Benjamin Franklin; Self-Revealed Volume Ii Part 18

CHAPTER IV

Franklin as a Man of Science

Franklin, as we have said, was primarily a man of action. If we do not always think of him as deeply involved in what Goethe calls "being's ocean, action's storm," it is only because he moved from appointed task to appointed task with such frictionless self-command and ease. But, throughout his life, his mind was quick to make excursions into the domain of philosophical speculation and experiment, whenever business cares or political responsibilities allowed it to do so. Poor Richard would seem to have little in common with Prometheus, but Prometheus, if Condorcet is to be believed, as well as Poor Richard, Franklin was; to say nothing of other transmigrations. That his interest in natural phenomena began at a very early age, is disclosed by his _Journal of a Voyage from London to Philadelphia_ in 1726, when he was in his twenty-first year. Throughout the course of this voyage, his faculties were intently concentrated upon all the marvels of the sea and its setting. With sedulous minuteness, he registers the state of the winds each day, and records the impression made on him by every object with a secret at its heart, to be plucked out by an inquisitive mind. A lunar rainbow, an eclipse of the sun, which darkened ten twelfths of his disk, an eclipse of the moon, which spread over six digits of her surface, dolphins in their bright mail of mixed green, silver and gold, a shark moving around the ship in a slow, majestic manner, and attended by an obsequious retinue of pilot fish, schools of harried flying fish, groups of young crabs, clinging to seaweeds, with indented leaves about three quarters of an inch long, and small yellow berries filled with nothing but wind, a white, tropical bird, said never to be seen further north than lat.i.tude 40, and marked by short wings and a single tail feather, other birds, too near the western continent not to be Americans, are among the things that the open-eyed and thoughtful youth jotted down in his Journal in terms that plainly enough indicated not only the eager curiosity but the exact.i.tude of a future man of science. As almost always, the child was but the father of the man. Upon each of his subsequent six voyages across the Atlantic, Franklin exhibited the same, though severer, and more practised, vigilance in observing everything that the ocean, including the instruments of commerce afloat on it, have for a penetrating and suggestive intelligence. How essentially he was a man of science, is demonstrated by the fact that, whenever he was on the element, where alone he could hope for exemption from the political demands of his countrymen, his intellect turned at once with ardor to the study of Nature. Old and feeble as he was, he wrote no less than three valuable dissertations on his last voyage across the Atlantic, one on the causes and cure of smoky chimneys, one on his smoke-consuming stove, and a third, distinguished by an extraordinary wealth of knowledge and observation, on the construction, equipment and provisioning of ships, and the winds, currents and temperature of the sea; which was accompanied by valuable thermometric tables, based upon observations made by him during three of his transatlantic voyages. The maritime essay was written with the closest regard to detail, and contains such a ma.s.s of information and luminous comment as has rarely been condensed into the same s.p.a.ce. It makes up some thirty-four quarto pages of Smyth's edition of Franklin's works, exclusive of the thermometric notes. The other two essays occupy some forty-nine pages more. All three are elucidated by numerous explanatory charts and ill.u.s.trations, and are marked by the mastery of scientific principles, which no mere artificer or artisan could have displayed in discussing such topics; but, at the same time, they could not have been more intensely practical, as respects minutiae of construction, if Franklin had been a professional sailor, mason or stove-maker. The maritime observations range from the Chinese method of dividing the hulls of vessels into separate compartments, which is now regarded as one of the most efficient devices for securing the safety of ocean greyhounds, to an inquiry into the reason why fowls served up at sea are usually too tough to be readily masticated and the best means of dishing soup on a rolling and pitching vessel.

After his return in his youth from London to Philadelphia, Franklin was for a long time too much immersed in business and civic projects to give much attention to natural phenomena. "Why does the flame of a candle tend upward in a spire?", "whence comes the dew, that stands on the outside of a tankard that has cold water in it in the summer time?", are among the few questions of a scientific nature that he appears to have framed for the discussions of the Junto; and they are elementary enough. But with the coming of pecuniary ease, the natural bent of his mind soon a.s.serted itself. While in Boston in 1746, he happened to see some electrical experiments performed by a Dr. Spence, who had recently arrived from Scotland. They were clumsily conducted, but crude as they were, they filled his mind with mixed sensations of surprise and delight; so much so that, when, shortly after his return to Philadelphia from Boston, the Library Company found itself the owner of a gla.s.s tube, for the production of electricity by friction, given to it by Peter Collinson, then a Fellow of the Royal Society of London, with instructions for its use, he eagerly availed himself of Collinson's generosity to repeat the experiments that he had witnessed at Boston, and, by continuous practice, became very expert in making them as well as others. Indeed, his house was soon overrun to such an extent with eager visitors that he was compelled in self-defence to relieve it of its congestion by supplying some of his friends with similar tubes blown at the Philadelphia gla.s.s-house. One of these friends was his ingenious neighbor, Kinnersley, who chanced at the time to be out of business. Franklin advised him to exhibit the experiments for profit, and followed up the advice by preparing two lectures for him, in which the details of the experiments were clearly set forth. Kinnersley himself employed skilled workmen to make the necessary electrical apparatus for him, modelled upon the rough agencies designed by Franklin for himself, and used in his own exhibitions. The lectures, when delivered by him in Philadelphia, were so well attended that he made a tour of all the chief towns of the Colonies with a considerable degree of pecuniary success. Some years later, similar instructions given by Franklin to Domien, a Greek priest, proved so useful to him on a long tramp that he wrote to his benefactor that he had lived eight hundred miles upon electricity, and that it had been meat, drink and clothing to him. When Franklin last heard from him, he was contemplating a journey from Havana to Vera Cruz, thence through Mexico to Acapulco, on its western coast, and from Acapulco to Manila, and from Manila through China, India, Persia and Turkey to his home in Transylvania; all with electricity as his main _viatic.u.m_.

