A hoop when trundled would tumble on its side if the force of gravitation was not overcome by the centrifugal force which imparts to it a motion in the direction of a tangent (_tango_, to touch) to a circle. The same principle applies to the spinning-top--this toy cannot be made to stand upon its point until set in rapid motion. Returning again to the subject of gravitation, we may now consider it in relation to other and more magnificent examples which we discover by studying the science of astronomy. [Page 19] CHAPTER III. THE SCIENCE OF ASTRONOMY. In a work of this kind, professedly devoted to a very brief and popular view of the different scientific subjects, much cannot be said on any special branch of science; it will be better, therefore, to take up one subject in astronomy, and by discussing it in a simple manner, our young friends may be stimulated to learn more of those glorious truths which are to be found in the published works of many eminent astronomers, and especially in that of Mr. Hind, called "The Illustrated London Astronomy." One of the most interesting subjects is the phenomenon of the eclipse of the sun; and as 1858 is likely to be long remembered for its "annular eclipse," we shall devote some pages and illustrations to this subject. Eclipses of the sun are of three kinds--partial, annular, and total. Many persons have probably seen large partial eclipses of the sun, and may possibly suppose that a total eclipse is merely an intensified form of a partial one; but astronomers assert that no degree of partial eclipse, even when the very smallest portion of the sun remains visible, gives the slightest idea of a total one, either in the solemnity and overpowering influence of the spectacle, or the curious appearances which accompany it. The late Mr. Baily said of an eclipse (usually called that of Thales), which caused the suspension of a battle between the Lydians and Medes, that only a total eclipse could have produced the effect ascribed to it. Even educated astronomers, when viewing with the naked eye the sun nearly obscured by the moon in an annular eclipse, could not tell that _any part_ of the sun was hidden, and this was remarkably verified in the annular eclipse of the 15th March of this year. During the continuance of a total eclipse of the sun, we are permitted a hasty glance at some of those secrets of Nature which are not revealed at any other time--glories that hold in tremulous amazement even veteran explorers of the heavens and its starry worlds.