could now be offered in proof of the Copernican doctrine, there is an air of truth about it sufficient to command the assent of a thoughtful mind. Proceeding upon the assumption, which was universally admitted, that the earth is a mere point when compared with the distance of the fixed stars, he very naturally remarked upon the improbability of such a vast circumference revolving in twenty-four hours, instead of the infinitesimal point by which the whole phenomena would be equally as well explained. It had been urged in support of the earth's immobility, that, if it revolved on its axis, objects on its surface would be scattered and dispersed in space by the extreme rapidity of the motion overcoming the force of gravity. He did not see the true reply to this, that such effects would not take place unless the velocity of rotation was greater than the force of gravity, which was an arbitrary assumption, but still he reasonably turned the argument against the objector by observing that the diurnal revolution of the sphere of the universe would be far more likely to derange the situations of the heavenly bodies, and produce their displacement. "Why, then," he exclaims, "do we hesitate to give to the earth the mobility suitable to its form, rather than that the universe, whose bounds we do not and cannot know, should revolve? Why should we not confess that the diurnal revolution is apparent only in the heavens, and real in the earth? Thus Æneas, in Virgil, exclaims, • Provehimur portu, terræque, urbesque recedunt.'

Since, while the ship glides tranquilly along, all external objects appear to the sailors to move in proportion as their vessel moves, and they alone, and what is with them, seem to be at rest."

The other conditions of the problem of the celestial motions, the sun's path in the ecliptic, are as exactly answered by the supposition of the earth's orbital motion. The annual revolution of the earth round the sun causes the apparent annual revolution of the sun round the earth; and when we consider the vast magnitude of the solar orb, and the enormous waste of force implied in moving the greater body around the less, when precisely the same effects are produceable by moving the less about the greater, our common sense is at once enlisted in favour of the latter hypothesis as Nature's "wise and frugal" plan. The apparent eccentricities of the planets likewise, their direct and retrograde movements, that mysterious puzzle which called the epicycles of the Ptolemaists into existence, are explained upon their own principle of two combined motions: an observer on the earth in ceaseless translation sees them performing a similar orbital course, and apparent irregularity and involution are the consequences of the combined prosecution of direct and regular paths. Suppose s the sun, A B C D part of the earth's orbit in the direction of the arrow, a dcb part of the orbit

M

FHG

N

of a superior planet, and M N an arc of the celestial sphere. When the earth is at A, and the planet at a, a terrestrial spectator will see it projected to a place in the heavens at E. The angular motion of a superior planet being less rapid than that of an inferior, when the earth is at B the planet may be supposed to be at d, and its place will be projected in the heavens at F, thus apparently retrograding in the sphere from E to F, while accomplishing the direct movement from a to d. The next movement

of the earth, B C, and of the planet, dc, will produce a

further retrogradation of the latter in the heavens from F to G; but, when the earth has arrived at D, and the planet at b, the retrocession of the planet will appear to have ceased, and the direct movement, GH, to have taken place. As both the earth and the planet proceed in their orbits, the planet will appear stationary among the fixed stars, then to

pursue a direct course, and afterwards to retrograde again. As an hypothesis, therefore, the Copernican theory had strong presumptive evidence in its favour, accounting for the celestial movements, and being in beautiful congruity with Nature's frugal and simple plan of general operation. Still presumptive evidence is not positive proof, nor could the truth of the theory at that time be demonstrated. It detracts not from the glory of its author, that to others the merit belongs of establishing his leading views as a real expression of the phenomena of the universe. He had not the instruments by which alone this could be done. He seems to have entertained a noble confidence that he had conceived the true system, and that future discoveries would remove the mechanical difficulties then in its way, and a more enlarged observation of physical facts place it upon the basis of incontrovertible evidence, a confidence which has been amply justified.

The sixteenth century, rendered memorable at its commencement by the foundation being laid of true views respecting the constitution of the universe, was distinguished at its close by the labours of TYCHO BRAHÉ, a Dane, born at Knudsthorp, near the Baltic, three years after Copernicus terminated his career. His attention was called to astronomy by a great eclipse of the sun, August 21. 1560, when quite a child. Upon being sent to the university for his education, he was accustomed to watch the constellations while his tutor slept. Of noble extraction, and strongly influenced

JUPITER

himself by prevailing aristocratic prejudices, he at length conquered the pride of his order, devoted himself to public usefulness in the particular department to which his natural genius was inclined, became a student, an author, an astronomical lecturer, and finally completed his offences against the pride of life by marrying a plebeian. This last step, probably, rendered exile desirable, in order to escape from the slights of his relatives. He found a welcome reception at the court of the Landgrave William of Hesse-Cassel, a prince who was himself an ardent student of astronomy, of whom it is related that, while observing the brilliant new star of 1572, his servants ran to tell him that the house was on fire: but he quietly pursued his task to its completion. The fame of Tycho has been obscured by his rejection of the Copernican doctrine, and the construction of a system of his own, combining the elements of the Ptolemaic and Copernican theories. He maintained the earth to be the immoveable centre of the universe, but supposed the planets to revolve round the sun, and to be carried with their centre in revolution round the earth.

