Another property of the new compass discovered by Dr. Gilbert was the magnetic “declination,” or the deviation of the compass from the true north. This deviation he attributed to the irregular distribution of land and sea. He thought that the mountains attracted the magnetic needle towards themselves, and so disturbed the action of the earth as a whole. Here again he was mistaken. [Illustration: Fig. 22.--The Compass Needle does not point due North and South except at a few parts of the earth. The deviation from the true North is shown in the diagram at a few Places in England.] The observations made since his time have proved that the deviation, like the “dip,” is not constant, but varies from century to century. It is, therefore, absurd to suppose that the mountains of the earth, which keep their outline unchanged for many centuries, should be the cause of the observed deviation. Dr. Gilbert, like those who went before him, made mistakes. But his mistakes we can easily forgive. So far as he was able, he reasoned only from actual facts. He could not make personal observations extending over a century, and he had at his hand no records of observations extending over long periods. So far as the facts known to him extended, he reasoned wisely and well. New facts observed by others who came after him would naturally lead in many cases to new conclusions, and Dr. Gilbert, had he lived in our own time, would have been the first to acknowledge and welcome any new facts duly proved and established, even though they contradicted some of his own theories. In the world of science there can be no absolute master or dictator. Gilbert himself was the first to overthrow the opinion that any master mind, however gifted or distinguished, can govern the thoughts and ideas of all men who come after him, and in this way he established one of the principles of modern science. When all the facts known are collected carefully together, it is right to arrange them all in a certain order, and look at them from a certain point of view, guided by certain principles. This principle, or point of view, is called a scientific theory. The establishment of such a theory enables us to survey the various facts and bewildering details at a glance. It is useful as a means of lessening the brain work required to remember a large number of facts. It also indicates the directions in which new facts may be looked for, and when such new facts are found it indicates probable explanations for them. But at that point the usefulness of a scientific theory ends.