes arising from the aequable motion of the Earth's pole in the arctick circle of the Zodiack is here demonstrated. Let A B C D be the Ecliptick line; I E G the arctic circle of the Zodiack. Then if the Earth's pole look to E, the æquinoxes are at D, C. Let this be at the time of Metho, when the horns of Aries were in the æquinoctial colure. Now if the Earth's pole have advanced to I; then the æquinoxes will be at K, L; and the stars in the ecliptick C will seem to have progressed, in the order of the signs, along the whole arc K C: L will be moved on by the præcession, against the order of the signs, along the arc D L. But this would occur in the contrary order, if the point G were to face the poles of the earth, and the motion were from E to G: for then the æquinoxes would be M N, and the fixed stars would anticipate the same at C and D, counter to the order of the signs.

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{236} CHAP. IX.

On the anomaly of the Præcession of the Equinoxes, _and of the obliquity of the Zodiack._

At one time the shifting of the æquinoxes is quicker, at another slower, being not always equal: because the poles of the earth travel unequally in the arctick and antarctick circle of the Zodiack; and decline on both sides from the middle path: whence the obliquity of the Zodiack to the Æquator seems to change. And as this has become known by means of long observations, so also has it been perceived, that the true æquinoctial points have been elongated from the mean æquinoctial points, on this side and on that, by 70 minutes (when the prostaphæresis is greatest): but that the solstices either approach the equator unequally 12 minutes nearer, or recede as far behind; so that the nearest approach is 23 degrees 28 minutes, and the greatest elongation 23 degrees 52 minutes. Astronomers have given various explanations to account for this inequality of the præcession and also of the obliquity of the tropicks. Thebit, with the view of {237} laying down a rule for such considerable inequalities in the motion of the stars, explained that the eighth sphære does not move with a continuous motion from west to east; but is shaken with a certain motion of trepidation, by which the first points of Aries and Libra in the eighth heaven describe certain small circles with diameters equal to about nine degrees, around the first points of Aries and Libra in the ninth sphære. But since many things absurd and impossible as to motion follow from this motion of trepidation, that theory of motion is therefore long since obsolete. Others therefore are compelled to attribute the motion to the eighth sphære, and to erect above it a ninth heaven also, yea, and to pile up yet a tenth and an eleventh: In the case of mathematicians, indeed, the fault may be condoned; for it is permissible for them, in the case of difficult motions, to lay down some rule and law of equality by any hypotheses. But by no means can such enormous and monstrous celestial structures be accepted by philosophers. And yet here one may see how hard to please are those who do not allow any motion to one very small body, the Earth; and notwithstanding they drive and rotate the heavens, which are huge and immense above all conception and imagination: I declare that they feign the heavens to be three (the most monstrous of all things in Nature) in order that some obscure motions forsooth[251] may be accounted for. Ptolemy, who compares with his own the observations of Timocharis and Hipparchus, one of whom flourished 260 years, the other 460 years before him, thought that there was this motion of the eighth sphære, and of the whole firmament; and proved by help of numerous phenomena that it took place over the poles of the Zodiack, and, supposing its motion to be so far æquable, that the non-planetary stars in the space of 100 years completed just one degree beneath the _Primum Mobile_. After him 750 years Albategnius discovered that one degree was completed in a space of 66 years, so that a whole period would be 23,760 years. Alphonsus made out that this motion was still slower, completing one degree and 28 minutes only in 200 years; and that thus the course of the fixed stars went on, though unequally. At length Copernicus, by means of the observations of Timocharis, Aristarchus of Samos, Hipparchus, Menelaus, Ptolemy, Mahometes Aractensis, Alphonsus, and of his own, detected the anomalies of the motion of the Earth's axis: though I doubt not that other anomalies also will come to light some ages hence. So difficult is it to observe motion so slow, unless extending over a period of many centuries; on which account we still fail to understand the intent of Nature, what she is driving after through such inequality of motion. Let A be the pole of the Ecliptick, B C the Ecliptick, D the Æquator; when the pole of the Earth near the arctick circle of the Zodiack faces the point M, then there is an anomaly of the præcession of the æquinox at F; {238} but when it faces N, there is an anomaly of the præcession at E. But when it faces I directly, then the maximum obliquity G is observed at the solstitial colure; but when it faces L, there is the minimum obliquity H at the solstitial colure.

