It need not be at all surprising that much uncertainty should exist with regard to the actual condition of the surface of Mars. The circumstances in which we are able to see that planet at the best are, indeed, hardly sufficient to warrant us in propounding any hard and fast theories. One of the most experienced of living observers, the American astronomer, Professor E.E. Barnard, considers that the view we get of Mars with the best telescope may be fairly compared with our naked eye view of the moon. Since we have seen that a view with quite a small telescope entirely alters our original idea of the lunar surface, a slight magnification revealing features of whose existence we had not previously the slightest conception, it does not seem too much to say that a further improvement in optical power might entirely subvert the present notions with regard to the Martian canals. Therefore, until we get a still nearer view of these strange markings, it seems somewhat futile to theorise. The lines which we see are perhaps, indeed, a foreshortened and all too dim view of some type of formation entirely novel to us, and possibly peculiar to Mars. Differences of gravity and other conditions, such as obtain upon different planets, may perhaps produce very diverse results. The earth, the moon, and Mars differ greatly from one another in size, gravitation, and other such characteristics. Mountain-ranges so far appear typical of our globe, and ring-mountains typical of the moon. May not the so-called "canals" be merely some special formation peculiar to Mars, though quite a natural result of its particular conditions and of its past history? THE ASTEROIDS (OR MINOR PLANETS) We now come to that belt of small planets which are known by the name of asteroids. In the general survey of the solar system given in Chapter II., we saw how it was long ago noticed that the distances of the planetary orbits from the sun would have presented a marked appearance of orderly sequence, were it not for a gap between the orbits of Mars and Jupiter where no large planet was known to circulate. The suspicion thus aroused that some planet might, after all, be moving in this seemingly empty space, gave rise to the gradual discovery of a great number of small bodies; the largest of which, Ceres, is less than 500 miles in diameter. Up to the present day some 600 of these bodies have been discovered; the four leading ones, in order of size, being named Ceres, Pallas, Juno, and Vesta. All the asteroids are invisible to the naked eye, with the exception of Vesta, which, though by no means the largest, happens to be the brightest. It is, however, only just visible to the eye under favourable conditions. No trace of an atmosphere has been noted upon any of the asteroids, but such a state of things is only to be expected from the kinetic theory.