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the satellites of Jupiter, and their motion, the horned phases of the planet Venus, the extremities of the ring of Saturn, and the spots in the sun's disk, which showed its motion round its axis. For these discoveries, which tended to confirm the Copernican heresy, Galileo was thrown into prison by the inquisition, and forced to purchase his liberty by retracting his opinions.

Kepler, much about the same time, that is, towards the beginning of the seventeenth century, added to these discoveries the knowledge of the laws which regulate the motions of the planets. Copernicus and Tycho Brahé believed that they moved in a circular orbit round the sun. Kepler demonstrated that they move in ellipses, of which the sun forms one of the foci; that their motion is slower in their aphelion than in their peribelion; that is, slower when at a distance from the sun than when nearer-in such a proportion, that a ray or line drawn from the planet to the sun, would in the course of the planet's revolution pass over equal spaces in equal times. He discovered likewise the analogy between the distances of the several planets from the sun, and their periodical revolutions; and he found the great law that regulates the planets, that the squares of their periodical times were in the same proportion as the cubes of their mean distances from the sun.

The age of Kepler and Galileo was the era of great discoveries in the arts and sciences. The invention of the telescope gave rise to a thousand experiments by means of glasses; and the science of optics received great improvements, The

new discoveries in astronomy led to improvements in navigation; and geometry, of course, made rapid advances towards perfection. The science of algebra, which Europe is said to have owed to the Arabians as well as the numeral ciphers, contributed greatly to abridge the labour of calculation; as did still more the invention of logarithms discovered in the year 1614, by Napier of Merchiston. The improvement of mechanics kept pace with the advancement of geometry; and the science of natural philosophy was successfully cultivated in all its branches. The Torricellian experiment, made about the year 1640, determined the height of the atmosphere. Experiments upon the oscillations of pendulums which were found always to preserve an equal time, though the spaces described were unequal, suggested the idea of applying the pendulum to regulate the motions of a clock; and the observation that adding to its weight adds nothing to the celerity of its motion led to the conclusion that the velocity with which a body gravitates to the centre is not in proportion to its weight. Galileo had discovered the laws which determine this velocity.

The ardour of prosecuting discoveries extended itself through the whole of the sciences. In the year 1616, Dr. Harvey made the great discovery of the circulation of the blood: at least he was the first who brought direct demonstration of the truth of that theory, which before his time had been only a matter of conjecture to some of the ablest anatomists. Hippocrates speaks of the usual motion of the blood, but had no idea of a constant and regular circulation. Servetus, about

the middle of the sixteenth century, had remarked that the whole mass of blood passes through the lungs by the pulmonary artery and veins; but the discovery of the complete circulation of this fluid, passing from the heart by the arteries to every part of the body, and thence returning to the heart by the veins, is due to Harvey alone.

About the middle of the seventeenth century, the spirit of sound philosophy was vigorously promoted in England by the institution of the Royal Society. Some time after the civil wars, a few learned men, particularly Mr. Boyle, Dr. Wilkins, Mr. Evelyn, Drs. Wallis and Wren, held private meetings for the sake of philosophical conversation. Cowley, the poet, had proposed in his works a very ingenious plan for a philosophical society, the idea of which he had probably borrowed from Lord Bacon's House of Solomon, described in his fanciful work of the New Atalantis. This plan of Cowley's contributed to the institution of a regular society by those gentlemen we have mentioned, which soon attracted the notice of Charles II., who granted to them his letters patent, and declared himself the founder and patron of the Royal Society of London. Experimental philosophy and natural history were the objects which deservedly engrossed their principal attention. The former of these Mr. Boyle prosecuted with great ingenuity and with the most successful industry. The world owed to him many valuable discoveries in chemistry, in mechanics, and in natural philosophy. He is distinguished by the invention of, or at least a great

improvement in, the air-pump, and the experiments made in vacuo, which have thrown light upon almost every branch of the study of nature. Το Mr. Evelyn, one of the first and most respectable members of the Royal Society, the world owes many ingenious works on agriculture, gardening, architecture, and sculpture. His excellent treatise, entitled Sylva, on the culture of trees, was read as one of the first discourses delivered before this society, and contributed to introduce a laudable and forward spirit in that most valuable of improvements through the whole of the island.

In short, that emulation which characterises all new institutions, gave rise to many ingenious treatises on a variety of branches of experimental philosophy and the study of nature.

Foreign nations began now to imitate the English in the foundation of similar societies for the improvement of philosophy. The Academia del Cimento at Florence, was established by the cardinal Leopold de Medicis, about the year 1665. Eleven years afterwards, in the year 1656, Lewis XIV., at the request of several of the French literati, founded the Royal Academy of Sciences. Colbert invited Cassini from Italy, and Huygens from Holland, to reside in Paris, and bestowed on them very liberal pensions. Soon after, the Royal Observatory of Paris was built at the king's expense, and Picard and Cassini employed themselves in the construction of a meridian line. Picard was employed, in the year 1670, by the French Academy, to measure a degree of the meridian, which he found to be 57,060 French toises; and thence

he made the first computation which approached to certainty of the size of the earth. Some of those great discoveries we have mentioned, and particularly that of the laws of the planetary motions laid down by Kepler, and the optical experiments of Galileo paved the way for the immortal Newton.

This great man, whose genius far outshone all who have gone before him in the path of philosophy, and who has, perhaps, exhausted the most important discoveries of the laws of nature, so as not to leave to posterity the possibility of eclipsing his fame, had, it is certain, made the greatest of his discoveries before he had attained the age of twenty-four. Before that early period of life he had discovered the theory of universal gravitation. Dr. Pemberton, who has given an excellent view of his philosophy, informs us that Newton, as he sat one day alone in a garden, fell into a reverie or speculation on the power of gravity. It occurred to him, that as this power is not found sensibly to diminish at the remotest distance to which we can ascend from the centre of the earth, for instance, at the top of the highest mountains, it was not unreasonable to suppose that it might extend much farther than was usually thought. Why not (said he to himself) as high as the moon? and if so, her motion must be influenced by it. Perhaps it is that which retains her in her orbit! However, though the power of gravity is not sensibly weakened in the little change of distance at which we can place ourselves from the centre of the earth, yet it is very possible that so high as the moon this power may differ much in strength

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