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the density may be as great as that of the earth or any other solid planet, and that the small average density of the entire mass is due to the vapourous character of the matter which surrounds this nucleus. In all probability the nucleus is at a very high temperature, even approximating that at the surface of the sun, but this temperature gradually diminishes as we ascend through the gaseous atmosphere, as we suppose to be the case with the sun; hence it may happen that, at the surface, none of the material that we see is at a high enough temperature to radiate a sensible amount of heat.

On the whole we may describe Jupiter as a small sun of which the surface has cooled off till it no longer emits light.

When Galileo first turned his little telescope on the planet Jupiter he was delighted and surprised to find it accompanied by four minute companions. Watching them from night to night, he found them to be in revolution around their central body as, upon the theory not fully accepted in his time, the planets revolve around the sun. This remarkable resemblance to the solar system was a strong point in favor of the Copernican Theory.

These bodies can be seen with a common spy-glass, or even a good opera glass. It has even been supposed that good eyes sometimes see them without optical assistance. They are certainly as bright as the smallest stars visible to the naked eye, yet the glare of the planet would seem to be an insuperable obstacle to their visibility, even to