Before the destruction of the great Arabian Empire, which put an end to the cultivation of science by the Arabs, and transferred their work to the new nations of Europe, they had reached the summit of fame in the whole world. Their last achievement was one which gave rise to many remarkable developments. It is described in a book called _The Book of the Balance of Wisdom_. The Arabian author says in this book that even air must have some weight, and that the true weight of a body cannot be the weight as measured in air, because air, like water, must somewhat reduce its weight. This was the observation upon which later the great inventions of the barometer and the air-pump were based. CHAPTER IV DR. GILBERT OF COLCHESTER THE ancients were in the habit of consulting books written by old masters whenever they were puzzled by an unknown power or force of nature. Instead of making experiments they went to their bookshelves, and read what the wise men of long ago had written on the same or similar subjects. They thought that they could not surpass the old masters in wisdom, and that what the old masters did not know was not worth knowing. This habit accounts for the slow progress made in science in Europe during the Middle Ages, that is, from about the sixth to the fifteenth centuries. But when Columbus discovered America, and broke through the traditions of the ancients by his courageous exploration of the unknown, a new era began to dawn. One of the heralds of this new era was Dr. William Gilbert. He was born in Colchester, in Essex, in the year 1540, and was the son of the Recorder of that city. He was educated at Cambridge, and became a medical doctor at the age of twenty-nine. He was appointed physician to Queen Elizabeth, and was elected president of the Royal College of Physicians in the year 1600. It was Dr. Gilbert who founded the science of Magnetism and the science of Electricity, collecting all that was known on these subjects, and making many new and valuable experiments of his own. All these facts he expounded in a great book which was published in London in the year 1600. It is called, _On the Magnet and Magnetic Bodies, and on the Great Magnet, the Earth_. In his introduction he lays down new principles to guide scientific work in the future, in words somewhat as follows:-- [Illustration: Fig. 17.--Dr. William Gilbert, who founded the Sciences of Magnetism and Electricity.]