« 上一頁繼續 »
the province; and in a short time the number of sign. ers amounted to ten thousand. Franklin was chosen colonel of the Philadelphia regiment; but he did not think proper to accept of the honour.
Pursuits of a different nature now occupied the greatest part of his attention for some years. He engaged in a course of electrical experiments, with all the ardour and thirst for discovery which characterized the philosophers of that day. Of all the branches of experimental philosophy, electricity had been least ex. plored. The attractive power of amber is mentioned by Theophrastus and Pliny, and, from them, by later naturalists. In the year 1600, Gilbert, an English phy. sycian, enlarged considerably the catalogue of substances which have the property of attracting light bodies. Boyle, Otto Guericke, a burgomaster of Magdeburgh, celebrated as the inventor of the air pump, Dr. Wall, and sir Isaac Newton added some facts: Guericke first observed the repulsive power of electricity, and the light and noise produced by it.
In 1709, Hawkesbec communicated some important observations and experiments to the world. For several years electricity was entirely neglected, until Mr. Gray applied himself to it, in 1728, with great assiduity. He and his friend Mr. Wheeler, made a great variety of experiments ; in which they demonstrated that electricity may be communicated from one body to an. other, even without being in contact, and in this way may be conducted to a great distance. Mr. Gray afterwards found, that by suspending rods of iron by silk or hair lines and bringing an excited tube under them, sparks might be drawn, and a light perceived at the extremities in the dark. M. Du Faye, intendant of the French King's gardens, made a number of experiments, which added not a little to the science. He made the discovery of two kinds of electricity, which he called vitreous and resinous ; the former produced by rubbing glass, the latter from excited sulphur, sealing wax, &c. But this idea he afterwards gave up as erroneous. Between the year 1732 and 17-49, Defaguliers made a num
ber of experiments, but added little of importance. He first used the terms conductors and electrics, per se. In 1742, several ingenious Germans engaged in this suba ject. Of these the principle were, professor Boze of Whittemberg, professor Winkler of Leipsic, Gordon, a Scotch Benedictine monk, professor of philosophy at Erfurt, and Dr. Ludolf of Berlin. The result of their researches astonished the philosophers of Europe. Their apparatus was large, and by means of it they were enabled to collect large quantities of electricity, and thus to produce phenomena which had been hitherto unobserved. They killed small birds, and set spirits on fire. Their experiments excited the curiosity of other phi. losophers. Collinson about the year 1745, sent to the library company of Philadelphia an account of these experiments, together with a tube, and directions how to use it. Franklin, with some of his friends, immediately engaged in a course of experiments; the result of which is well known. He was enabled to make a number of important discoveries, and to propose theories to account for various phenomena; which have been universally adopted, and which bid fair to endure for ages. His observations he communicated, in a series of letters, to his friend Collinson; the first of which is dated March 28th, 1747. In these he makes known the power of points in drawing and throwing off the electrical matter, which had hitherto escaped the nofice of electricians. He also made the grand discovery of a plus and minus, or of a positive and negative state of electricity. We give him the honour of this, without hesitation ; although the English have claimed it for their countryman Dr. Watson. Watson's paper is dated January 21, 1748; Franklin's July 11, 1747; several months prior. Shortly after, Franklin, from his principles of plus and minus state, explained, in a satisfactory manner, the phenomena of the Leyden phial, first observed by Mr. Cuneus, or by professor Muschenbroeck of Leyden, which had much perplexed phi. losophers. He showed clearly that the bottle, when charged, contained no more electricity than before, but
that as much was taken from the one side as was thrown on the other; and that, to discharge it nothing was necessary but to make a communication between the two sides, by which the equilibrium might be restored, and that then no signs of electricity would remain.
