de of the rings for a space
of nearly fifteen years--a day fifteen years long! And then, when that
face of the ring is turned away from the sun, there ensues a night of
fifteen years' duration also.

Whatever appearance the rings may present from the equator and the
middle latitudes on Saturn, from the polar regions they would be totally
invisible. As one passed toward the north, or the south, pole he would
see the upper part of the arch of the rings gradually sink toward the
horizon until at length, somewhere in the neighborhood of the polar
circle, it would finally disappear, hidden by the round shoulder of the
great globe.

URANUS, NEPTUNE, AND THE SUSPECTED ULTRANEPTUNIAN PLANET

What has been said of Jupiter and Saturn applies also to the remaining
members of the Jovian group of planets, Uranus and Neptune, viz., that
their density is so small that it seems probable that they can not, at
the present time, be in a habitable planetary condition. All four of
these outer, larger planets have, in comparatively recent times, been
solar orbs, small companions of the sun. The density of Uranus is about
one fifth greater than that of water, and slightly greater than that of
Neptune. Uranus is 32,000 miles in diameter, and Neptune 35,000 miles.
Curiously enough, the force of gravity upon each of these two large
planets is a little less than upon the earth. This arises from the fact
that in reckoning gravity on the surface of a planet not only the mass
of the planet, but its diameter or radius, must be considered. Gravity
varies directly as the mass, but inversely as the square of the radius,
and for this reason a large planet of small density may exercise a less
force of gravity at its surface than does a small planet of great
density.

The mean distance of Uranus from the sun is about 1,780,000,000 miles,
and its period of revolution is eighty-four years; Neptune's mean
distance is about 2,800,000,000 miles, and its period of revolution is
about 164 years.

Uranus has four satellites, and Neptune one. The remarkable thing about
these satellites is that they revolve _backward_, or contrary to the
direction in which all the other satellites belonging to the solar
system revolve, and in which all the other planets rotate on their axis.
In the case of Uranus, the plane in which the satellites revolve is not
far from a position at right angles to the plane of the ecliptic; but in
the case of Neptune, the plane of revolut