The curious darkness of the inner or crape ring is easily explained. The particles composing it are not packed together so closely as in the outer ring, and therefore reflect less sunlight. Indeed, so sparsely strewn are the particles in this ring that it is in great measure transparent to the sunlight, as is shown by a recorded observation of one of the satellites which was distinctly although faintly seen while moving through the shadow of the dark ring, but disappeared in total eclipse when it entered the shadow cast by the bright ring. 144. THE BALL OF SATURN.--The ball of the planet is in most respects a smaller copy of Jupiter. With an equatorial diameter of 76,000 miles, a polar diameter of 69,000 miles, and a mass 95 times that of the earth, its density is found to be the least of any planet in the solar system, only 0.70 of the density of water, and about one half as great as is the density of Jupiter. The force of gravity at its surface is only a little greater (1.18) than on the earth; and this, in connection with the low density, leads, as in the case of Jupiter, to the conclusion that the planet must be mainly composed of gases and vapors, very hot within, but inclosed by a shell of clouds which cuts off their glow from our eyes. Like Jupiter in another respect, the planet turns very swiftly upon its axis, making a revolution in 10 hours 14 minutes, but up to the present it remains unknown whether different parts of the surface have different rotation times. 145. THE SATELLITES.--Saturn is attended by a family of nine satellites, a larger number than belongs to any other planet, but with one exception they are exceedingly small and difficult to observe save with a very large telescope. Indeed, the latest one is said to have been discovered in 1898 by means of the image which it impressed upon a photographic plate, and it has never been _seen_. Titan, the largest of them, is distant 771,000 miles from the planet and bears much the same relation to Saturn that Satellite III bears to Jupiter, the similarity in distance, size and mass being rather striking, although, of course, the smaller mass of Saturn as compared with Jupiter makes the periodic time of Titan--15 days 23 hours--much greater than that of III. Can you apply Kepler's Third Law to the motion of Titan so as to determine from the data given above, the time required for a particle at the outer or inner edge of the ring to revolve once around Saturn?