Mercury’s force of gravity is small. It is smaller than that of any of the other planets. It is a little less than one-quarter that of the earth. The same weight of feathers that would compose a pillow here would make a whole feather bed on Mercury. Any object weighing one hundred pounds here would weigh only twenty-four there. The materials composing our earth and all the planets are held together only by the force of gravity. The air we breathe would dart off into space with almost incredible fleetness if the earth had not sufficient gravitative force to hold it. Its particles are struggling all the time to get beyond this power. The lightest of them do get beyond it and are lost, and the less power we have to hold them the sooner they leave us. The greater the mass of a body, the rarer the gases it can hold in its atmosphere, for this mysterious force which pulls everything toward the center of a planet depends upon its mass, or the quantity of material in it. The planet may be very large because it is very much expanded. It may be gaseous even, and its mass would then be very small in proportion to that of a solid body of the same size. As it condenses, the particles draw closer and closer together, the density increases; but the mass is the same. It is only the size that diminishes. So a planet with a small mass starts out in life with a disadvantage. It not only has little power to grow by drawing in particles from its environment, but also has little power to hold such as by their nature are volatile and swift of motion, as the molecules of gases are. The mass of Mercury is not exactly known. The only way we have of measuring the masses of the planets is by their influence through gravitation on other bodies near them. When a planet has satellites, the movements of the satellites tell the story, and by mathematical calculation the amount of material in the planet can be determined. But Mercury has no satellite, and the only way to determine his mass is by observation of his influence on Venus, and on an occasional comet which passes near enough to be disturbed by the planet. The particular comet which has been useful in determining the mass of Mercury is Encke’s. On passing near the sun it comes sometimes near Mercury, and the pull it has repeatedly received from that little planet on such occasions is thought to be largely responsible for the comet’s having become a part of the solar system. The changes in its orbit caused by these encounters show the power of Mercury, and hence the mass.