Now, from observations of the apparent places of a planet, it is possible to determine the relative path of the planet in space as compared with the path of the Earth. This Kepler did from observations of Tycho Brahe's, and showed the wanderers to belong to a system of bodies, all revolving about the Sun in various elliptic orbits, the Sun being at the focus of each ellipse. He also found that the line connecting each planet with the Sun passed over equal areas in equal times, and thirdly, that the squares of the times were as the cubes of the major axes of the orbits. From these three "laws" Newton deduced the fact that the force controlling the planets was directed toward the Sun, that it varied inversely as the square of the distance, and that it was the same in origin for all. This is the so-called law of gravitation, and this is the way in which it was discovered. We do not yet know why gravity so acts, but it is interesting to note that it follows the simple law of geometrical expansion, diminishing in exact ratio to the space it fills, just like electricity or light. It may, therefore, also be a wave motion.
Thus all the wanderers proved to be associated in common dependence on the Sun, and among the members of the solar family thus recognized Mars was found to hold the position next exterior to the Earth, and the path he fol-
