In violation of the decree of the Church, to which he had submitted, he published his most famous work in which he defended the theory that the earth moves about the sun. The book was the outcome of his life-work, but the Church believed it dangerous. He was summoned to Rome. Confined to a sick-bed, he pleaded for delay, which was granted. Before he recovered, however, the summons was made imperative. He must go to Rome, or be carried in irons. He went in a litter, carried by servants of the Grand Duke. In Rome he was to appear before the Inquisition. There he was treated with a consideration never before accorded to a prisoner of the Inquisition. Nor was he subjected to torture, as has been stated by some. He was found guilty of teaching the doctrine that the sun does not move, and that the earth moves about the sun. He was compelled to recant, and sentenced to the prison of the Holy Office and, by way of penance, to repeat once a week for three years the seven penitential Psalms. He yielded without reserve to the decree of the Inquisition, renounced his "errors and heresies," and, with his hand on the Bible, took oath never again to teach the forbidden doctrine. And now, though a shattered old man of seventy-four, enjoined to silence on the chief results of his life-work, nothing could quench his devotion to science. In these last years, he published a new book which, with his earlier work, entitles him to be regarded as the founder of the science of mechanics. In his study of machines Galileo found that no machine will do work of itself. Whenever a machine is at work, a man or a horse, or some other power, is at work upon the machine. In no case will a machine do work without receiving an equal amount of work. Torricelli and the Barometer Galileo had a pump which he found would not work when the water was thirty-five feet below the valve. He thought the pump was injured, and sent for the maker. The maker assured him that no pump would do better. This led Torricelli, one of Galileo's pupils, to the discovery of the barometer. Men had said that water rises in a pump because nature abhors a vacuum. Torricelli believed that air-pressure and not nature's "horror of a vacuum" is the cause of water rising in a pump. He invented the barometer to measure air-pressure. The first barometer was a glass tube filled with quick-silver or mercury (Fig. 4). The tube was closed at the upper end, and the lower end, which was open, dipped in a dish of mercury. He allowed the tube to stand, and saw that the height of the mercury changed. This he believed was because the air-pressure changed. Wind, Torricelli said, is caused by a difference of air-pressure, which is due to unequal heating of the air. For this reason a cool breeze blows from the mountain top to the heated valley, or from sea to land on a summer day. [Illustration: FIG. 4--TORRICELLI'S EXPERIMENT] Otto Von Guericke and the Air-Pump