With this great telescope, Herschel discovered two satellites of Saturn, Mimas and Enceladus; one on August 27, 1789, and the other on September 17 of the same year. Two years before this, January 11, 1787, he discovered two satellites of Uranus, Oberon and Titania. Sixty years afterwards, Mr. Lassell, of England, discovered the remaining two satellites of Uranus, called Ariel and Umbriel. From this time his work went forward grandly. He had already completed more than two hundred seven-foot, one hundred and fifty ten-foot, and eighty twenty-foot mirrors. For many of the telescopes sent abroad he made no stands, but provided the drawings. He wrote much about Saturn and its rings, and showed that its most distant satellite, Japetus, turns once on its axis in each revolution about its primary, as our moon does about the earth. He studied carefully the nature of the sun, its probable gaseous surface, and its spots, and was the first to suspect their periodic character. What would Herschel have said to the wonderful photographic representations of these spots given by Professor Langley, in his New Astronomy; spots which are one billion square miles in size; more than five times the surface of the land and water on the earth? He saw, as astronomers to-day see, that heat cannot be produced without expenditure of force; and that the sun is probably cooling, even though scarcely perceptibly for ages to come. He saw what science now generally concedes, the rise and fall of the solar system; its gradual fitness for the coming of man, through almost countless centuries; and its final unfitness, when his generations shall have gone forever. He wrote much about the Milky Way, believing at first that it could be completely resolved into stars, about eighteen millions of them; but later he changed his theory, having found so much nebulous matter--in a state of condensation as though new worlds were forming, possibly to be the homes of some new race, or of man in the ages to come. His study of the variable stars attracted wide attention. He found that the star _Mira Ceti_ was for several months invisible to the naked eye; then it grew brighter and brighter, and finally disappeared for months, as before. He saw that other stars are periodic, and came to the conclusion that this is occasioned by the rotation of the star upon its axis, by which different parts of its surface are presented to us periodically. He made a catalogue of double stars, and found by laborious calculations that such stars have a common centre of gravity; that one sun revolves about another. He found that our solar system has a motion of its own; a grand orbit round some as yet unknown centre, and that other systems have a like motion. What this centre may be, whether a great sun like Sirius, one hundred times larger than ours, with unknown powers and unknown uses, is of course only conjecture.