planets did not revolve in a perfect circle round the Earth, but that they described a peculiar path called an epicycloid, of the nature of which some idea may be formed from the following illustration:-If we had a large coach wheel, and by any contrivance could make a smaller wheel roll round the outside of it, at its circumference, then any particular point on the small wheel would describe that curve which is called an epicycloid: the axle of the small wheel would describe a perfect circle, but it is easy to see that any point on the edge would not describe a circle. Ptolemy was obliged to multiply these epicycloids to a most perplexing extent, in order to account for the various appearances of the planets. It is to this circumstance that Milton alluded, when he spoke of the shifts and difficulties which beset the progress of those who build their opinions on a wrong foundation :

When they come to model Heaven
And calculate the stars, how they will wield
The mighty frame; how build, unbuild, contrive,
To save appearances; how gird the sphere
With centric and eccentric scribbled o'er,

Cycle and epicycle, orb in orb.-Par. Lost. b. viii.

There is nothing which more beautifully shows the power and force of truth, than the embarrassments which retard the progress of those who do not take truth for their guide. In religion, in morals, in science, he whose steps are guided by the light of truth, can arrive, by a short and pleasant path, at results which others can scarcely obtain by a complex and wearying track. Thus Ptolemy was forced to assume the existence of much unwieldy machinery in the scheme of the heavens, in order to account for those motions which

are most simple and beautiful; this was because he placed (in his own imagination,) the earth in the midst of the heavenly bodies, all of which were made to revolve round it.

Great as were the difficulties which attended the adoption of the Ptolemaic system, it yet retained its ground, with some slight modifications, until the time of Nicholas Copernicus, an eminent astronomer, who was born at Thorn, in Polish Prussia, in 1473. This distinguished individual. perceived the unreasonable results which follow from the theory of Ptolemy. That all the planets, the Sun, the Moon, and all the stars and comets, should revolve round the Earth, seemed to him much more unnatural and complex than that the Earth should revolve on its own axis, and move in an orbit round the Sun. All appearances of the heavenly bodies can be explained with much greater ease and simplicity by the latter supposition than by the former; and Copernicus was thus led, after the study of forty years, to the adoption of a theory which had been advocated by Pythagoras and Thales of Greece, five or six hundred years before the time of Ptolemy.

Figure 4, is a representation of the solar system, according to the theory of Copernicus. In this system the sun, the glorious source of light and heat to us, is placed in the centre. Round him the planets revolve in the following order :-Mercury, Venus, Earth, Mars, Jupiter, and Saturn. Since the time of Copernicus, five more planets have been discovered, namely, Uranus, Pallas, Vesta, Juno, and Ceres. In order to account for the phenomena presented by the Moon, Copernicus assumed, (what has since been confirmed,) that the Moon has a twofold motion,-round the Earth and

THE SOLAR SYSTEM.

round the Sun; a small circle, therefore, surrounds the Earth, which circle represents the path of the Moon round the Earth, while the Earth and the accompanying Moon together revolve round the Sun. This figure of course represents the Copernican system with the addition of the five planets since discovered. The circles represent the orbits of the several planets; that is, the paths in which they travel in their progress round the Sun. The distances of these circles from the centre at which the Sun is placed, could not conveniently be in the same proportion one to another, as the real distances of the planets are respectively from the Sun, because the innermost orbits would be too small to be conveniently seen. The real proportions, however, which exist between the distances of the planets from the Sun, admit of being easily understood. For instance, the Earth, (as we shall hereafter explain more fully,) is about ninety-five millions of miles from the Sun. If now we call that distance 1, the distances of the other ten planets from the Sun, will be represented by the following numbers, with sufficient nearness for our present purpose..