We rapidly approach, and when we have arrived within a distance comparable with that of its planets, we see that the sun has increased in apparent magnitude.

And now we observe that this star that we have drawn near to has attending it a number of minute satellites, faintly shining specks, that circle about it as if charmed, like night-wandering insects, by its splendor.

It is manifest to us at the first glance that without the sun these obedient little planets would not exist; it is his attraction that binds them together in a system, and his rays that make them visible to one another in the abyss of space.

One striking fact that commands our attention as soon as we have entered the narrow precincts of the solar system is the isolation of the sun and its attendants in space.

From the sun to the nearest star, or other sun, the distance is approximately five thousand times the entire diameter of the solar system. But the vast majority of the stars are probably a hundred times yet more remote.

The former class comprises Mercury, Venus, the earth, and Mars, and the latter Jupiter, Saturn, Uranus, and Neptune. I have named them all in the order of their distance from the sun, beginning with the nearest.

This alone might suffice to render some of the planets impossible places of abode for us, unless a decided change were effected in our present physical organization.

One of the first questions that we should ask about a foreign world to which we proposed to pay a visit, would relate to its atmosphere. We should wish to know in advance if it had air and water, and in what proportions and quantities.

The nearer a planet is to the sun the more rapidly it is compelled to move in its orbit, and Mercury, being the nearest to the sun of all the planets, is by far the swiftest footed among them.

Next let us consider the effects of this rapid fall, or approach, toward the sun, which is so foreign to our terrestrial experience, and so appalling to the imagination.

We might, for instance, suppose that, owing to the rarity of its atmosphere, the excessive heat was quickly radiated away, or that there was something in the constitution of the atmosphere that greatly modified the effective temperature of the sun's rays.

Schiaparelli had satisfied himself, by a careful series of observations, of the truth of his strange announcement, but before giving it to the world he determined to make doubly sure. Early in 1890 he withdrew the pledge of secrecy from his friends and published his discovery.

As it is difficult to understand how water can exist on the night hemisphere, except in the shape of perpetual snow and ice, so it is hard to imagine that on the day hemisphere water can ever be precipitated from the vaporous form.

In truth, there can be very little water on Mercury even in the form of vapor, else the spectroscope would have given unquestionable evidence of its presence. Those who think that Mercury is entirely waterless and almost, if not quite, airless may be right.