The interesting suggestion was made and urged by Mr. Proctor that if 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 firmament twenty degrees in diameter, or, in round numbers, forty times the diameter of the full moon as seen from the earth! It would shed a great amount of light and heat, and thus would more or less effectively supply the deficit of solar radiation, for we must remember that Jupiter and his satellites receive from the sun less than one twenty-fifth as much light and heat as the earth receives. The maze of contending motions, the rapid flow and eddying of cloud belts, the outburst of strange fiery spots, the display of rich, varied, and constantly changing colors, which astonish and delight the telescopic observer on the earth, would be exhibited to the naked eye of an inhabitant of Jupiter's nearest moon far more clearly than the greatest telescope is able to reveal them to us.