_Copernicus' contorted circlet in the Arctick circle of the Zodiack._

Let F B G be the half of the Arctick circle described round the pole of the Zodiack: A B C the solstitial colure: A the pole of the Zodiack; D E the anomaly of longitude 140 minutes at either side on both ends: B C the anomaly of obliquity 24 minutes: B the greater obliquity of 23 degrees 52 minutes: D the mean obliquity of 23 degrees 40 minutes: C the minimum obliquity of 23 degrees 28 minutes.

{239}

{240} The period of motion of the præcession of the æquinoxes is 25,816 Ægyptian years; the period of the obliquity of the Zodiack is 3434 years, and a little more. The period of the anomaly of the præcession of the æquinoxes is 1717 years, and a little more. If the whole time of the motion AI were divided into eight equal parts: in the first eighth the pole is borne somewhat swiftly from A to B; in the second eighth, more slowly from B to C; in the third, with the same slowness from C to D; in the fourth, more swiftly again from D to E; in the fifth, with the same swiftness from E to F; again more slowly from F to G; and with the same slowness from G to H; in the last eighth, somewhat swiftly again from H to I. And this is the contorted circlet of Copernicus, fused with the mean motion into the curved line which is the path of the true motion. And thus the pole attains the period of the anomaly of the præcession of the æquinoxes twice; and that of the declination or obliquity once only. It is thus that by later astronomers, but especially by Copernicus (the Restorer of Astronomy)[252], the anomalies of the motion of the Earth's axis are described, so far as the observations of the ancients down to our own times admit; but there are still needed more and exact observations for anyone to establish aught certain about the anomaly of the motion of the præcessions, and at the same time that also of the obliquity of the Zodiack. For ever since the time at which, by means of various observations, this anomaly was first observed, we have only arrived at half a period of the obliquity. So that all the more all these matters about the unequal motion both of the præcession and of the obliquity are uncertain and not well known: wherefore neither can we ourselves assign any natural causes for it, and establish it for certain. Wherefore also do we to our reasonings and experiments magnetical here set an end and period.[253]

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{241}

INDEX.

Abano, Pietro di (Apponensis or Apianus), 2.
Abbas, Hali ('Alí ibn Al 'Abb[=a]s, _Al Majúsi_, 2, 6.
Abohalis, 47. _See also_ Avicenna.
_aciarium_ or _acies_, also _aciare_, 18, 23, 33, 36.
Acosta, Josephus, 5.
adamant, 11.
æquator, the magnetick, 13, 79.
Aetius Amidenus, 2.
Affaytatus, Fortunius, 6.
agate, non electrick, 51, 53.
Agricola, Georgius, 2, 3, 10, 19, 26, 111, 112.
Agrippa, H. Cornelius, 3.
_aimant_, 11.
Albategnius (Muhammad ibn J[=a]bir, _Al-Batt[=a]ni_, 237.
Albertus Magnus, 2, 7, 18, 111.
Alexander Aphrodiseus, 3, 48, 92.
Alexandria, Hero of, 58.
Alfonso, Diego, 178.
Alfonsus the Wise (Alphonsus X.), 237.
Amalfians said to have first constructed the compass, 4.
Amatus Lusitanus, 2.
amber, 47, 49-60, 85, 112, 116.
amethyst, electrical properties of, 48.
amianth, 11.
Amidenus, Aetius, 2.
amphitane, 111.
Anatolismus, or Northeasting, 167.
Anaxagoras, 61, 208.
Andrea Doria (Admiral), 4.
Antonius de Fantis, 107.
Antonius Musa Brasavolus, 2.
Antony, the denarius of, 110.
Apianus. _See_ Abano.
Apponensis. _See_ Abano.
Aquinas, Thomas, 3, 64.
Aractensis, Mahometes, 234, 237.
Archelaus, 208.
Ardoynis, Santes de, 2.
Arias Montanus, 4.
Aristarchus, 214, 237.
Aristotle:
_De Anima_, 1, 11, 61, 210.
_De Coelo_, 226, 232.
_De Mirabilibus Auscultationibus_, 22.
_Meteorologica_, 35, 39.
on material of the metals, 19, 20.
on the element of earth, 43.
on motions, 45, 219, 225.
on primary form, 65.
on the _Primum Mobile_, 220.
on animate nature of planets, 208.
armature, 87.
armed loadstones, 86, 87, 88, 89.
Arnaldus de Villa nova, 2, 7.
Arsinoe, Temple of, 2.
Attraction, 46, 60, 64, 68, 90, 98, 109.
Avicenna (Abu 'Ali Husain ibn 'Abd Allah, _Ibn Síná_; also called
Abohalis):
writes on the magnet, 2.
on falling masses of iron, 26.
alleges loadstone an antidote to iron poison, 35.
on the property of attraction, 49.
Augsburgers (Augustani), the, prescribe loadstone in plaster, 33.
axis, the magnetick, 13, 81, 212.
Azores, variation of compass at the, 4, 154, 156, 167.

Bacon, Roger, 5.
Bambola, or Bilbilis, 23.
Baptista Montanus, 2.
Baptista Porta. _See_ Porta.
Barbarus, Hermolaus, 3.
Barlowe, William (Rev. Archdeacon), his book, _The Navigators Supply_, 8.
basil leaves alleged not to be attracted, 48.
belemnites are electrical, 48.
Bencora (Th[=a]bit ibn Kurrah, _Al Harrani_; also called Thebitius), 117,
236.
Benedictus, Joannes Baptista (Giambattista Benedetti), 167.
beryl, electrick properties of, 48.
Bessardus (Toussaincte de Bessard), 5, 116, 153.
Blondus, Flavius, the historian, 4.
Borough, William, his book on the _Variation of the Compass_, 8.
Borrholybicum (North-north-west), 160.
Brahe, Tycho, 174, 229.
Brandoe, the island of, 181.
Brasavolus, Antonius Musa, 2.
Bristolla, or Bristol gem, 48.
burnt clay, magnetick properties of, 26, 43.
{242}

Cabot, Sebastian, 4.
Cælius Calcagninus, 7.
Cæsare, or Cesare, Giulio, 141.
Calaber, Hannibal Rosetius, 3,
_calamita_ or _kalamita_, 11.
Calcagninus, Cælius, 7.
Camillus Leonhardus, 3.
Candish, or Cavendish, Thomas, *iij, 117.
cap of iron for a loadstone, 86, 89, 90, 95.
_carabe_, or _karabe_, 47.
carbuncle, electrick properties of, 48, 111.
Cardan, Hieronymo, 2.
_De Proportionibus_:
on iron and earth, 43, 62, 67.
on distance of centre of cosmos, 169.
_De Rerum Varietate_:
on fall of meteorick iron, 26.
on attraction of amber, 49.
on a perpetual motion engine, 107.
_De Subtilitate_:
alleges magnet to feed on iron, 37, 63, 92.
on magnet that draws silver, 110.
on magnetick influence of star in tail of _Ursa Minor_, 5, 116, 153.
carnelian, the, 51, 55.
_catoblepas_, the antelope called, 63.
Cesare, Giulio, 141.
_chalybs_, 18, 25, 33.
chatochitis, 111.
chemists, the, 19, 20, 21, 24, 37, 66.
China, 4, 8, 9, 11, 17, 32, 119.
Chinocrates, 2.
circumpulsion, doctrine of, 3, 61.
clamps (open kilns), 26.
clay when burnt is magnetick, 26, 43, 97.
clepsydra, 231.
Coimbra, College of, 5.
coition (mutual attraction), 45, 46, 60, 65, 67, 68, 81, 98, 99, 103,
109, 131.
definition of, *vj, 68.
orbe of, *vj.
colours of loadstones, 9, 10, 27.
Como, 23.
compass, alleged invention of, by Amalfians, 4.
origin of the compass-card, 4, 165.
the mariners' (_pyxis_), 3, 115, 147, 165, 172.
the little (_pyxidula_), 181, 202.
different forms of, Italian, Baltic, Portuguese, English, 165, 166,
177, 181.
conduction, magnetick, 85, 104, 125.
consequent poles, 129, 142.
Copernican system, 231.
Copernicus, Nicolas, 212, 214, 216, 231, 237, 238, 240.
