Thus Galileo came very near to inventing both the thermometer and the barometer, but yet invented neither! It seems incredible that he should have failed to invent the barometer, having come so near it; for he had been engaged for a long period in investigating the weight of air, and finally had succeeded in ascertaining it. The barometer was invented or rather discovered by Galileo's successor, Torricelli, in 1645. Torricelli, in investigating the action of suction pumps, constructed what now we call a barometer; but it was not until _after_ he had constructed it that he realized that the height of mercury in his tube indicated the pressure of the air outside. Seventy-five years later, Fahrenheit made a great improvement in the thermometer by substituting mercury for spirits. Meanwhile, Otto von Guericke, following in the footsteps of Galileo and Torricelli, had invented the air-pump, by means of which he succeeded in getting a fairly perfect vacuum in a glass receiver. This seems to have been an invention of the most clear-cut kind, resulting from an idea that occurred to Guericke that he seized upon promptly and put to work to serve mankind. Its influence in giving impetus to the science and art of pneumatics, and the influence of pneumatics on the progress of civilization, are too obvious to need more than to be pointed out. The invention of Guericke is a simple and clear illustration of the "power of an idea"; an illustration of seed falling on good ground and bringing forth fruit an hundred fold. One of the greatest inventors that ever lived was Isaac Newton, who lived from 1642 till 1728. Even as a child he busied himself with contriving and constructing mechanical appliances, mostly toys. As a young man he occupied himself mostly with studies in mathematics and experiments in physics, especially optics. In 1671 he invented a special form of the reflecting telescope, called after him the Newtonian telescope. He made many experiments in optics, in consequence of which he discovered and announced that white light consists of seven colors, having different degrees of refrangibility. The influence of this discovery on the advancement of learning since that time, it is unnecessary to point out; but we cannot realize too clearly that without it much of the most important progress in optics since that time would have been impossible. The invention by reason of which Newton is most generally known is his theory or law of gravitation, which he announced in his _Principia_, published in 1686. In 1609, Kepler had announced his famous laws, that reads: "1. The orbits of planets are ellipses having the sun at one focus. "2. The area swept over per hour by the radius joining sun and planet is the same in all parts of the planet's orbit. "3. The squares of the periodic times of the planets are proportional to the cubes of their mean distances from the sun."