to science than Huyghens. His improvement of the tele scope and his application of the pendulum to clocks were valuable additions to the machinery of astronomical inves tigation. By means of his telescopes he discovered that the extraordinary appearance exhibited by Saturn was occasioned by a ring surrounding the body of the planet, and inclined to the ecliptic in an angle which he estimated at 21. He published his observations of this planet in a work entitled Systema Saturnium, which still shows some traces of that species of reasoning from final causes which so greatly disfigures the writings of Kepler. For example, on discovering the satellite, he conceived that as the number of satellites now equalled the number of planets, it was vain to look for more, the equality being necessary to the harmony of the system. He lived, however, to witness the discovery of four more satellites belonging to the same
planet. (See {{9link|Huyghens|sc=1.)
The application of telescopes and micrometers to graduated instruments forms an important epoch in the history of astronomy. This happy improvement was first brought into use by Picard in 1667. Morin, indeed, had applied a telescope to the quadrant so early as 1634, and perceived the stars in full day in 1635. In 1669 Picard began to observe the stars on the meridian in the day time, with a quadrant, to which, in concert with Azout, he had applied an astronomical telescope having cross wires in its focus. Huyghens invented the plate micrometer in 1650; Malvasia that with the fixed wires in 1662; and Azout that with the movable wire in 1666. (See Delambre, Astronomic du Moyen Age, p. 618; note by Bouvard.) It is principally to these ingenious inventions and the fine application of the pendulum to clocks by Huyghens in 1656, that we must attribute the rapid progress since made in practical astro nomy, and the extreme precision of modern observations. Picard was also the first who introduced the modern method of determining the right ascensions of the stars, by observing their meridional passages, and employed the pendulum for that purpose. He likewise introduced the method of cor responding altitudes, and is entitled to be regarded as the founder of modern astronomy in France. Roemer, the friend and pupil of Picard, discovered the progressive motion of light in 1675, and measured its velocity by means of the eclipses of Jupiter s satellites. He was the first who erected a transit instrument, which gave a new accuracy to observations of right ascension.
The Royal Observatory of Paris was completed in 1670, and its direction intrusted to Dominic Cassini, who enriched astronomy with a great number of valuable observations and new discoveries. He determined the motions of Jupiter s satellites from observations of their eclipses, and constructed tables of them, which were found to be remarkably exact. He observed that the ring of Saturn is double, and discovered four of the satellites of that planet. He also determined the rotation of Jupiter and Mars, and made a number of observations on Venus with the same view. He observed the zodiacal light, and made a near approximation to the parallax of the sun. We also owe to him the first table of refractions calculated on correct principles, and a complete theory of the libration of the moon. Galileo had only observed the libration in latitude ; Hevelius explained the libration in longitude, by supposing that the moon always presents the same face to the centre of her orbit, of which the earth occupies a focus. Cassini made the important remark, that the axis of rotation of the moon is inclined to the ecliptic, and that its nodes coincide with those of the lunar orbit, so that the poles of the orbit, ecliptic, and equator of the moon, are on the same circle of latitude, the pole of the ecliptic being situated between the other two. Though the greater number of these discoveries were only of secondary importance, Cassini, nevertheless, obtained an extraordinary reputation. Lalande remarks, that in his hand astronomy underwent the most signal re volutions, and that his name is, in France, almost synony mous with that of creator of the science. Delambre has, however, expressed a different and far more accurate view of the real services of Cassini in the following terms:—
" The revolution in astronomy was brought about by Copernicus, by the laws of Kepler, by the pendulum of Huyghens, by the micro meters of Azout and Picard, by the sectors and mural of Picard and his method of corresponding altitudes, by the transit instruments of Roemer ; and Cassini appears to us an entire stranger to all these innovations, lie followed another route; he devoted a long life to painful observations, which at last deprived him of sight. Let us not refuse hkn the praise which he has so well merited, but let us reserve a place in our esteem for labours less brilliant perhaps, but of greater and more permanent utility, and which evince at least equal talent and sagacity."
Cassini was assisted in his observations by his nephew, James Philip Maraldi, who determined the regression of the nodes, and the progressive motion of the apsides of the orbit of Jupiter. This astronomer also corrected the theory of Mars, and observed the sun s parallax. He rejected the hypothesis of the progressive motion of light, as being insufficient to explain the inequalities of Jupiter s satellites ; and he conceived the design of forming a new catalogue of the stars, which, however, was never executed. He died in 1729.
There is no period in the history of mankind so distinguished by great and important discoveries, or so remarkable for the rapid development of the human intellect, as the 17th century. We have already noticed the invention of the pendulum, and its application to regulate the motion of timekeepers ; the invention of the telescope, bringing within the range of vision the phenomena of new worlds; of logarithms, by which computations are so much abridged ; and of the mechanical contrivances for measuring minute angles in the heavens. The same century witnessed the application of algebra to geometry, the discovery of the laws of the planetary motions, the infinitesimal calculus, the acceleration of falling bodies, the sublime theory of central forces, and the great principle of gravitation which connects the celestial orbs, and regulates the motions which it had been the business of the astronomer to observe since the earliest ages of the world. The service which the dis covery of these primary laws rendered to the progress of astronomy can scarcely be exaggerated. Many of the inequalities of the planetary motions, in consequence of their minuteness and the slowness with which they occur, could not have been detected by observation ; others might perhaps have been perceived, but we should still have been ignorant whether their constant accumulation might not ultimately change the state of the system, and, by destroy ing all confidence in the tables, demolish the fabric which had been reared at such a vast expense of time and labour. But when these inequalities are detected by theory, and separated from the mean motions with which they were blended, it becomes an object of the highest interest to confirm their existence by the most delicate and accurate observations. Hence, a more refined practice has con stantly followed every theoretical discovery. Besides, it is the perfection of theory, and not the mere knowledge of isolated facts, which gives astronomy its greatest value in the eyes of the philosopher. Numerous and important as its applications are, they have but a subordinate interest, in comparison with the knowledge of those general laws to which every particle of matter in the universe is subject, and by the discovery of which man has penetrated so deeply into the mysteries of nature.
