In the 17th century appeared Torricelli, the inventor of the barometer; Guericke, the German, inventor of the air pump; Fahrenheit, the inventor of the mercurial thermometer bearing his name; Leibnitz, eminent in every department of science and philosophy; Huygens, the great Dutch astronomer and philosopher; Pascal of France and Sir Isaac Newton of England, the worthy successors of Kepler, Galileo and Copernicus; and yet, with the exception of philosophical discoveries and a few experiments, the field of invention in the way of motor engines still remained practically closed. But slight as had been the discoveries and experiments referred to, they were the mine from which the inventions of subsequent times were quarried. One of the earliest, if not the first of pneumatic machines, was the bellows. Its invention followed the discovery of fire and of metals. The bladders of animals suggested it, and their skins were substituted for the bladders. The Egyptians have left a record of its use, thirty-four centuries ago, and its use has been continuous ever since. Mention has been made of the cannon. It was probably the earliest attempt to obtain motive power from heat. The ball was driven out of an iron cylinder by the inflammatory power of powder. Let a piston be substituted for the cannon ball, as was suggested by Huygens in 1680 and by Papin in 1690, and the charge of powder so reduced that when it is exploded the piston will not be thrown entirely out of the cylinder, another small explosive charge introduced on the other side of the piston to force it back, or let the cylinder be vertical and the piston be driven back by gravity, means provided to permit the escape of the gas after it has done its work, and means to keep the cylinder cool, and we have the prototype of the modern heat engines. The gunpowder experiments of Huygens and Papin were not successful, but they were the progenitors of similar inventions made two centuries thereafter.