“There is therefore a conflict between the carrying power of the sun and the impotence or material sluggishness (_inertia_) of the planet; each enjoys some measure of victory, for the former moves the planet from its position and the latter frees the planet’s body to some extent from the bonds in which it is thus held, ... but only to be captured again by another portion of this rotatory virtue.”[93] The annexed diagram is given by Kepler in illustration of this rather confused and vague theory. [Illustration: FIG. 63.—Kepler’s idea of gravity. From the _Epitome_.] He believed also in a more general “gravity,” which he defined[94] as “a mutual bodily affection between allied bodies tending towards their union or junction,” and regarded the tides as due to an action of this sort between the moon and the water of the earth. But the speculative ideas thus thrown out, which it is possible to regard as anticipations of Newton’s discovery of universal gravitation, were not in any way developed logically, and Kepler’s mechanical ideas were too imperfect for him to have made real progress in this direction. 151. There are few astronomers about whose merits such different opinions have been held as about Kepler. There is, it is true, a general agreement as to the great importance of his three laws of planetary motion, and as to the substantial value of the _Rudolphine Tables_ and of various minor discoveries. These results, however, fill but a small part of Kepler’s voluminous writings, which are encumbered with masses of wild speculation, of mystic and occult fancies, of astrology, weather prophecies, and the like, which are not only worthless from the standpoint of modern astronomy, but which—unlike many erroneous or imperfect speculations—in no way pointed towards the direction in which the science was next to make progress, and must have appeared almost as unsound to sober-minded contemporaries like Galilei as to us. Hence as one reads chapter after chapter without a lucid still less a correct idea, it is impossible to refrain from regrets that the intelligence of Kepler should have been so wasted, and it is difficult not to suspect at times that some of the valuable results which lie imbedded in this great mass of tedious speculation were arrived at by a mere accident. On the other hand, it must not be forgotten that such accidents have a habit of happening only to great men, and that if Kepler loved to give reins to his imagination he was equally impressed with the necessity of scrupulously comparing speculative results with observed facts, and of surrendering without demur the most beloved of his fancies if it was unable to stand this test. If Kepler had burnt three-quarters of what he printed, we should in all probability have formed a higher opinion of his intellectual grasp and sobriety of judgment, but we should have lost to a great extent the impression of extraordinary enthusiasm and industry, and of