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observation of the kind. Horrox, an english astronomer, also made a similar observation of Venus in 1639. John Baptist Morin, who was a long time professor of mathematics in the College of France, composed several works, which do no honour to his memory: yet, on the other hand, it must not be forgotten, that he was the first who pointed out the manner of solving the celebrated problem of the longitude by means of astronomical observations; and that, in order to make these observations with greater accuracy, he proposed the application of a telescope to the quadrant; an idea, which has been erroneously ascribed to later
Hevelius, who was born in 1611, and died in 1688, rendered himself eminent for his numerous and delicate observations of the spots on the Sun, the motions of comets, &c. To him likewise we are indebted for the first accurate description of the spots on the Moon.
Riccioli, a jesuit, left a great work after the example of Ptolemy, entitled The new Almagest; in which he has collected together all the astronomical theories known in his time, with his own observations and remarks. He was much assisted by his friend Grimaldi; who, independently of this work, published a Selenography, in which the spots on the Moon are distinguished by the names of philosophers. This nomenclature was adopted at first with applause, and is still retained, though time has introduced into it some corrections.
Mouton a canon of Lyons, determined with dexterity and success the apparent diameters of the Sun. and Moon, by means of the telescope and a simple
pendulum; and to him we owe the first idea of connecting together, by interpolation, the observations made of one object at different times. He likewise calculated a table of logarithms of sines and tangents for every second as far as four degrees, which was printed in the edition of Gardiner published at Avignon by the jesuits Pezenas and Dumas, in 1770.
After the discovery of the satellites of Jupiter, this branch of astronomy remained almost stationary for more than forty years, either because it demanded extreme attention in the observer, or because the telescope was not yet brought to sufficient perfection. Galileo had imagined in 1615, that he saw two satellites very near the body of Saturn. They appeared immovable for three years, always retaining the same figure; but at length they ceased to be visible altogether, and it was supposed, that Galileo had been deceived by some optical illusion.
In 1655, Huygens having himself accomplished the construction of two excellent telescopes, one twelve feet in length, the other twenty-four, he discovered one of the satellites of Saturn; which is that now called the fourth. He determined it's distance from Saturn, the position of it's orbit, the period of it's revolution, &c., with such clearness and accuracy, as left no doubt of the existence and motion of this new secondary planet. Astronomers at that time were so fixed in the prejudice, that the number of satellites could not exceed that of the primary planets, as to lead Huygens, after this discovery, which made the number of satellites and of planets
equal*, to observe, in the epistle dedicatory of his Systema Saturnium to the grand duke of Tuscany, that the number of satellites was complete, and we must not hope to see any new ones in future. Let us forgive a great man, who enriched the accurate sciences with so many immortal discoveries, this metaphysical errour. Perhaps indeed it may be ascribed to the high opinion he entertained of his telescopes; for their having enabled him to see phenomena in the heavens, which no one before had remarked, might have induced him to think, that none of the bodies in our planetary system had escaped his view.
The discovery of this satellite gradually led Huygens, as he informs us, to the knowledge of the ring, by which Saturn is environed. Several astronomers after Galileo had observed Saturn under different irregular and variable forms, for which they could give no satisfactory reason. Huygens perceived with his telescopes, and soon demonstrated, that Saturn was a round body, encircled by a flat ring every where detached from it, which, being seen obliquely from the Earth, ought, conformably to the rules of optics, to appear in the form of an ellipsis, more or less open according as our eye is more or less elevated above it's plane, the inclination of which to the ecliptic is about thirty degrees. Hence followed a simple and natural explanation of all the appearances of Saturn.
* On the one hand there were six primary planets, Mercury, Venus, the Earth, Mars, Jupiter, and Saturn: on the other six satellites, the Moon, the four satellites of Jupiter, and one of Saturn.
The ring becomes entirely invisible to our eyes, when it's thickness is too small to reflect the solar rays toward us in sufficient quantity to be perceived. Huygens found, that the extreme semidiameter of the ring is to that of Saturn as nine to four; and that it's breadth is equal to the space between it's inner surface and the body of the planet. This system, attacked at first by ignorance or envy, is at present a fundamental truth in astronomy.
At this period two grand establishments for the promotion of science were formed, the Royal Society at London, in 1660, and the Royal Academy of Sciences at Paris, in 1666. These two illustrious bodies have produced men of the first order in every branch of science: at their commencement they were chiefly useful in astronomy, which has more need of being encouraged by the attention and remuneration of princes, than all the other sciences.
One of the first cares of Lewis XIV, or rather of his great minister Colbert, in founding the Academy of Sciences, was not only to introduce into it the men of learning in France, but likewise to attract to it foreigners of the greatest celebrity, and such as were most capable of contributing to the splendour of the establishment, and the progress of the sciences. Among the former we may remark Claude Perrault, Mariotte, Pecquet, Auzout, Picard, Richer, &c.; among the latter, John Dominic Cassini, Roemer, &c.
John Dominic Cassini, who had already acquired a great name in science, by his meridian line at the church of St. Petrona at Bologna; by tables of the
Sun and of the satellites of Jupiter; and by other astronomical labours, as well as by the hydraulic works in which the popes had employed him, before he came to settle in France, found himself at perfect liberty in this country to indulge his genius and inclination, which led him to astronomy, In this science he made a great many important discoveries ; the most striking of which was that of four more satellites of Saturn, which are in the order of their distances the 1st, 2d, 3d, and 5th; so that with the 4th, discovered by Huygens, this planet had now five satellites, fully known.
The hypothesis of the elliptical motion of the planets, which Kepler had advanced, was not perfectly comprehended by every astronomer. Cassini com
bated it on an unfounded supposition.
that Kepler, while he placed the Sun in one of the foci of the common ellipsis, made the other focus the centre of the mean motions, or the vertex of the areas proportional to the times; which gave results by no means accordant with observation. To correct this fault, Cassini substituted instead of the common ellipsis another curve, which he called an ellipsis likewise, in which the product of two lines, drawn from two fixed points to one and the same point in the curve, forms every where an invariable quantity: whereas in the common ellipsis it is the sum of two lines drawn from the two foci, which is an invariable quantity. But Kepler did not really fall into the mistake ascribed to him by Cassini; he places the centre of the mean motions at the focus which the Sun occupies, and in this case all the observations are very