Cordus, Valerius, 10.
Cornelius Agrippa, 3.
Cornelius Gemma, 63.
Cornelius Tacitus, 25.
_corolla insorta_, or contorted circlet, 238, 240.
Cortes, Martin, 5, 116, 152.
Corvo, Island of, 167.
Costa, Filippo (of Mantua), 141.
Costæus, Joannes, 3, 62, 227, 228.
_creagus_, the, or flesh-magnet, 110.
crystal, rock, 48, 52, 59, 111
Curtius, Nicolaus, 35.
Cusan (Michael Khrypffs), Cardinal de Cusa, 3, 64, 108.
Cynosure, the, or Pole-star, 14, 81, 117, 222, 235.

Dean, Forest of, loadstone found in the, 11.
decay of the magnetick virtue, 18, 37, 124, 138, 149.
declination, the, or dip, 184.
denarius of Antony, 110.
diamond, an electrick, 48, 50, 59, 111.
alleged power to attract iron, 109, 112.
alleged antipathy to magnet, 2, 7, 109, 143.
experiments upon, 143.
Diego Alfonso, 178.
Differences between electricks and magneticks, 47, 60, 65.
Dioscorides, 1, 2, 9, 32.
dip, the, also called declination, 8, 46, 184-204.
dipping-needle, or declination instrument, 185, 203.
direction, or directive force, 41, 46, 115, 119.
dividing a loadstone, 16, 72, 100, 121, 122, 127, 130, 136, 145, 146.
Dominicus Maria Ferrariensis, 212, 213.
Doria, Andrea (Admiral), 4.
Drake, Sir Francis, *iij _bis_, 117.
Du Puys (also called Puteanus), 3, 63.

Earth, the, a great magnet, 38, 39, 40, 41, 44, 119, 211.
_echeneis_ (the sucking fish), 7, 63, 110.
Ecphantus, 214.
effluvia, electrical, 52, 53, 59, 66.
magnetical, 61.
electrical attraction, 50, 51, 111.
electrick force, definition of, 52.
electricks, *vj, 46-60.
_electrum_ ([Greek: êlektron]), 47.
emerald is non-electrick, 51.
emery, 22, 51.
Empedocles, 208.
Encelius (or Entzelt, Christoph.), 3, 111.
Epicurius, 61, 62.
Erasmus Rheinholdus, 213.
Erastus, Thomas, 3, 22.
errors in navigation, 166, 177.
Evax, King of Arabia, 111.
Euripides, 9, 11, 18.
{243}

Fallopius, Gabriellus, 3, 34, 35, 112.
Fantis, Antonius de, 107.
Fernelius, Joannes Franciscus, 4.
Ficinus, Marsilius (or Marsiglio Ficino), 3, 7, 116 153.
filings of iron, 37, 69, 90, 91, 92, 104.
Filippo Costa. _See_ Costa.
fire destroys magnetick properties, 66, 67, 91, 124.
flame destroys electrification, 59.
flame hinders not magnetick attractions, 66.
Flavius Blondus. _See_ Blondus.
flies in amber, 47.
form _versus_ matter, 52, 65.
Fra Paolo, 6.
Fracastorio, Hieronymo, 5, 50, 67, 71, 91, 110, 113, 152.
Franciscus Maurolycus. _See_ Maurolycus.
Franciscus Rueus. _See_ Rueus.

