There was an astronomer of very great skill, who lived about the same time as Copernicus, who thought that a mean might be struck between the theories of Ptolemy and Copernicus, by which the favourite idea of a celestial revolution round the Earth should still be retained, and so concession might be made both to the growing authority of science, and to the persuasion of men's senses. This astronomer was Tycho Brahé, who was born in 1546, at Knudstrup, in Denmark. The system advocated by him, and which is named from the inventor the Tychonic system, is represented in the annexed diagram.

In this system, which was published about the year 1586, the Sun is considered as a centre, round which five of the planets revolve; namely, Mercury, Venus, Mars, Jupiter, and Saturn; while, at the same time, the Sun himself, with all those planets, revolves round the Earth. It certainly appears strange, that a man of such eminent abilities as Tycho was, should have preferred this ponderous arrangement to the more simple theory advocated by Copernicus. After the death of Tycho, his theory gradually sank in the estimation of philosophers, who found, in the arrangement advocated by Copernicus, the means of explaining celestial phenomena by less complex reasonings than by the theories either of Ptolemy or of Tycho Brahé.

Here, then, we arrive at an important part of our subject; that there are, as we shall hereafter show more clearly, immense bodies revolving in orbits which measure millions of miles across. The appearances presented to the eye lead to an opinion that these bodies revolve round the earth; but a further inquiry into the accompanying circumstances, have led men universally, at the

present day, to conclude that the Earth and the other planets revolve round the Sun.

What then can keep these splendid bodies in motion? How did they first begin to move? And why do they revolve round the Sun instead of moving in a straight line? These questions naturally occur to our minds,. for our humble faculties can give us but a faint idea of the powers necessary to keep such immense masses in motion. If we see a carriage passing with great rapidity along a railway, we can account for its motion, by tracing the action of steam-pressure upon a piston; which by connecting machinery makes the axles of the wheels revolve, and thus sets the whole vehicle in motion. If we see a carriage passing along the street, we can assign a cause for its metion, by the muscular efforts of the horse which is attached to it. If a ball be shot from a cannon, its flight is so rapid as to render it invisible to us, but we can account for its motion by considering that a quantity of gunpowder, small in bulk, suddenly enlarges to about two thousand times its former dimensions, by being converted into gas, when heat is applied; and that the endeavour to obtain two thousand times as much room as it before occupied, acts with such pressure against the ball as to force it out with fearful velocity. All this we can understand, because we can trace the progress of the occurrences step by step. But when we turn to the heavens, we are lost in wonder! Our means of judging admit of no such details of comparison as those which before assisted us; and we are brought to the conclusion that the Almighty, for his own wise purposes, impressed upon the heavenly bodies those motions which we know they possess,-which we can calculate

and measure,—but the origin of which we can here never know.

But ought this circumstance to prevent us from studying the nature and extent of the motions impressed upon these glorious bodies? Is it presumptuous in us to endeavour to become acquainted with the laws which, once known, will give us additional proofs of the wisdom and power of the Divine Architect of the heavens? Assuredly not! The origin of those movements we know not, but the study of the nature and direction of them is a noble and fitting employment for the human mind.

In the century following the age of Copernicus and Tycho Brahé, there sprang up a genius, who was destined to add more to the amount of human knowledge in this path than any one, perhaps, who had previously existed. This was Sir Isaac Newton, who was born in 1642, at Woolsthorpe, in Lincolnshire. This distinguished man displayed from boyhood an ardent love for the study of the natural phenomena around him. When a child he made clepsydras, or clocks which told the hour by the descent of water, through an orifice at the bottom of a vessel. He also constructed a windmill, which was worked in part by a mouse placed inside. As he grew towards manhood, his soaring mind directed itself to the sublime phenomena presented by the heavens, and to the nature and composition of light. That part of his career which more particularly concerns us at present, we will now consider.

When Newton was about twenty-three years of age, he was forced to leave Cambridge where he had been residing, on account of the appearance of the plague