In this system, the earth is placed in the centre of the orbits of the sun and moon; but the sun is supposed to be the centre of the orbits of the five primary planets then known. Thus, according to Tycho Brahe, the sun and all the planets moved round the earth, in order to save the earth from revolving on its axis once in twentyfour hours!

This System is represented by the following figure.

It must be allowed, that all the phenomena purely astronomical may be accounted for on this hypothesis; and that the objections to it are rather derived from physical and mechanical considerations, than from the appearances themselves. It is simpler than the Ptolemaic system, and free from its inconsistencies; but it is more complex than the Copernican, and in no respect affords a better explanation of the phenomena. Its true place is between these two systems; an advance beyond the one, and a step short of the other. If its author had lived before Copernicus, it would have been a step in the advancement of knowledge; but coming after him, it was a step backward.

It is certainly not to his credit as a philosopher to have made this retrograde movement, yet he is not altogether without apology. For the physical arguments in favour of the Copernican system could not

be supposed to have much weight in an age when the laws of motion were unknown, and when it was not clearly understood that the sun and planets had any effect on each other. Having already fully and clearly established both the diurnal and annual motion of the earth, and having advanced some other convincing arguments in favour of the Copernican system, in the former part of this work, it would be superfluous to enter into a formal refutation of the Tychonic system, which is so different from that of the Copernican.

But as Tycho Brahe advanced a number of objections against the Copernican system, and particularly that part of it which supposes the earth in motion, we shall here notice a few of those objections which appear to have the greatest weight, although the arguments already advanced in support of the annual as well as the diurnal motion of the earth, would be considered as equivalent to a demonstration by any person who was not previously prepossessed against the possibility of such motions.

Tycho asked, how was it possible for a ball which was projected perpendicularly up in the air, to fall down on the same spot from which it was projected if the earth revolved on its axis? In answer to this objection, it will be sufficient to observe, that the atmosphere which surrounds the earth revolves with it, having received the same common motion, in the same direction; and that every thing on the earth revolves with it, and retains the same relative position. It is rather astonishing that Tycho, and all those who have repeated the same argument, in various forms, never observed that when balls or billiards are played in a vessel sailing with the greatest velocity, that the shock of the ball is there given with the same force the one way as the other; and that when a stone is dropped from the top of the mast, it falls just as near the foot of it when the vessel sails as when she is at rest. But though this be the case, the stone has actually been carried some distance, in the direction of the ship's motion, from the point perpendicularly under that from which it was dropt and if a spectator in another vessel at rest were to observe the stone during its descent, he would see it describe an oblique line, or the diagonal of a parallelogram, the sides of which would be the height of the mast, and the distance which the ship sailed during the time of the stone's descent. The motion of the vessel is communicated to the mast, to the stone, and to every thing on board, in such a way, that every thing thrown or moved arrives at the same point, and strikes with the same force, when the ship sails as when she is at rest, provided it be thrown with the same velocity in the one case as in the other.

The reason that the stone is seen to describe an oblique line, or even to have any motion at all, is its meeting or passing other objects in its descent which are known to be fixed. But as the earth meets with no other object, there is nothing at a distance from it, nor on its surface, which can by its situation, by its motion, or by its resistance, make us perceive or feel either its annual or diurnal motion. All bodies on or near the surface of the earth partake of the motion of the earth itself, and are carried smoothly through the air with the same

velocity, and continue to preserve the same relative position with respect to each other.

"The earth," said Tycho Brahe, to the followers of Copernicus, "is a heavy inert mass, very ill adapted to motion, and seems only fitted to be the fixed foundation of all stability; and yet you wish to make it a star, and travel about in the air! That is too strange an

idea!"

But there is nothing in this remark of Tycho's which carries with it the smallest degree of evidence against the motion of the earth. Why not the earth move which is so much less than the sun, even following the observations and demonstrations of Tycho himself? Why should it be viler or grosser than the planets, which are round like the earth, opaque and dark like it, when the sun does not shine on them, and some of them much larger, even according to Tycho?

Another thing which shocked Tycho very much was the enormous distance at which the fixed stars must be placed in the system of Copernicus; for the breadth of the earth's orbit is an insensible point compared with their distance. "It is not probable," said he, that the distance between Saturn and the fixed stars is seven hundred times greater than the distance between Saturn and the sun, without other stars being placed in the interval !" But this distance cannot be less, since it is found that the annual parallax of none of the fixed stars amounts to two seconds of a degree. And as this is the angle under which the earth's orbit appears when viewed from a fixed star, if it be equal to the angle under which we see the star, it follows that the star and orbit are equal in magnitude.

Now, as the parallax of the stars is so small as scarcely to amount to one second, they must either greatly exceed the earth's orbit in diameter, or be immensely distant from the earth.* But if Tycho had lived at the present day, he would not have advanced this objection against the motion of the earth; for he would have learned that the planet Uranus, as well as many of the comets, moved in orbits which extend far beyond that of Saturn, and fill a part of that immense space which appeared to him so inconceivable. He would have known by the discovery of telescopes, that the apparent diameter of stars of the first magnitude, does not amount to one second; and of course we are not under the necessity of supposing them so prodigiously great as if it amounted to two or three minutes, as he supposed. And though it should be admitted that there exists an immense interval void of stars and planets, and that the fixed stars are incomparably greater than the sun, it proves nothing against the system of Copernicus.

