The old Greeks had stated that large stones and small stones fall at different rates, and nobody seems to have taken the trouble to find out whether that is the case. Galileo did try on many occasions. He took up a large stone in one hand and a small stone in the other hand, and let them drop together. He found that they reached the ground at the same instant. When he showed the experiment to his friends, they said there was a difference in the rate at which the two stones fell, but that the distance through which they fell was not great enough to allow this difference to appear. To answer this objection Galileo went to the top of the tower of Pisa. He took with him a cannon ball weighing a hundred pounds and a shot weighing one pound. These were contained in a box, and were dropped by overturning the box. They therefore started together, and the people who watched the experiment also found that they arrived at the bottom of the tower at exactly the same moment. They seemed to hit the ground at the same time. This famous experiment was the starting-point of a revolution. [Illustration: Fig. 24.--The Leaning Tower of Pisa.] The fall of those two balls marked the fall of the old system by which the advance of science had been so long delayed. Henceforth Galileo did not stop to consult the old masters before he tried an experiment. Nature, he said, was always ready to answer questions. The only thing necessary was to put the question to her in a clear and unmistakable manner. Nature never hesitated with her answer. He therefore proceeded during the rest of his life to question Nature constantly and methodically. In this way he was able to discover a great number of new laws, and his method has since been accepted as the only sure method by which our knowledge of the structure and constitution of this world can be extended. Wishing to determine the speed of falling bodies more accurately, Galileo varied the experiment in different ways. In one method, which he used a great deal, he made spheres or balls of solid brass run down a groove cut into a wooden board. This groove was lined with smooth parchment, so that there should be as little resistance as possible to the motion of the balls. He measured accurately the time at which the balls were started and the time at which they arrived at the bottom of the groove. These measurements would nowadays be made by means of a watch, but at that time there were no watches except water-clocks, such as had been used in Babylon, and, later on, in Alexandria. Galileo measured time as best he could with the help of one of these. He attached a small spout to the bottom of a pail of water, so that the water ran out in a thin jet. At the instant when he started the ball on its way down the groove he placed a little cup under the water tap, and at the instant when the ball reached the bottom of the groove he took the cup away. [Illustration: Fig. 25.--A Simple Water-Clock.]