Gagates. _See_ jet.
Galen, 2, 9, 32, 35, 39, 46, 49, 61, 62, 63.
Gallus, Marbodæus, 2, 7.
garlick, its reputed antagonism to magnetism, 2, 32, 64.
Gartias ab Horto, 32.
Gaudentius Merula, 7.
Gauricus, Lucas, 7.
Geber (J[=a]bir ibn Háiyán, _Al-Tarsus[=i]_) 21.
Gemma, Cornelius, 63.
gems, electrick properties of, 48, 51.
_geniter_, 47.
Georgius Agricola. _See_ Agricola.
Gilbert, Adrian, 11.
Gilgil Mauritanus, 19.
Gioia, or Goia, of Amalfi, 4.
Giulio Cæsare, 141.
glass, an electrick by friction, 48, 54, 59.
use of loadstone in making, 111.
goat's blood, 7.
Gonzalus Oviedus, 4.
Goropius, Henricus Becanus, 4.
Grotius, Hugo, 167, 168.

Haematite, 22, 51.
Hali Abbas ('Ali ibn Al 'Abbás, _Al Masúfí_), 2, 6.
Hannibal Rosetius Calaber, 3.
Hariot, Thomas, 7.
Heat, effect of on loadstone, 66, 67, 93, 123, 124.
Helmshuda, 167.
Heraclea, the city of, 8.
Heraclean stone, or stone of Hercules, 8, 43, 61, 169.
Heraclides, 214.
Heraclitus, 208.
Hermes, 209.
Hermolaus Barbarus, 3.
Hero of Alexandria, 58.
Hipparchus, 213, 214, 234, 235, 237.
Hippocrates, 8, 35, 51, 61.
horizon, the magnetick, defined, 80.
Horto, Gartias ab, 32.
Horus, the bone of, or _Os Ori_, 9.
hot iron not magnetick, 66.
Hues, Robert, 7.
Hugo Grotius, 167, 168.

Inclination. _See_ dip.
interposition of bodies, 53, 66, 83, 85, 89, 137.
iris gem, the, 48.
iron, its nature and occurrence, 19, 20, 22, 25.
filings of, 37, 69, 90, 91, 92, 104.
its various names and qualities, 23, 33, 36.
its various uses, 23, 24, 39, 86, 90, 95.
medical uses of, 33, 35.
surpasses loadstone, 69, 95.
verticity in, 85, 123, 139.
iron ore is magnetick, 18, 27, 38, 43.
has poles, 28.
islands, magnetick influence of, 5, 153,161.

Jacobus Severtius, 5.
jet, 47, 48, 53, 55, 86.
Joannes Baptista Porta. _See_ Porta.
Joannes Baptista Montanus, 2.
Joannes Costæus. _See_ Costæus.
Joannes Franciscus Offusius, 46.
Joannes Goia. _See_ Gioia.
Joannes Langius, 3.
Joannes Taisner, or Taisnier. _See_ Taisnier.
Jofrancus Offusius, 46.
Josephus Acosta, 5.
Julius Cæsar Moderatus, 141.
Julius Cæsar Scaliger. _See_ Scaliger.

Kendall, Abraham, 7, 178.
Korrah, Thebitius Ben. _See_ Bencora.

