Kepler was one of the greatest philosophers that ever lived, and ought to be considered as the discoverer of the _true_ system of the world. He was born in Germany, at Wiel, near Wirtemberg, 27th December, 1571. He early imbibed the principles of Copernicus. After the death of Tycho, he was employed to finish the tables which he had begun to compose from his observations. Kepler took twenty years to finish them. He dedicated them to the emperor, under the title of the “Rodolphine Tables.” Kepler united optics with astronomy, and thus made the most important discoveries. He was the first who discovered that the _planets move not in a circle, but in an ellipse_; and that altogether they move sometimes faster and sometimes slower, yet that they describe equal areas in equal times; that is, that the spaces through which they move in different parts of their orbit, are of equal times, though of unequal length; yet when two straight lines are drawn from the extremity of either space to the centre of the sun, they form triangles which include equal areas. He likewise demonstrated that the squares of the periodical times of the revolution of the planets round the sun, are in proportion to the cubes of their distance from him; a theorem of the greatest use in astronomical calculations: for having the periodical times of two planets given, and if the distance of one of them be known, by the rule of proportion, the distance of the other can be ascertained. Kepler is said to have used logarithms in framing his “Rodolphine Tables.” This great man died in poverty, 15th November, 1631, at Ratisbon, whither he had gone to solicit the arrears of his pension, which had been very ill paid: he left nothing to his wife and children but the remembrance of his virtues. Contemporary with Kepler was Galileo, born at Pisa, in Italy, in 1564; illustrious for his improvements in mechanics, for his application of the effects of gravity, and for the invention, or at least, the improvement of telescopes. The use of spectacles, or reading glasses (convex for long-sighted; and concave for short-sighted persons,) had been invented by one Spina, a monk at Pisa, in 1290; or, as some say, by our countryman Roger Bacon. The use of telescopes or glasses for viewing objects at a distance, was invented by Zachary Janssen, a spectacle-maker, at Middleburg, or rather, as it is said, from the accidental discovery of a child. The honour of this invention is also claimed by others. It is certain that Galileo first improved them so as to answer astronomical purposes. He also first made use of the single pendulum for measuring time in making his observations; to which he was led, by considering one day the vibrations of a lamp suspended from the vaulted roof of a church. He likewise discovered the gravity of the atmosphere from the rising of water in a pump, by the action of a piston, which led the way to the invention of the barometer, by his scholar Toricelli.