his system ; and the actual discovery of their existence made it impossible to doubt the revolution of Venus round the sun. He detected the four satellites or moons of Jupiter, and, in honour of his patron, gave them the name of the " Medicean Stars." The discovery of these little bodies circulating round the huge orb of Jupiter afforded him a strong analogical proof of the annual revolu tion of the earth, accompanied by its moon. He perceived spots on the disk of the sun, from the motions of which he deduced the rotation of that body about its axis in the space of 27 days. The singular appearances of Saturn were beheld by him with no less pleasure than astonish ment. His telescope was not sufficiently powerful to separate the ring from the body of the planet ; and to explain the appearances he supposed Saturn to be composed of three stars almost in contact with one another. These discoveries proved that the substances of the celestial bodies are similar to that of the earth, and demolished the Aristotelian doctrine of their divine essence and incor ruptible nature. They enlarged the ideas of mankind respecting the planetary system, and furnished the most
convincing arguments in favour of the doctrines of Copernicus.Science is indebted to Galileo for two other discoveries of a different kind, less brilliant perhaps, but of far greater importance than those which we have yet enumerated. These are the isochronism of the vibrations of the pendulum, and the law of the acceleration of falling bodies. His telescopic discoveries could not have remained long un known ; in fact, with the exception of those of the phases of Venus, and of the triple form of Saturn, they were all fiercely disputed, even during his own lifetime. It is now universally admitted that he was the first who discovered the satellites of Jupiter, and the spots of the sun ; but the very circumstance of other claimants to these discoveries having arisen, proves that they were within the reach of ordinary observers. For an account of his eventful life see Galileo.
While astronomy was making these rapid advances in the hands of Kepler and Galileo, an event occurred in Scotland which contributed, though less directly, no less powerfully, to the acceleration of its progress. This was the invention of logarithms by Lord Napier, baron of Merchiston ; " an admirable artifice," says Laplace, " which by reducing to a few days the labour of many months, doubles the life of the astronomer, and spares him the errors and disgust inseparable from long calculations, an invention of which the human mind has the more reason to be proud, inasmuch as it was derived exclusively from its own resources." It may be added, that without this, or some equivalent artifice, the computations rendered necessary by more correct observations would far exceed the limits of human patience or industry, and astronomy could never have acquired that precision and accuracy by which it is now distinguished above all the other branches of human knowledge.
The same epoch presents to us a great number of excellent observers, who, although they did not produce any revolution in the state of astronomy, still rendered it useful service. Scheiner is celebrated for his observations of the solar spots, and his disputes with Galileo. John Bayer of Augsburg published a description of the constellations, accompanied by maps, in which the stars are marked by Greek letters, a simple idea, which has been universally adopted. Lansberg, a Flemish mathematician, published i:i 1632 a set of astronomical tables, which, though filled with inaccuracies, rendered good service to science by Horrox. apprising Horrox of the transit of Venus over the sun s disk, which that young astronomer and his friend Crabtree had the satisfaction of observing on the 24th of November 1639. They were the first who ever witnessed that rare phenomenon. Snellius is celebrated for his measurement of the earth. Gassendi, who had the merit along with Descartes of hastening the downfall of the Aristotelian philosophy in France, made some useful observations, particularly one of a transit of Mercury in 1631. His works, which fill six folio volumes, abound with curious and useful researches. Paccioli, a Jesuit, born at Ferrara in 1598, contributed to the progress of astronomy, not so much by his own discoveries, as by collecting and render ing an account of those of others. He rejected the system of Copernicus, and was more zealous in maintaining the doctrines of the- church than in investigating nature ; but his works form a vast repertory of useful information. His Novum Almagestum is a collection of the observations, opinions, and physical explanations of the phenomena, together with all the methods of computation then known. He was assisted in his labours by Grimaldi, who discovered the inflection of light, and he gave to the principal spots of the moon the names which are now used by astronomers.
The most accurate observations that were ever made prior to the adaptation of the telescope to astronomical instruments were those of Hevelius, a rich citizen of Dantzic, who devoted his life and a large fortune to the service of astronomy. Having fitted up an observatory, and furnished it with the best instruments which could be procured, he commenced a course of observations, which he followed assiduously upwards of forty years. In his Selenographia he has given an accurate description of tho face and spots of the moon, accompanied with excellent delineations of her appearance in her different phases and librations. The idea of making drawings of the different phases of the moon had previously occurred to Gassendi and Peiresc, but they had not been able to execute tho project ; indeed, the difficulty attending it was such, that it occupied Hevelius, who was an excellent draughtsman, as well as observer, during a great number of years. Hevelius made an immense number of researches on comets ; and finding that the observations could not be represented by rectilinear or circular orbits, he supposed them to move in parabolas. During a temporary absence from Dantzic he had the misfortune to lose, in a great fire which occurred in the city, his observatory, instruments, manuscripts, and almost the entire copy of the second volume of his Machina Ccelestis, which contained the results of his pro tracted labours. He was now in his old age, but his zeal did not give way under the terrible calamity. He patiently recommenced all his calculations, reconstructed tables of the sun, and prepared for publication his Firmamentum Soliescianum, or celestial chart, which did not appear till after his death. Towards the latter part of his life the use of telescopic sights began to be generally adopted. Heve lius, however, resisted the innovation, and continued to employ plain sights. This preference given to the ancient method by so skilful an observer induced Dr Halley to visit him at Dantzic, for the purpose of ascertaining, by a comparison of observations made at the same time and place, which of the two methods gave the most correct results. Dr Halley observed with the telescope, and Hevelius with his own instruments ; but such was the dexterity he had acquired through long practice, that the difference of their observations seldom amounted to more than a few seconds, and in no case to so much as a minute. Notwithstanding this agreement, it is to be regretted that Hevelius did not adopt the new method ; for, on account of the greater precision given to instruments by the use of the telescope, his observations, which were made without it, cannot now be admitted in the construction of tables, and consequently are for the most part useless to astronomy.
