THE LA}Y OF GRAVITATION. 479 



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 phiced 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 dis- 

 tance proportional to the comparative mass of the other. 



If the moon and the earth were not retained in their orbits by their an- 

 nual 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 tlie 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 investi- 

 gator of the phenomena of gravitation should be able, by his dis- 

 coveries, 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 specu- 

 lated 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 ease 

 was rectilinear, and in the other curvilinear. Again, not to over- 

 estimate 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 pre- 

 sent, 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 



