Nearly all the minor planets so far found have had names assigned to them principally legendary and mythological, and a nearly complete catalogue of them, containing the elements of their orbits (that is, all the mathematical data that tell us about their distance from the sun and the circumstances of their motion around him) is published each year in the "Annuaire du Bureau des Longitudes" at Paris. But these little planets require a great deal of care and attention, for some astronomers must accurately observe them every few years, and other astronomers must conduct intricate mathematical computations based on these observations; otherwise they get lost and have to be discovered all over again. Professor Watson, of the University of Michigan and later of the University of Wisconsin, endowed the 22 asteroids of his own discovery, leaving to the National Academy of Sciences a fund for prosecuting this work perpetually, and Leuschner is now ably conducting it. [Illustration: JUPITER, LARGEST OF THE PLANETS. The irregular belts change their mutual relation and shapes because they do not represent land, but are part of the atmosphere. (_Photo, Yerkes Observatory._)] [Illustration: THE PLANET NEPTUNE AND ITS SATELLITE. The photograph required an exposure of the plate for one hour. (_Photo, Yerkes Observatory._)] [Illustration: SATURN, AS SEEN THROUGH THE 40-INCH REFRACTOR, at the time when only the edge of the rings is visible, showing condensations. (_Photo, Yerkes Observatory._)] [Illustration: SATURN, PHOTOGRAPHED THROUGH THE 40-INCH REFRACTOR. The rings appear opened to the fullest extent they can be seen from the earth. The picture was made July 7, 1898. (_Photo, Yerkes Observatory._)] While the number of the asteroids is gratifyingly large, their individual size is so small and their total mass so slight that, even if there are a hundred thousand of them (as is wholly possible), they would not be comparable in magnitude with any one of the great planets. Vesta, the largest, is perhaps 400 miles in diameter, and if composed of substances similar to those which make up the earth, its mass may be perhaps one twenty-thousandth of the earth's mass. If we calculate the surface gravity on such a body, we find it about one-thirtieth of what it is here; so that a rifle ball, if fired on Vesta with a muzzle velocity of only 2,000 feet a second, might overmaster the gravity of the little planet entirely and be projected in space never to return.