Saturn has seven secondary planets re volving about him. One of them, which till lately was reckoned the fourth in or der from Saturn, was discovered by Huy gens, the 25th of March, 1655, by means of a telescope 12 feet long ; and the first, second, third, and fifth, at different times, by Cassini, viz. the fifth in October, 1671, by a telescope of 17 feet ; the third in December, 1672, by a telescope of Cam pani's, 35 feet long ; and the first and se cond in March, 1684, by help of Carh pani's glasses, of 100 and 136 feet. Final. ly, the sixth and seventh satellites were discovered by Dr. Herschel, with his 40 feet reflecting telescope, viz. the sixth on the 19th of Agust, 1787, and the se venth on the 17th of September, 1788. These two he has called the sixth and se venth satellites, though they are nearer to the planet Saturn than any of the former five, that the names or numbers of these might not be mistaken or con-" founded, with regard to former observa tions of them.
Moreover, the great distance between the fourth and fifth satellite gave occa sion to Huygens to suspect that there might be some intermediate one, or else that the fifth might have some other sa tellite moving round it, at its centre. Dr. Halley, in the Philosophical Transactions, gives a correction of the theory of the motions of the fourth or Huygenian satel lite. Its true period he makes 11d 22b
41' 6".
The periodical revolutions, and dis tances of the satellites from the body of Saturn, expressed in semidiameters of that planet, and in miles, are as fol low : The four first describe ellipses like to those of the ring, and are in the same plane. Their inclination to the eclip tic is from 30 to 31 degrees. The fifth describes an orbit inclined from 17 to 18 degrees with the orbit of Saturn; his plane lying between the ecliptic and those of the other satellites, &c. Dr. Herschel observes, that the fifth sa tellite turns once round its axis exact ly in the time in which it revolves about the planet Saturn ; in which re spect it resembles our Moon, which does the same thing. And he makes the angle of its distance from Saturn, at his mean distance, 17' 2". Philoso phical Transactions, 1792, p. 22. See a long account of observations of these satellites, with tables of their mean mo tions, by Dr. Herschel. Philosophical Transactions, 1790.
The Herschel has six satellites, or moons, that revolve about him, like those of Jupiter and Saturn. These satellites were discovered by Dr. Herschel, who gave an account of them in the Philoso phical Transactions.