t； or oversight； a great number of general readers as well as many scientists； through succeeding centuries； have been deprived of the benefit of writings that contained a good share of the fundamental facts about magnetism as known to…day。 Gilbert was the first to discover that the earth is a great magnet； and he not only gave the name of 〃pole〃 to the extremities of the magnetic needle； but also spoke of these 〃poles〃 as north and south pole； although he used these names in the opposite sense from that in which we now use them； his south pole being the extremity which pointed towards the north； and vice versa。 He was also first to make use of the terms 〃electric force；〃 〃electric emanations；〃 and 〃electric attractions。〃 It is hardly necessary to say that some of the views taken by Gilbert； many of his theories； and the accuracy of some of his experiments have in recent times been found to be erroneous。 As a pioneer in an unexplored field of science； however； his work is remarkably accurate。 〃On the whole；〃 says Dr。 John Robinson； 〃this performance contains more real information than any writing of the age in which he lived； and is scarcely exceeded by any that has appeared since。〃＇4＇ In the preface to his work Gilbert says： 〃Since in the discovery of secret things； and in the investigation of hidden causes； stronger reasons are obtained from sure experiments and demonstrated arguments than from probable conjectures and the opinions of philosophical speculators of the common sort； therefore； to the end of that noble substance of that great loadstone； our common mother （the earth）； still quite unknown； and also that the forces extraordinary and exalted of this globe may the better be understood； we have decided； first； to begin with the common stony and ferruginous matter； and magnetic bodies； and the part of the earth that we may handle and may perceive with senses； and then to proceed with plain magnetic experiments； and to penetrate to the inner parts of the earth。〃＇5＇ Before taking up the demonstration that the earth is simply a giant loadstone； Gilbert demonstrated in an ingenious way that every loadstone； of whatever size； has definite and fixed poles。 He did this by placing the stone in a metal lathe and converting it into a sphere； and upon this sphere demonstrated how the poles can be found。 To this round loadstone he gave the name of terrellathat is； little earth。 〃To find； then； poles answering to the earth；〃 he says； 〃take in your hand the round stone； and lay on it a needle or a piece of iron wire： the ends of the wire move round their middle point； and suddenly come to a standstill。 Now； with ochre or with chalk； mark where the wire lies still and sticks。 Then move the middle or centre of the wire to another spot； and so to a third and fourth； always marking the stone along the length of the wire where it stands still； the lines so marked will exhibit meridian circles； or circles like meridians； on the stone or terrella； and manifestly they will all come together at the poles of the stone。 The circle being continued in this way； the poles appear； both the north and the south； and betwixt these； midway； we may draw a large circle for an equator； as is done by the astronomer in the heavens and on his spheres； and by the geographer on the terrestrial globe。〃＇6＇ Gilbert had tried the familiar experiment of placing the loadstone on a float in water； and observed that the poles always revolved until they pointed north and south； which he explained as due to the earth's magnetic attraction。 In this same connection he noticed that a piece of wrought iron mounted on a cork float was attracted by other metals to a slight degree； and he observed also that an ordinary iron bar； if suspended horizontally by a thread； assumes invariably a north and south direction。 These； with many other experiments of a similar nature； convinced him that the earth 〃is a magnet and a loadstone；〃 which he says is a 〃new and till now unheard…of view of the earth。〃 Fully to appreciate Gilbert's revolutionary views concerning the earth as a magnet； it should be remembered that numberless theories to explain the action of the electric needle had been advanced。 Columbus and Paracelsus； for example； believed that the magnet was attracted by some point in the heavens； such as a magnetic star。 Gilbert himself tells of some of the beliefs that had been held by his predecessors； many of whom he declares 〃wilfully falsify。〃 One of his first steps was to refute by experiment such assertions as that of Cardan； that 〃a wound by a magnetized needle was painless〃； and also the assertion of Fracastoni that loadstone attracts silver； or that of Scalinger； that the diamond will attract iron； and the statement of Matthiolus that 〃iron rubbed with garlic is no longer attracted to the loadstone。〃 Gilbert made extensive experiments to explain the dipping of the needle； which had been first noticed by William Norman。 His deduction as to this phenomenon led him 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 generalize his observations； he attempted to find the law that governed the observed change of direction which a ray of light assumes in passing from one medium to another。 Kepler measured the angle of ref