Lactantius, Lucius, 219.
Lagos, Rodriguez de, 177.
Langius, Joannes, 3.
_lapis magnetis_, 8.
_lapis specularis_, muscovy stone, or mica, 11, 48, 52.
latitude in relation to dip, 196, 200.
Leonardus (or Leonhardus), Camillus, 3.
Levinus Lemnius, 3.
{244}
lifting power of loadstones, 86, 89, 97.
lily of the compass, 117, 152, 165, 177.
liquids, electrical attraction of, 55.
attraction on surface of, 57.
Livio Sanuto, 5, 153, 167.
loadstone armed and unarmed, 86, 87, 88.
as medicine, 32.
in plasters, 33.
rock, the, 5, 6, 18, 116, 152.
various names of, 11.
colours of, 9, 10, 27.
various sources of, 8, 25, 32.
London, magnetick variation at, 154, 163.
longitude, magnetick finding at, 166.
long magnets, advantage of, 82, 83, 99, 101
Lucania, fall of meteorick stones in, 26.
Lucas Gauricus, 7.
Lucretius, 2, 3, 8, 49, 61.
Lusitanus, Amatus, 2.
Lynschoten, Hugo van, *iiij.

Magnes, [Greek: magnês], [Greek: magnêtis], 11.
Magnesia, 8.
Magnetick axis of terrella, 81, 212.
axis of earth, 13, 81, 212.
horizon, 80.
meridian, 79, 152.
mountains or rocks, 5, 6, 18, 116, 152.
islands, 5, 153, 161.
motions, the five, 45.
Magnus, Albertus. _See_ Albertus.
Magnus, Olaus, 5, 6.
Mahometes Aractensis, 234, 237.
Mahomet's tomb, 2.
Manardus, Joannes, 35.
Marbodæus Gallus, 2, 7.
Marcellus Empiricus, 2.
Marco Polo (Paulus Venetus), 4.
mariners' compass. _See_ compass.
Mars, saffron of (_Crocus Martis_), 34, 91.
Marsiglio Ficino. _See_ Ficinus.
Martin Cortes, 5, 116, 152.
matter and form, 52, 65.
Matthæus Silvaticus, 3.
Matthiolus, Petrus, 2, 3.
Mauritanus, Gilgil, 19.
Mauritanus, Serapio, 2, 6.
Maurolycus, Franciscus, 5, 42, 153, 180.
medicinal use of iron, 33.
of loadstone, 32.
Medina, Pedro de, 166.
Menelaus, 234, 237.
meridian, magnetick, 79, 152, 163.
Merula, Gaudentius, 7.
meteorick stones, falls of, 26, 27.
mica (or muscovy stone), 11, 48, 52.
[Greek: mikrogê]. _See_ terrella.
moisture stops electrick action, 53, 56.
Montagnana, B., 35.
Montanus, Arias, 4.
Montanus, Joannes Baptista, 2.
Moors, Serapio and the, 6.
mountains, magnetick, 5, 6, 18, 116, 152.
movement of trepidation, 117.
Musa Brasavolus, Antonius, 2.
muscovy stone, 11, 48, 52. _See also_ mica.
myths of the magnet, 2, 3, 5, 6, 7, 18, 32, 63, 107, 109, 110, 111, 116,
143, 153, 228
motions, the various magnetical, 46.

Names of amber, 47.
names of the loadstone, 11.
names given to the magnetick poles, 15, 115, 125, 129.
Nicander of Colophon, 8, 9.
Nicetas, 214.
Nicolas Copernicus, 212, 214, 216, 231, 237, 238, 240
Nicolaus Myrepsus, or Præpositas, 33.
non-electrick bodies, 51, 55.
Nonius, Petrus (Pedro Nuñez), 166.
Norman, Robert, 5, 8, 153, 161, 162.
supposes a point respective, 5, 153, 161, 162.
his _Newe Attractive_, 8.
discoverer of the dip, 8.
Norumbega, the city of, 154.
Nova Zembla, 152, 179.

Offusius, Jofrancus, 46.
Olaus Magnus, 5, 6.
opal becomes electrical, 48.
orbe of virtue, 76, 96, 191, 205
orbes of planets, 208, 215.
Oribasius, 2.
Orpheus, 11, 61, 209.
Oviedus, Gonzalus (Gonzalo Fernandez de Oviedo y Valdès), 4.

