The law of gravity, or of the mutual attraction of masses of matter upon each other, accounts so perfectly for all the observed motions of the heavenly bodies, that we are apt to regard Newton's discovery of the great law as though it had finally solved the mystery of these motions. Many accept the verdict given by the poet Pope in the famous epitaph which he suggested for Newton,-- "Nature and Nature's laws lay hid in night: God said, _Let Newton be!_ and all was Light." But Newton, who probably knew as much about his work as Pope, was of another opinion. Every one knows how he compared himself to a child who had picked up a few shells on the shore, while the ocean of truth lay unexplored before him. He has, however, spoken definitely of the great discovery which has rendered his name illustrious, in terms which show that _he_ did not find that all was light. Among the questions which he specially would have had answered, amongst the secrets of nature concealed beneath the ocean of truth, the mystery of gravity was probably the chief. When Newton asked of the Ocean of Truth what Mrs. Hemans later said, and in another sense, of the natural sea-- "What hidest thou in thy treasure-caves and cells, Thou hollow-sounding and mysterious main?" he had in his thoughts the very power which he is commonly supposed to have explained, but which was in truth for him, more than for any man that had ever lived, the mystery of mysteries. It may be well to consider the very words of the great philosopher, so far at least as our more diffuse language can present the concise expressions of the original Latin: