Jupiter itself is in no condition at present to bear life, its
satellites may be, in that respect, more happily circumstanced. It can
not be said that very much has been learned about the satellites of
Jupiter since Proctor's day, and his suggestion is no less and no more
probable now than it was when first offered.

There has been cumulative evidence that Jupiter's satellites obey the
same law that governs the rotation of our moon, viz., that which compels
them always to keep the same face turned toward their primary, and this
would clearly affect, although it might not preclude, their
habitability. With the exception of the minute fifth satellite
discovered by Barnard in 1892, they are all of sufficient size to retain
at least some traces of an atmosphere. In fact, one of them is larger
than the planet Mars, and another is of nearly the same size as that
planet, while the smallest of the four principal ones is about equal to
our moon. Under the powerful attraction of Jupiter they travel rapidly,
and viewed from the surface of that planet they would offer a wonderful
spectacle.

They are continually causing solar eclipses and themselves undergoing
eclipse in Jupiter's shadow, and their swiftly changing aspects and
groupings would be watched by an astronomer on Jupiter with undying
interest.

But far more wonderful would be the spectacle presented by Jupiter to
inhabitants dwelling on his moons. From the nearer moon, in particular,
which is situated less than 220,000 miles from Jupiter's surface, the
great planet would be an overwhelming phenomenon in the sky.

Its immense disk, hanging overhead, would cover a circle of the