Pantarbes, 111.
Paolo (Paulus Æginæ), 35.
Paolo, Rev. Maestro (Fra Paolo Sarpi), 6
Paolo the Venetian (Marco Polo), 4.
Paracelsus (Bombast von Hohenheim).
asserts the stars to attract iron, 3.
his emplastrum of loadstone, 33.
his method of strengthening loadstones, 93.
Parmenides, 208.
pearls are not electrick, 51, 55.
Pedro de Medina, 166.
percussion excites verticity, 139.
Peregrinus, Peter,
his book, 5.
on cause of magnetick direction, 5, 116, 153.
on perpetual motion engine, 107.
affirms a terrella to revolve daily, 223.
{245}
Peripateticks, the, 20, 41, 43, 45, 65, 218, 222, 225, 227, 228.
perpetual motion machine, 107.
Peter Peregrinus. _See_ Peregrinus.
Peter Plancius. _See_ Plancius.
Petrus Apponensis. _See_ Abano, Pietro di.
Petrus Nonius. _See_ Nonius or Nuñez.
Philolaus, 214.
Philostratus, 111.
Pictorio, G., 6, 49.
_piedramant_, 11.
Plancius, Peter, *v _bis._
planets, influence of, 20, 137, 142.
plasters, magnetick, 32, 33.
Plato, 3.
in the _Io_, discusses name and properties of the magnet, 1, 9, 11, 18.
in the _Timæus_, suggests the theory of circumpulsion, 61.
his Atlantis, 159.
on life in the universe, 208.
Pliny (C. Plinius Secundus).
on loadstone fables, 1, 2, 9, 18.
his mistake about Æthiopian loadstones, 17.
on the five kinds of loadstones, 9.
on the alleged discovery of the loadstones, 8.
on the alleged magnetick mountains, 18.
on a locality where loadstone was found, 11.
on the occurrence of iron in Spain, 25.
on the Sagda and the Catochites, 111.
on the silver denarius of Antony, 110.
on the use of loadstone by glass-makers, 111.
on the shadow of a gnomon of a sun-dial at Rome, 213.
Plotinus, 218.
Plutarch, Claudius.
on the garlick fable, 32.
says something flammable exists in amber, 54.
his theory of circumpulsion, 3, 62.
polarity. _See_ verticity.
pole, the, elevation of, 200, 213.
poles, magnetick, of a loadstone, 13, 41, 72, 81, 144.
poles are not points, 12, 41, 72, 96.
Polo, Marco, 4.
Porta, Joannes Baptista (Giambattista della Porta).
his narration of marvels, 6.
on various tempering of iron, 24.
asserts loadstone a mixture of stone and iron, 63.
on his assertion that loadstones have hairs, 66.
asserts vapour to be cause of attraction, 67.
his error as to change of verticity, 73.
suspends iron upwards by a thread, 92.
his error as to centre of the orbe of virtue, 95.
his error as to the polarity which causes repulsion, 102.
his error as to magnetick opposing forces, 103.
experiment with a balance, 108.
his error as to iron being intoxicated, 138.
his error as to iron excited by a diamond, 143.
his error as to the pointing of a magnet, 144.
proportion between loadstone and iron, 149.
his error as to variation and longitude, 166.
præcession of the Æquinoxes, 234, 236.
_primum mobile_, the, 79, 214, 216, 218, 220, 226, 232, 237.
prostaphæresis, 174, 236.
Prutenical Tables, the, 235.
Ptolemæus, Claudius.
on loadstone fables, 2, 32.
on the occurrence of loadstone and of iron, 9,25.
on the dissolution of the earth, 91, 217, 218.
alleged relation of regions with the planets, 137.
on the elevation of the pole at different latitudes, 213, 214.
on the _Primum Mobile_, and the diurnal movement of the stars, 216,
228, 234.
on the anomalies of the earth's motion, 237.
Puteanus, Gulielmus (Du Puys), 3, 63.
pyrimachus (_i.e._, pyrites), 23.
Pythagoras, 57, 208.
_pyxidula_, 4, 181.
_pyxis_, 3, 115, 147, 165, 172.

