There is no such thing in science as absolute truth. A theory is only true in so far as it covers all the facts. As soon as new facts are discovered which do agree with it the theory falls to the ground. As thousands of people are seeking every day to discover new facts, no theory can be considered absolutely perfect. Actual facts, or results of accurate observation, remain the same; but the explanation of the effects observed changes as knowledge of nature grows from more to more. This is a matter in which we nowadays differ from the ancients. We want our science to be living and progressive. We do not want to lose the fruits of the work of our ancestors. We find a place for it in our theories, and keep those theories unchanged until we are compelled to change them by the discovery of a new truth. But we insist that the new truth shall be a real truth proved as carefully and laboriously as the older truths were proved. And so we advance, step by step, to the conquest of realms the limits of which no man can see. CHAPTER V GALILEO THE man who is considered the greatest of all pioneers of science was born in the year 1564 at the city of Pisa, in the north of Italy. He first studied medicine, but after a few years he turned his attention to Mathematics and Science, which suited him much better. When only twenty-five years of age he was appointed for three years as Professor of Mathematics in Pisa. He soon showed that he did not intend to follow slavishly in the footsteps of the old masters. He lived at a time when a new spirit of independent inquiry was beginning to be felt in the universities of Europe. Galileo’s most remarkable early achievement was connected with the famous leaning tower of Pisa. That tower had been built on a weak foundation, and after it was finished it began to lean over towards one side, owing to the ground giving way beneath it. However, the tower did not fall, but remained in a leaning position. It was used by Galileo in order to prove that the old Greeks were wrong when they said that a heavy body would fall more quickly than a light body. [Illustration: Fig. 23.--Galileo.] This is true, of course, when we compare a feather with a stone. But that is due to the resistance of the air. If no air were present the stone and the feather would fall to the ground together, and the feather would make a little rattling noise just as if it were a piece of wire. This, of course, could not be proved by Galileo, as he had no means of obtaining a space free from air. But at all events he was well able to compare the rates at which a large stone and a small stone fell.