faculty comes from profound reflection on the nature of the subject itself, from a sort of transmuting power which changes the words of the books into the things of reality. Herschel's paper on Saturn, in 1790, is an admirable example of this.
Herschel's observations on Saturn began in 1772. From 1790 to 1808 he published six memoirs on the figure, the ring, and the satellites of this planet. The spheroidal shape of the ball was first discovered by him, and we owe much of our certain knowledge of the constitution of the rings to his work. The sixth and seventh satellites, Mimas and Enceladus, were discovered by him in 1789. The periods of rotation of the ball and of the ring were also fixed. In his conclusions as to the real figure of the rings, there is a degree of scientific caution which is truly remarkable, and which to-day seems almost excessive.
In his paper of 1792, Herschel shows that the most distant satellite of Saturn—Japetus—turns once on its axis in each revolution about its primary, just as our moon does. He says of this:
