They made a large number of experiments, and gradually allowed the results of the experiments to force them into a certain way of looking at things. One man of science who adopted the second method was Robert Boyle, an Irishman, who made himself famous by his studies on what he called “the spring of the air,” meaning the pressure which compressed air can exert. He took a long glass tube and bent it into a hook at the lower end, which was closed. He then poured in mercury through the top, until the bent end contained only a little air, which was compressed by the weight of the mercury in the long tube. He poured in more and more mercury, and found that the space filled up by the air became smaller, the greater the pressure of the mercury was. He found, in fact, that the air behaves very much like an elastic spring, which can be stretched and compressed. After some experience of this kind, Boyle heard of an apparatus which a German had invented, and with which he performed the most astonishing experiments. This man’s name was Guericke. Guericke had the idea of finding out all about the stars, and how they moved through space. He believed that they did not move through air, because if they did they would gradually be stopped by the resistance of the air. He decided that if he wished to find out how they moved, he would have to obtain a space similar to that in which the heavenly bodies moved. This space, he said, would have to be an empty space. He therefore tried to make an empty space. The first method Guericke adopted was a very curious one. He took a big cask filled with water, and containing only one opening. Into that opening he screwed a pump, and tried with all his might to pump the water out of the cask. This was more difficult than he had expected, for the air, pressing on the water with the force of fifteen pounds per square inch, held the water in the cask. If the opening had been free, the air would have got into the cask in bubbles, and would have pressed the water out through the opening. But Guericke did not allow any air bubbles to get into the cask, and so he had to work against the whole pressure of the air in trying to pump the water out of the cask. If he could but pump it out, he was convinced that he would leave only empty space in the cask. [Illustration: Fig. 37.--Guericke’s experiment of trying to draw Water out of a full cask in order to leave a Vacuum.]