SATURN

MARS

VENUS

MOON

OEARTH

The adoption of this hypothesis has been usually deemed discreditable to Tycho; but it will be only fair to recollect that the Copernican theory was, in his day, quite incapable of proof. He argued, against the diurnal motion of the earth, that, upon that assumption a stone dropped from the summit of a high tower would not fall at the base, as we see it does, because the velocity of rotation would carry the tower several hundred feet during the descent of the stone,

T

which would, therefore, fall at that distance behind it. This argument was employed by Ptolemy, who stated that, if the earth revolved with great rapidity from west to east, it would leave behind it the clouds, birds flying in the air, and, generally, all objects suspended in the atmosphere. The answer to this is, that a falling body will partake of a rectilinear and circular motion; the former tending to the centre of gravity, the latter proceeding in the direction of the circumference described by the point from which it falls. Gassendi tried experiments in the harbour of Marseilles, and proved, what every one now knows, that a stone dropped from the mast of a vessel in full sail will partake of the advance of the mast, and fall at its foot, as though the vessel were at rest. It may also be proved, that a stone falling from a considerable elevation, so far from being left behind during its descent, will fall in advance of the base of the perpendicular, upon the theory of the earth's rotation. Let the circle E be the equatorial circumference of the earth, the line T a tower perpendicular to c the centre, and the circle м will then be the circumference described by the summit of the tower s, in the course of one rotation of the earth upon its axis. If we suppose the base of the tower, b, to pass to c, the summit, s, will, in the same time, pass to a, and, this being the larger arc, it follows that the summit must travel faster than the base. Assuming, then, the earth's rotation eastward, a stone dropped from the summit will leave it with its momentum, and will move faster eastward, through the whole of its descent, than the base. The result will be, that it will deviate from the plumb-line, and reach the ground a little to the east of c, the foot of the perpendicular. Experiment has confirmed the accuracy of this reasoning, though obviously a very difficult matter to test, owing to the comparatively small height of buildings suitable to the purpose. Newton first threw out the idea, and calculated that a ball would deviate about half an inch from the plumb-line, to the east, from the height of three hundred feet. Thirty balls descended from the height of two hundred and thirty-five feet, in St. Michael's tower Hamburg, and deviated from the perpendicular four lines eastward, swerving also one line and a half southward, owing probably to a current of air in the tower. The experiment was, therefore, repeated in a coal-pit, in the county of Mark, two hundred and sixty feet deep. There the balls fell five lines eastward of the perpendicular, but neither northward nor southward, so that the theory of the earth's motion on its axis may now be said to have received a complete and sensible confirmation.

M E

M

The reason which led Tycho to reject the doctrine of the earth's orbital motion was of great force in his time. Supposing the earth to revolve round the sun, two points of the orbit will be distant from each other by the whole diameter of the orbit; yet lines drawn from those points to the nearest fixed star discovered no appreciable angle, or annual parallax. It is sufficient to reply now, that the distance of the stars is so great as to render insensible the diameter of the earth's orbit; but, previously to the invention of the telescope, this answer could not be deemed so satisfactory. To the naked eye the stars of the first magnitude present a diameter equal to two minutes of space. The telescope has shown this to be an optical delusion, that no star has an apparent diameter of a second. But Tycho, knowing that if the earth moved, its change of place from one extremity of the diameter of its orbit to another produced no sensible alteration in the place of the stars, so that they must be at an enormous distance, and yet seeing them present a diameter to the eye varying from a quarter to as much as two minutes, had little option but to assign a magnitude utterly inconceivable to the nearest fixed star, or reject the theory of the earth's motion in space. He chose the latter alternative; and, though undoubtedly it would have been a wiser course to have paused before coming to such a conclusion,

which the Copernican system as then developed might well have justified when viewed as a whole, we are not warranted in interpreting his rejection of it much to his disparage

ment.