He afterwards demonstrated, by experiments, that the electricity did not reside in the coating, as had been supposed, but in the pores of the glass itself. After a phial was charged, he removed the coating, and found that upon applying a new coating the shock might still be received. In the year 1749, he first suggested his idea of explaining the phenomena of thundergusts, and of the aurora borealis, upon electrical principles. He points out many particulars in which lightning and electricity agree ; and he adduces many facts, and reasoning from facts, in support of his positions. In the same year he conceived the astonishingly bold and grand idea of ascertaining the truth of his doctrine, by actually drawing down the forked lightning, by means of sharppointed iron rods raised into the region of the clouds. Even in this uncertain state, his passion to be useful to mankind displays itself in a powerful manner. Admitting the identity of electricity and lightning, and knowing the power of points in repelling bodies charged with electricity, and in conducting their fire silently and imperceptibly, he suggests the idea of securing houses, ships, &c. from being damaged by lightning, by erecting pointed iron rods, which should rise some feet above the most elevated part, and descend some feet into the ground or the water. The effect of these, he concluded, would be either to prevent a stroke by repelling the cloud beyond the striking distance, or by drawing off the electrical fire which it contained; or, if they could not effect this, they would at least conduct the stroke to the earth, without any injury to the building.
It was not until the summer of 1752, that he was enabled to complete his grand and unparalleled discovery by experiment. The plan which he had original
iy proposed, was to erect on some high tower, or other elevated place, a centry-box, from which should rise a pointed iron rod, insulated by being fixed in a cake of resin. Electrified clouds passing over this, would he conceived, impart to it a portion of their electricity, which would be rendered evident to the senses by sparks being emitted, when a key, a knuckle, or other con. ductor, was presented to it. Philadelphia at this time afforded no opportunity of trying an experiment of this kind. Whilst Franklin was waiting for the erection of a spire, it occurred to him, that he might have more ready access to the region of clouds by means of a common kite. He prepared one by attaching two cross sticks to a silk handkerchief, which would not suffer so much from the rain as paper. To his upright stick was fixed an iron point. The string was, as usual, of hemp, except the lower end, which was silk. Where the hempen string terminated, a key was fastened. With this apparatus, on the appearance of a thundergust approaching, he went into the commons, accompanied by his son, to whom alone he communicated his intentions, well knowing the ridicule which too generally for the interest of science, awaits unsuccessful experiments in philosophy.
He placed himself under a shed to avoid the rain. His kite was raised. A thunder cloud passed over it. No signs of electricity appeared. He almost despaired of success; when suddenly he observed the loose fibres of his string move towards an erect position. He now presented his knuckle to the key, and received a strong spark. How exquisite must his sensations have been at this moment! On this experiment depended the fate of his theory. If he succeeded, his name would rank high amongst those who have improved science ; if he failed, he must inevitably be subjected to the derision of mankind, or, what is worse, their pity, as a well-meaning man, but a weak, silly projector. The anxiety with which he looked for the result of his experiment, may easily be conceived. Doubts and despair had begun to prevail, when the
fact was ascertained in so clear a manner, that even the most incredulous could no longer withhold their assent. Repeated sparks were drawn from the key, a phial was charged, a shock given, and all the experiments made, which are usually performed with electri. city.
About a month before this period some ingenious Frenchinan had completed the discovery, in the manner originally proposed by Dr. Franklin. The letters which he sent to Mr. Collinson, it is said, were refused a place amongst the papers of the Royal Society of London. However, this may be, Collinson published them in a separate volume, under the title of New Experiments and Observations on Electricity, made at Philadelphia in America. They were read with avidity, and soon translated into different languages. A very incorrect French translation fell into the hands of the celebrated Buffon, who notwithstanding the disadvantages under which the work laboured, was much pleased with it, and repeated the experiments, with suc
He prevailed upon his friend, M. D'Alibard, to give his countrymen a more correct translation of the work of the American electrician. This contributed much towards spreading a knowledge of Frank. lin's principles in France.-The king, Louis XV, hearing of these experiments expressed a wish to be a spectator of them. A course of experiments was given at the seat of the Duc D’Ayen, at St. Germaine, by M. de Lor. The applauses which the king bestowed upon Franklin, excited in Buffon, D'Alibard, and De Lor, an earnest desire of ascertaining the truth of his theory of thunder-gusts. Buffon erected his ap. paratus on the tower of Montbar. M. D'Alibard at Mary-la-ville, and De Lor at his house in the Estrapade at Paris, some of the highest ground in that capital. D'Alibard's machine first showed signs of electricity. On the tenth of May, 1752, a thunder cloud passed over it, in the absence of M. D'Alibard ; and a number of sparks were drawn from it by Coiffier, a joiner, with whom D'Alibard had left directions how