If, as is likely, some of the smallest asteroids are not more than ten miles in diameter, their gravity must be so feeble a force that it might be overcome by a stone thrown from the hand. There is no reliable evidence that any of the asteroids are surrounded by atmospheric gases of any sort. Probably they are for the most part spherical in form, although there is very reliable evidence that a few of the asteroids, being variable in the amount of sunlight that they reflect, are irregular in form, mere angular masses perhaps. The network of orbits of the asteroids is inconceivable complicated. Nevertheless, there is a wide variation in their average distance from the sun, and their periods of traveling round him vary in a similar manner, the shortest being only about three years. While the longest is nearly nine years in duration, the average of all their periods is a little over four years. The gap in the zone of asteroids, at a distance from the sun equal to about five-eighths that of Jupiter, is due to the excessive disturbing action of Jupiter, whose periodic time is just twice as long as that of a theoretical planet at this distance. The average inclination of their orbits to the plane of the ecliptic is not far from 8 degrees. But the orbit of Pallas, for example, is inclined 35 degrees, and the eccentricities of the asteroid orbits are equally erratic and excessive. Both eccentricity and inclination of orbit at times suggest a possible relation to cometary orbits, but nothing has ever been definitely made out connecting asteroids and comets in a related origin. No comprehensive theory of the origin of the asteroid group has yet been propounded that has met with universal acceptance. According to the nebular hypothesis the original gaseous material, which should have been so concentrated as to form a planet of ordinary type, has in the case of the asteroids collected into a multitude of small masses instead of simply one. That there is a sound physical reason for this can hardly be denied. According to the Laplacian hypothesis, the nearness of the huge planetary mass of Jupiter just beyond their orbits produced violent perturbations which caused the original ring of gaseous material to collect into fragmentary masses instead of one considerable planet. The theory of a century ago that an original great planet was shattered by internal explosive forces is no longer regarded as tenable. To astronomers engaged upon investigation of distances in the solar system, the asteroid group has proved very useful. The late Sir David Gill employed a number of them in a geometrical research for finding the sun's distance, and more recently the discovery of Eros (433) has made it possible to apply a similar method for a like purpose when it approaches nearest to the earth in 1924 and 1931. Then the distance of Eros will be less than half that of Mars or even Venus at their nearest.