The career of Tycho is far more satisfactory as a practical astronomer than as a theorist; indeed, few men have done so much for the advance of the science, or are more worthy of praise for the amount of toil they have undergone, and the success of their labours. In the year 1572, the remarkable event of a stellar apparition attracted the attention of Europe, and excited universal astonishment and speculation. It was early in November, when Tycho, who then resided at his paternal home, observed a star of great splendour in the constellation Cassiopeia, which he had never seen before, as he was walking across the fields about ten o'clock in the evening. It beamed with a lustre quite unwonted in that part of the heavens. It could not have escaped his observation had it previously been there. He suspected at first an optical illusion occasioned by some defect of his own vision, but found a group of peasants gazing upon it with as much astonishment as himself. Its place he at once fixed by his instruments, and noted the fact with all its circumstances in his journal. Soon afterwards, when at Copenhagen, he found that the scientific men of the university had not observed the stranger, and excited some derision at a convivial party by mentioning the phenomenon, which, however, he soon turned into surprise by pointing out the star. One of his works is devoted to this object. The same star was observed by Cornelius Gemma, who had particularly examined that part of the heavens two nights previous, and was confident of its not being present then. It continued visible for the space of sixteen months, gradually diminishing in lustre, until it finally vanished in March 1574. The brilliance of this star was so great as at first to cause Tycho's staff to deflect a shadow. Its light changed from white when the brightest to a yellowish hue, and afterwards had a fiery tinge like Mars, becoming livid like Saturn before its disappearance. During the whole time of its visibility, its place in the heavens remained unaltered; it had no annual parallax, consequently its locality was far beyond the bounds of our system, at a remote distance in the region of the fixed stars. The appearance of a new star had been observed in Europe about three centuries previous, and such occurrences are mentioned by the ancients; but this taking place in a comparatively enlightened age naturally excited serious observation and inquiry. The idea of the Danish astronomer, never happy in his philosophical speculations, was not very fortunate. He supposed that it was produced by a condensation of the matter collected in the Via Lactea, in which it was situated.

The appearance of a comet in 1577 was industriously watched by Tycho, and he was led to some important conclusions. Unable to detect any sensible parallax, it became obvious that these bodies move at a remote distance from the earth, and were not, according to the common opinion of the times, sublunary objects; and observing the spheres of

the planets cut by them in every direction, the inference was clear that the planetary spheres had no material existence. It may seem idle now to notice so absurd a notion as that of the planets moving in solid transparent spheres, but it was not so then; and even Newton deemed it necessary to argue against the monstrous doctrine in the Principia. The labours of Tycho were also directed towards forming a catalogue of the stars, and he determined the relative and absolute positions of 777, a work inferior as to numbers to preceding catalogues, but vastly superior on account of its scrupulous exactness. He amassed, likewise, a regular series of observations on the planets, which afterwards, in the hands of Keppler, materially contributed to the framing of his famous laws. To him, also, we owe the discovery of that inequality of the moon called the variation, that of the inequalities of the motion of the nodes, and of the inclination of the lunar orbit; and by a comparison of his own observations with previous ones, he was the first to announce the slow diminution of the obliquity of the ecliptic, which the, theory of universal gravitation now teaches, while he commenced the systematic application to the observed altitudes of the sun, moon, and stars, of the correction required for the refraction of light. In practical astronomy the refraction of the atmosphere is an important element, for, owing to it, all the heavenly bodies appear to us considerably higher than they really are. Let a b, a b, a b, a b, be strata, or layers of the atmosphere, increasing in density towards mn, the surface of the earth. A ray of light from the star s, impinging on the atmosphere, will be refracted, or bent, so as to move

in the curve rrr A; and as an object is seen in the direction of the ray that meets the eye, the star which is actually at s, will seem, to a spectator at A, to be in the direction c. This refraction, which always acts in a vertical direction, elevates objects above their real place, and hence a body at D, below the horizon Ho, will be raised and seen as if at o. Ptolemy, as has been already noticed, was acquainted with the refraction of the atmosphere, and mentions it in a treatise on optics, but as it is not alluded to in his astronomical work, it is presumed that he had not then discovered it, and made no practical use of it in observation. The Arab astronomers, likewise, were aware of the fact through him, but to Tycho Brahe the honour belongs of calculating its effects, correcting altitudes by them in a systematic manner, and forming the first table of refractions. He estimated its amount at 34' with reference to bodies in the horizon, which is nearly correct, but erroneously supposed that it did not exist at elevations greater than forty-five degrees. This was owing to the effects of refraction above that altitude being insensible to his instruments.

The urgent recommendation of Tycho to his own sovereign by the Landgrave William, induced the King of Denmark to offer him an asylum in his own country, and an annual provision for the prosecution of his scientific pursuits. Embracing the offer, he received a grant of the small island of Hoëne in the Baltic, opposite Landscrona, and an annual allowance of two thousand dollars, with the proceeds of a fief in Norway, and a canonry in the church. Here he laid the foundation of a house for himself and an observatory, which ultimately grew into a fantastic castle, resembling rather the abode of an eastern magician than the home of a sober astronomer. Uraniberg, or the castle of the heavens, expressed the large views, feudal spirit, and undisciplined mind of its architect and tenant. He was an admirable observer, had high thoughts of the dignity of his possession, never forgot his own nobility, was abundantly superstitious likewise, and unskilful at systematising. These are characteristics plainly stamped upon the edifice which he constructed. Its front elevation, shown in the vignette, extended sixty feet in length, and seventy-five in height. It was surrounded by a wall twenty-two feet high, in the form of a square, each