A History of Science-2 [44]
to believe that this was also explained by the magnetic attraction of the earth, and to predict where the vertical dip would be found. These deductions seem the more wonderful because at the time he made them the dip had just been discovered, and had not been studied except at London. His theory of the dip was, therefore, a scientific prediction, based on a preconceived hypothesis. Gilbert found the dip to be 72 degrees at London; eight years later Hudson found the dip at 75 degrees 22' north latitude to be 89 degrees 30'; but it was not until over two hundred years later, in 1831, that the vertical dip was first observed by Sir James Ross at about 70 degrees 5' north latitude, and 96 degrees 43' west longitude. This was not the exact point assumed by Gilbert, and his scientific predictions, therefore, were not quite correct; but such comparatively slight and excusable errors mar but little the excellence of his work as a whole. A brief epitome of some of his other important discoveries suffices to show that the exalted position in science accorded him by contemporaries, as well as succeeding generations of scientists, was well merited. He was first to distinguish between magnetism and electricity, giving the latter its name. He discovered also the "electrical charge," and pointed the way to the discovery of insulation by showing that the charge could be retained some time in the excited body by covering it with some non-conducting substance, such as silk; although, of course, electrical conduction can hardly be said to have been more than vaguely surmised, if understood at all by him. The first electrical instrument ever made, and known as such, was invented by him, as was also the first magnetometer, and the first electrical indicating device. Although three centuries have elapsed since his death, the method of magnetizing iron first introduced by him is in common use to-day. He made exhaustive experiments with a needle balanced on a pivot to see how many substances he could find which, like amber, on being rubbed affected the needle. In this way he discovered that light substances were attracted by alum, mica, arsenic, sealing-wax, lac sulphur, slags, beryl, amethyst, rock-crystal, sapphire, jet, carbuncle, diamond, opal, Bristol stone, glass, glass of antimony, gum-mastic, hard resin, rock-salt, and, of course, amber. He discovered also that atmospheric conditions affected the production of electricity, dryness being unfavorable and moisture favorable. Galileo's estimate of this first electrician is the verdict of succeeding generations. "I extremely admire and envy this author," he said. "I think him worthy of the greatest praise for the many new and true observations which he has made, to the disgrace of so many vain and fabling authors."
STUDIES OF LIGHT, HEAT, AND ATMOSPHERIC PRESSURE We have seen that Gilbert was by no means lacking in versatility, yet the investigations upon which his fame is founded were all pursued along one line, so that the father of magnetism may be considered one of the earliest of specialists in physical science. Most workers of the time, on the other band, extended their investigations in many directions. The sum total of scientific knowledge of that day had not bulked so large as to exclude the possibility that one man might master it all. So we find a Galileo, for example, making revolutionary discoveries in astronomy, and performing fundamental experiments in various fields of physics. Galileo's great contemporary, Kepler, was almost equally versatile, though his astronomical studies were of such pre-eminent importance that his other investigations sink into relative insignificance. Yet he performed some notable experiments in at least one department of physics. These experiments had to do with the refraction of light, a subject which Kepler was led to investigate, in part at least, through his interest in the telescope. We have seen that Ptolemy in the Alexandrian time, and Alhazen, the Arab, made studies of refraction. Kepler repeated their experiments, and, striving as always to
STUDIES OF LIGHT, HEAT, AND ATMOSPHERIC PRESSURE We have seen that Gilbert was by no means lacking in versatility, yet the investigations upon which his fame is founded were all pursued along one line, so that the father of magnetism may be considered one of the earliest of specialists in physical science. Most workers of the time, on the other band, extended their investigations in many directions. The sum total of scientific knowledge of that day had not bulked so large as to exclude the possibility that one man might master it all. So we find a Galileo, for example, making revolutionary discoveries in astronomy, and performing fundamental experiments in various fields of physics. Galileo's great contemporary, Kepler, was almost equally versatile, though his astronomical studies were of such pre-eminent importance that his other investigations sink into relative insignificance. Yet he performed some notable experiments in at least one department of physics. These experiments had to do with the refraction of light, a subject which Kepler was led to investigate, in part at least, through his interest in the telescope. We have seen that Ptolemy in the Alexandrian time, and Alhazen, the Arab, made studies of refraction. Kepler repeated their experiments, and, striving as always to