Abbildungen der Seite
PDF
EPUB
[ocr errors][merged small][ocr errors]

and Indians had made inroads upon the frontier inhabitants of the Province, who were unprovided for such an attack. It became necessary that the citizens should arın for their defence. Governor Thomas recommended to the Assembly, who were then sitting, to pass a militia law. To this they would agree only upon condition that he should give his assent to certain laws, which appeared to them calculated to promote the interests of the people. As he thought these laws would be injurious to the Proprietaries, he refused his assent to them; and the Assembly broke up without passing a militia law. The situation of the Province was at this time truly alarming; exposed to the continual inroad of an enemy, destitute of every means of defence. At this crisis Franklin stepped forth, and proposed to a meeting of the citizens of Philadelphia, a plan of a voluntary association for the defence of the Province. This was approved of, and signed by twelve hundred persons immediately. Copies were instantly circulated throughout the Proyince, and in a short time the number of signers amounted to ten thousand. Franklin was chosen Colonel of the Philadelphia regiment; but he did not think proper to accept of the honor.

Pursuits of a different nature now occupied the greater part of his attention for some years. He engaged in a course of electrical experiments, with all the ardor and thirst for discovery which characterised the philosophers of that day. Ci all the branches of experimental philosophy electricity had been least explored. The attrac tive power of amber is mentioned by Theophras tus and Pliny, and from them by later naturalists In the year 1600, Gilbert, an English physician, enlarged considerably the catalogue of substan. ces which have the property of attracting light

[ocr errors][merged small][ocr errors][ocr errors][ocr errors][merged small]

bodies. Boyle, Otto Guericke, a burgomaster of Magdeburg, celebrated as the inventor of the air pump, Dr. Wall, and Sir. Isaac Newton, added some facts. Guericke first observed the repul. sive power of electricity, and the light and noise produced by it. In 1709, Hawesbec communi. cated some important observations and experi. ments to the world. For several years electri. city was entirely neglected, until Mr. Grey applied himself to it in 1728, with great assiduity. He and his friend Mr. Wheeler made a great va. riety of experiments; in which they demonstra. ted, that electricity may be communicated from one body to another, even without being in contact, and in this way may be conducted to a great distance. Mr. Grey 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

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, sealingwax, &c. But this idea he afterwards gave up as erroneous.

Be. tween the years 1739 and 1742, Desauguliers made a number of experiments, but added little of importance. He first used the terms conductors and electrics per se. In 1742, several ingepious Germans engaged in this subject; of these, the principal were, Professor Boze, of Wittemberg, 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 philoso. phers of Europe. Their apparatus was large, and

science.

[ocr errors]

by means of it they were enabled to collect large quantities of the electric fluid, and thus to produce phenomena wbich had been bitherto unobserved. They killed small birds, and set spirits on fire. Their experiments excited the curiosity of other philosopliers. Collinson, about the year 1745, sent to the Library Company of Philadel. phia 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 28, 1747. In these he shows the power of points in drawing and throwing off the electrical matter, which had hitherto escaped the notice of elece tricians. He also made the grand discovery of a plus and minus, or of a positive and negalive state of electricity. We give him the honor of this without hesitation; althongh 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 the plus and minus state, explained in a satisfactory man. ner, the phenomena of the Leyden phial, first observed by Mr. Cuneus, or by Professor Muse chenbroeck, of Leyden, which had much perplexed philosophers. He showed clearly, that the bottle, when charged, contained no more electricity than before, but that as much was taken from one side as was thrown on the other; and that, to discharge it, nothing was necessary

[merged small][ocr errors]

but to produce 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 sug. gested his idea of explaining the phenomena of thunder gusts, and of the aurora borealis, upon electrical principles. He points out many par. ticulars in which lightning and electricity agree; and he adduces many facts, and reasonings 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 lightning, by means of sharp pointed iron rods raised into the regions 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 suggested the idea of securing houses, ships, fc. from being damaged by lightning, by erecting pointed rods, that should rise some feet above the most eleva. ted 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 electric matter 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 unparalfeled discovery by experiment. This plan which he had originally proposed, was, to erect on some high tower, or other elevated place, a sentrybox, from which should rise a pointed iron rod, insulated by being fixed in a cake of resin. Electri. fied clouds passium 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, the knuckle, or other conductor, was presented to it. Philadelphia at this time afforded no opportunity of trying an experiment of this kind. While 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 fastening two cross sticks to a silk handkerchief, which would not suffer so much from the rain as paper. To the upright stick was affixed an iron point. The string was, as usual, cf hemp, except the lower end, which was silk. Where the bempen string terminated, a key was fastened. With this apparatus, on the appearance of a thunder gust approaching, he went out into the commons, accompanied by his son, to whom aione 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 shade, to avoid the rain-his kite was raised-a thundercloud passed over it--no sign of electricity appeared. He almost despaired of success, when, suddenly, he observed the loose fibres of his string to move towards an erect position. He now presented his knuckle to the key, and received a strong spark. How exquisite must his

[ocr errors][merged small]
« ZurückWeiter »