If the earth were not round, heavy bodies would not tend from every side toward its center, but to different points from different sides.
If two stones were placed in any part of the universe, near each other, and beyond the sphere of the influence of a third cognate body, these stones would come together at an intermediate point, each approaching the other at a distance proportional to the comparative mass of the other.
If the moon and the earth were not retained in their orbits by their annual force, or some other equivalent, the earth would mount to the moon by a fifty-fourth part of their distance from each other, and the moon would fall toward the earth through the other fifty-three parts, that is, assuming that the substance of the earth is of the same density.
The sphere of the attractive virtue which is in the moon, extends to the earth and entices up the waters, but as the moon flies rapidly across the zenith and the waters cannot follow so quickly, a flow of the ocean is occasioned toward the westward.
If the attractive virtue of the moon extends to the earth, it follows, with greater reason, that the attractive virtue of the earth extends to the moon and much farther, and in short, nothing which consists of earthy substance, however constituted, although thrown up to any height, can ever escape the powerful operation of this attractive virtue.These views of Kepler—so novel at the time they were announced by him and yet which we now know to be in the main so correct—were published more than thirty years before Newton was born. As we read them, our first feeling is one of surprise that any subsequent investigator of the phenomena of gravitation should be able, by his discoveries, to achieve for himself a fame which should not only render his name immortal but should almost wholly hide from view the merit of the great pioneer in this field of inquiry. To appreciate, however, the importance of the work which yet remained to be performed, we should bear in mind that whilst Kepler's views in regard to terrestrial gravity were so remarkably just, he at the same time, in common with the age in which he lived, and, we may say, with all preceding ages—regarded the tendency of bodies near the earth to fall toward its center, and the motions of heavenly bodies, as entirely different phenomena and not at all referable to the same physical cause. He indeed speculated on the possibility of referring the motions of the planets to an attractive force emanating from the sun, similar to that which caused bodies near the earth to tend toward its center, and concluded that such a hypothesis was untenable, inasmuch as the motion in one case was rectilinear, and in the other curvilinear. Again, not to overestimate the merit of Kepler in connection with the discovery of the law of gravitation, we should remember that a theory as to the physical cause of natural phenomena, even if it be in the main correct, will furnish no complete solution of the problems which phenomena present, unless it express accurately and precisely the measure as well as the mode of the action of the assigned cause. For example, to know merely that all matter attracts all matter, would not enable us to
