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of Saturn. But, by their mutual attraction, the planes of the orbits are all kept together as if they were solidly attached to the planet.

There is a remarkable resemblance between the physical make-up of this planet and that of its neighbour Jupiter. They are alike remarkable for their small density, that of Saturn being even less than that of water. Another point of likeness is the rapid rotation, Saturn turning on its axis in 10 hours 14 minutes, a little more than the period of Jupiter. The surface of the planet also seems to be variegated with cloud-like forms, similar to those of Jupiter, but far fainter, so that they cannot be seen with any distinctness.

What has been said of the probable cause of the small density of Jupiter applies equally to Saturn. The probability is that the planet has a comparatively small but massive nucleus, surrounded by an immense atmosphere, and that what we see is only the outer surface of the atmosphere.

A curious fact which bears on this view is that the satellite Titan is far denser than the planet. Its cubical contents are about one ten-thousandth those of the planet. But its mass, as found from the motion of Hyperion, is one forty-three-hundredth that of the planet.