Radius, the, of the earth's orbit, 218.
Rasis. _See_ Rhazes.
rays of magnetick virtue, 95.
Reinoldus, Erasmus (or Rheinholdus), 213.
_remora_, the (or sucking fish), 7, 63, 110.
resin becomes electrical by friction, 48, 52.
respective points, 5, 153, 161, 162.
reversal of polarity, 101, 137.
revolution of the globe, 46, 81, 91, 220.
repulsion, electrical, denied to exist, 113.
Rhazes (Muhammad ibn Zakar[=i]y[=a]), 34, 35.
rings, on the verticity of, 129.
Rodriguez de Lagos, 177.
Rosetius Calaber, Hannibal, 3.
Ruellius, Joannes, 7.
Rueus, Franciscus (de la Rue), 6.

Saffron of Mars, 34, 91, 93.
sagda, or sagdo, the, 111.
Sanuto, Livio, 5, 153, 167.
sapphire, the, 48.
scales of iron, 22.
Scaliger, Julius Cæsar.
on cause of magnetick direction, 5, 64, 153.
on a fall of meteorick iron, 26.
on preservation of loadstones, 37.
on amber, 47.
on magnetick attraction, 70.
admits the loadstone to have a soul, 68.
on diamond attracting iron, 112.
scoria or slag of iron, 34, 35.
sealing wax is electrical, 48, 53.
Sebastian Cabot. _See_ Cabot.
Serapio, or Serapio Mauritania (Yuhanná ibn Sarapion), 2, 6.
Severtius, Jacobus, 5.
shielding, magnetick, by iron plate, 83, 85.
{246}
_siderites_ ([Greek: sideritês]) 8, 11, 143.
_siegelstein_ 11.
silk suspension for magnetick iron, 29, 30.
Silvaticus, Matthæus, 3.
silver, loadstone for, 109, 110.
similars, doctrine of attraction of, 50, 62.
Simon Stevinus, *v _bis_, 167, 168.
slate, magnetick properties of, 43.
smeargel (emery), 22.
Solinus, Caius Julius, 1, 9, 111.
Solomon the King, 4.
Sotacus, 9.
Stadius, 213.
stars are at various distances, 215.
steel, 23, 39, 69, 71, 93, 95, 147.
Stevinus, Simon, *v _bis_, 167, 168.
_stomoma_ ([Greek: stomôma]) 23, 33, 36.
Strabo, 25.
_succinum_. _See_ amber.
Sudini, or Sudavienses, 47.
sulphur, electrical by friction, 48, 53, 56, 59.
[Greek: sundromê], *vj.
[Greek: sunentelecheia], 68.
Sussex, iron ore in, 22.
sympathy and antipathy, 65, 68, 112.

Tacitus, Cornelius, 25.
Taisner, or Taisnier, Joannes, 5, 107.
Tariassiona or Tarazona, 23.
terrella.
definition of, *vj, 12, 13.
poles and axis of, 13, 72, 81, 144.
divided into two parts, 72.
magnetick vigour, diagram of, 74, 75.
how small pieces of iron behave toward, 75, 76.
orbe of virtue of, 76, 77, 104.
"geography" of, 78.
æquinoctial circle of, 79, 144.
parallels of, 80, 211.
magnetick horizon of, 80.
proportion of the forces in, 81, 82.
experiment with iron sphere, 85.
small iron sphere and rod, 94, 102.
centre of magnetick virtue in, 95.
irregular terrella to exhibit variation, 155, 157.
to illustrate the dip of the needle, 190, 192.
analogy of, with the earth, 41, 78, 119, 211.
testing loadstones, methods of, 108.
Thales of Miletus, 11, 61, 68, 208, 210.
_theamedes_, the, 18.
Thebitius, or Thebit ben Korrah, 117, 236.
Themistius, 71.
Theophrastus, 1, 9, 11.
Thomas Aquinas, 3, 64.
tides, the cause of, 86.
Tycho Brahe, 174, 229.