a history of science-2-第50章
按键盘上方向键 ← 或 → 可快速上下翻页，按键盘上的 Enter 键可回到本书目录页，按键盘上方向键 ↑ 可回到本页顶部！
————未阅读完？加入书签已便下次继续阅读！
te incentive for this offer was the obvious fact that with such an instrument the determination of the longitude of places would be much simplified。 Encouraged by these offers； a certain carpenter named Harrison turned his attention to the subject of watch…making； and； after many years of labor； in 1758 produced a spring time…keeper which； during a sea…voyage occupying one hundred and sixty…one days； varied only one minute and five seconds。 This gained for Harrison a reward Of L5000 sterling at once； and a little later L10；000 more； from Parliament。 While inventors were busy with the problem of accurate chronometers； however； another instrument for taking longitude at sea had been invented。 This was the reflecting quadrant； or sextant； as the improved instrument is now called； invented by John Hadley in 1731； and independently by Thomas Godfrey； a poor glazier of Philadelphia； in 1730。 Godfrey's invention； which was constructed on the same principle as that of the Hadley instrument； was not generally recognized until two years after Hadley's discovery； although the instrument was finished and actually in use on a sea…voyage some months before Hadley reported his invention。 The principle of the sextant； however； seems to have been known to Newton； who constructed an instrument not very unlike that of Hadley； but this invention was lost sight of until several years after the philosopher's death and some time after Hadley's invention。 The introduction of the sextant greatly simplified taking reckonings at sea as well as facilitating taking the correct longitude of distant places。 Before that time the mariner was obliged to depend upon his compass； a cross…staff； or an astrolabe； a table of the sun's declination and a correction for the altitude of the polestar； and very inadequate and incorrect charts。 Such were the instruments used by Columbus and Vasco da Gama and their immediate successors。 During the Newtonian period the microscopes generally in use were those constructed of simple lenses； for although compound microscopes were known； the difficulties of correcting aberration had not been surmounted； and a much clearer field was given by the simple instrument。 The results obtained by the use of such instruments； however； were very satisfactory in many ways。 By referring to certain plates in this volume； which reproduce illustrations from Robert Hooke's work on the microscope； it will be seen that quite a high degree of effectiveness had been attained。 And it should be recalled that Antony von Leeuwenboek； whose death took place shortly before Newton's； had discovered such micro…organisms as bacteria； had seen the blood corpuscles in circulation； and examined and described other microscopic structures of the body。
XIV。 PROGRESS IN ELECTRICITY FROM GILBERT AND VON GUERICKE TO FRANKLIN We have seen how Gilbert； by his experiments with magnets； gave an impetus to the study of magnetism and electricity。 Gilbert himself demonstrated some facts and advanced some theories； but the system of general laws was to come later。 To this end the discovery of electrical repulsion； as well as attraction； by Von Guericke； with his sulphur ball； was a step forward； but something like a century passed after Gilbert's beginning before anything of much importance was done in the field of electricity。 In 1705； however； Francis Hauksbee began a series of experiments that resulted in some startling demonstrations。 For many years it had been observed that a peculiar light was seen sometimes in the mercurial barometer； but Hauksbee and the other scientific investigators supposed the radiance to be due to the mercury in a vacuum； brought about； perhaps； by some agitation。 That this light might have any connection with electricity did not； at first； occur to Hauksbee any more than it had to his predecessors。 The problem that interested him was whether the vacuum in the tube of the barometer was essential to the light； and in experimenting to determine this； he invented his 〃mercurial fountain。〃 Having exhausted the air in a receiver containing some mercury； he found that by allowing air to rush through the mercury the metal became a jet thrown in all directions against the sides of the vessel； making a great； flaming shower； 〃like flashes of lightning；〃 as he said。 But it seemed to him that there was a difference between this light and the glow noted in the barometer。 This was a bright light； whereas the barometer light was only a glow。 Pondering over this； Hauksbee tried various experiments； revolving pieces of amber； flint； steel； and other substances in his exhausted air…pump receiver； with negative； or unsatisfactory； results。 Finally； it occurred to him to revolve an exhausted glass tube itself。 Mounting such a globe of glass on an axis so that it could be revolved rapidly by a belt running on a large wheel； he found that by holding his fingers against the whirling globe a purplish glow appeared； giving sufficient light so that coarse print could be read； and the walls of a dark room sensibly lightened several feet away。 As air was admitted to the globe the light gradually diminished； and it seemed to him that this diminished glow was very similar in appearance to the pale light seen in the mercurial barometer。 Could it be that it was the glass； and not the mercury； that caused it？ Going to a barometer he proceeded to rub the glass above the column of mercury over the vacuum； without disturbing the mercury； when； to his astonishment； the same faint light； to all appearances identical with the glow seen in the whirling globe； was produced。 Turning these demonstrations over in his mind； he recalled the well…known fact that rubbed glass attracted bits of paper； leaf…brass； and other light substances； and that this phenomenon was supposed to be electrical。 This led him finally to determine the hitherto unsuspected fact； that the glow in the barometer was electrical as was also the glow seen in his whirling globe。 Continuing his investigations； he soon discovered that solid glass rods when rubbed produced the same effects as the tube。 By mere chance； happening to hold a rubbed tube to his cheek； he felt the effect of electricity upon the skin like 〃a number of fine； limber hairs；〃 and this suggested to him that； since the mysterious manifestation was so plain； it could be made to show its effects upon various substances。 Suspending some woollen threads over the whirling glass cylinder； he found that as soon as he touched the glass with his hands the threads； which were waved about by the wind of the revolution； suddenly straightened themselves in a peculiar manner； and stood in a radical position； pointing to the axis of the cylinder。 Encouraged by these successes； he continued his experiments with breathless expectancy； and soon made another important discovery； that of 〃induction；〃 although the real significance of this discovery was not appreciated by him or； for that matter； by any one else for several generations following。 This discovery was made by placing two revolving cylinders within an inch of each other； one with the air exhausted and the other unexhausted。 Placing his hand on the unexhausted tube caused the light to appear not only upon it； but on the other tube as well。 A little later he discovered that it is not necessary to whirl the exhausted tube to produce this effect； but simply to place it in close proximity to the other whirling cylinder。 These demonstrations of Hauksbee attracted wide attention and gave an impetus to investigators in the field of electricity； but still no great advance was made for something like a quarter of a century。 Possibly the energies of the scientists were exhausted for the moment in exploring the new fields thrown open to investigation by the colossal work of Newton。
THE EXPERIMENTS OF STEPHEN GRAY In 1729 Stephen Gray （died in 1736）； an eccentric and irascible old pensioner of the Charter House in London； undertook some investigations along lines similar to those of Hauksbee。 While experimenting with a glass tube for producing electricity； as Hauksbee had done； he noticed that the corks with which he had stopped the ends of the tube to exclude the dust； seemed to attract bits of paper and leaf…brass as well