That the stars become nearer and smaller in the system of Tycho, are things too vague to prove any thing in his favour; because we have no more knowledge respecting their real size than their true distance.

Tycho also asked, how it was possible for the earth to preserve the

* According to Tycho, the apparent diameter of the stars of the first magnitude was two or three minutes!

parallelism of its axis during the whole of its revolution round the sun, or how one and the same body could have two different motions, one of which transports the centre of the globe, and the other which changes the position of its axis? But although the axis continues to maintain its parallelism during the whole of its annual revolution, this is not a peculiar motion as Tycho supposed, and forming a third motion, in the sense which Copernicus views motion. It is a situation natural to the axis, which does not change, for there is no cause to make it change.

It is sufficient that the axis has once been directed to a particular point in the heavens, for it still continues to be directed there, though the earth itself be constantly moving forward in its orbit. There is no physical nor mathematical reason from whence it can be concluded, that the axis would be perpendicular to the plane of the orbit, if the earth had a motion on its axis-there is no connection between these two motions.

In the time that all parts of the earth are thrown from the same side by a projectile force, they all acquire the same velocity, as well as a parallel direction; but this change does not affect the parts with respect to each other, nor in the least alter the position which they ought to have. All parts receive the same impression; there is a perfect equilibrium between the superior and inferior parts; and they all preserve the motion of rotation which they had before, or each particle moves in a direction parallel to that which it had when the earth was at rest.

A body having begun to move round its axis, the two poles, or the two immoveable points round which the body turns, receive the same motion by impulse from the centre which produces the motion of translation: and if they receive the same motion, there is no reason for supposing that one of these points advances faster than the other; and as the two make equal advances, they will necessarily be always in a line parallel to that in which they were when they began to

move.

When a top turns upon a table by a rotatory motion, the table may be moved either up or down, or from right to left, obliquely or circularly, without occasioning any difference in the movement of the top. The top may also be thrown in any direction whatever, and yet continue to turn on the same axis. A ball thrown from the mouth of a cannon almost always turns round an axis, and although that axis may sometimes be vertical, horizontal, or inclined, yet this depends on the obstacles which it meets with in one or other of these directions before it leaves the mouth of the cannon. But as the earth meets with no obstacle in its revolution, the axis always continues to point in the same direction.

As the annual and diurnal motions of the earth are the foundation of all astronomy, both physical and practical, we have been at particular pains to refute the objections which have been started against these motions by the celebrated Tycho Brahe, because they are the strongest philosophical arguments that have ever been urged against the Copernican system. But had Tycho known what has been dis

covered since his death he would not have started many of the objections which he has done against this system.

CARTESIAN SYSTEM.

The next system was the Cartesian. Its founder, Rhène Des Cartes, flourished about the beginning of the 17th century. He supposed that every thing in the universe was formed from very minute bodies, called atoms, which had been floating in open space. To each atom he attributed a motion on its axis; and he also maintained that there was a general motion of the whole universe round like a vortex, or whirlpool. In the centre of this vortex was the sun, with all the planets circulating round him at different distances; those that were nearer the sun circulating faster than those at a greater distance, as the most distant parts of a vortex or whirlpool are known to do. Besides this general vortex, each of the planets had a particular vortex of its own, by which its satellites, if it had any, were whirled round, and any other body that came within its reach.

This is the celebrated system of vortices, invented by Des Cartes. The fabric, it must be confessed, is raised with great art and ingenuity, and is evidently the produce of a lively fancy and a fertile imagination. But then, it can be considered only as a philosophical romance, which amuses without instructing us; and serves principally to shew that the most shining abilities are frequently misemployed.

In this hypothesis, Des Cartes supposes extension to constitute the essence of matter, and wholly neglects solidity, as well as the inertia by which it resists any change in its state of motion or rest, which principally distinguishes body from space; and, consequently, the doctrine of an universal plenum, deduced from this definition, is founded upon false principles.

That there is such a thing as a vacuum in nature, or a space void of body, may be demonstrated from various experiments. By means of the air-pump, we can so far exhaust the air from a glass receiver, that a piece of gold and a feather, let fall together, from the top of the vessel, shall both descend equally swift, and come to the bottom at the same time: which evidently shews, that, the air being taken away, there remains no other matter sufficient to cause any sensible resistance, or that in the least impedes or obstructs their passage.

It was said by many of the ancient philosophers, that nature abhors a vacuum; and by means of this dogma, and others of a like nature, they attempted to prove and illustrate the doctrine of an universal plenum, like that of Des Cartes. But this is an assertion, unsupported by facts, and too idle a notion to require any formal refutation: and in nearly the same predicament are most of the other arguments which have been used in defence of this doctrine. They are all sufficiently exposed, not only by the Torricilian experiment, and the nature of pumps in general, but likewise from the most obvious phenomena of the constant and free motion of bodies, whether celestial or terrestrial, which come continually under our inspection.

On the system of Des Cartes, and all others that depend on the