con- with; and the mass of the heavenly bodies is determined by this power of disturbing other bodies. In the case of the comets, no disturbing power has been detected, and no mass can be assigned to them. How stupendous an object must Jupiter have appeared to an eye in Lexell's comet, when it swept through its satellites. The disc must have covered a great part of the heavens as a brilliant canopy, and the rotation would be distinctly sensible, as some marked cloud would be seen appearing at one edge of the disk, and, in five hours, disappearing at the other. Jupiter is by far the largest planet in the system. His dimensions are such, that it would take 1491 terrestrial globes to equal it. The density is, however, only that of water, so that the increased weight of objects on its surface, is not so great as might be supposed. A man's weight would be little more than doubled.
In setting out again on our journey, we might expect a new station by halving the distance between Jupiter and the sun, as this process served us in the case of our last two stages. We are so far successful, that we find not one large planet, but thousands of small ones. We have, from the earth, discovered only seventy-one, but, in all probability, there are many yet to be discovered; there may be thousands too small to be detected. In sweeping through the zone of asteroids, we are like a ship threading her way through innumerable icebergs, large and small.