Franklin's own experiments fortunately ended in something better than vagabondage, however respectable or profitable. Grateful to Collinson for his timely gifts, he wrote to him several letters, laying before him the results of the Philadelphia experiments. Collinson procured for these letters the privilege of being read before the Royal Society, where they did not excite enough notice to be printed among its Transactions. Another letter, one to Kinnersley, in which Franklin propounded the ident.i.ty of lightning and electricity, he sent to Dr. Mitch.e.l.l, an acquaintance of his, and also a member of the Royal Society, who replied by telling him that it had been read before the Society, but had been laughed at by the connoisseurs. Then it was that the happy obstetric suggestion of Dr.

Fothergill that the letters were of too much value to be stifled led Collinson to gather them together for publication by Cave in the _Gentleman's Magazine_. They were not published in this magazine, but Cave did bring them out in pamphlet form with a preface by Dr. Fothergill. The event showed that he and the general public had more ac.u.men than the sages of the Royal Society, for the letters, when subsequently published in a quarto volume, with additions by Franklin, ran through five editions, without the cost of a penny to Cave for copyright. It was from France, however, that they first received the full meed of prompt approbation that they deserved. A copy of them happened to fall into the hands of Buffon, who prevailed upon D'Alibard to translate them into French. Their publication in that language provoked an attack upon them by the Abbe Nollet, Preceptor in Natural Philosophy to the Royal Family, and the author of a popular theory of Electricity. At first, the Abbe could not believe that America was capable of producing such letters, and insisted that they must have been fabricated at Paris for the purpose of discrediting his system. In fact, he even doubted whether there was such a person as Franklin, but, afterwards, being convinced upon that point, he published a volume of letters, mainly addressed to Franklin, in which he defended his own theory, and denied the accuracy of Franklin's experiments and conclusions. Le Roy, of the Royal Academy of Sciences, rejoined on behalf of Franklin, who had decided to let the truth be its own champion, and easily refuted the Abbe. The papers could not have asked for a better advertis.e.m.e.nt than this controversy. They were further translated into the Italian, German and Latin languages, and Franklin's theory of electricity was so generally adopted by the learned men of Europe, in preference to that of the Abbe, that the latter lived, Franklin tells us, to see himself the last of his sect, except Monsieur B. of Paris, his _eleve_ and immediate disciple. It is surprising that even the solitary _eleve_ should have been left clinging to his master; for, in the meantime, the most momentous experiment, suggested by Franklin in his letters, had been performed, substantially in the manner outlined by him, with brilliant success, by D'Alibard, on a hill at Marly-la-Ville, where a pointed rod of iron, forty feet high, and planted on an electric stand, had been erected for the purpose of carrying it into execution. When a thundercloud pa.s.sed over the rod on May 10, 1752, between 2 and 3 o'clock in the afternoon, the persons, set by D'Alibard to watch it, had drawn near "and attracted from it sparks of fire, perceiving the same kind of commotions as in the common electrical experiments." A week later, the fire and crackling sound, elicited by M. de Lor from a rod, erected at his house in Paris on a cake of resin, and electrified by a cloud between 4 and 5 o'clock in the afternoon, told the same story. He had previously performed what he called the "Philadelphia experiments" in the presence of Louis XV., who seems to have been as much delighted with them as if they had been a new mistress.

In a short time, they became so popular that we are told by Franklin that "all the curious of Paris flocked to see them." One of the results of the fame acquired by him in France was a letter written by Dr. Wright, an English physician, then in Paris, to a member of the Royal Society, apprising the latter of the excitement that the experiments had created in France, and expressing his astonishment that Franklin's papers had been so little noticed in England. Quickened by Dr. Wright's words, the Society reconsidered the letters which had been read before them, and caused an abstract of them and the other letters on electricity, sent to England by Franklin, to be printed among its Transactions. Afterwards, when several members of the Society had themselves drawn down lightning from the clouds, it elected Franklin a member, and, in view of the fact that the honor had not been sought by him, voted that he "was not to pay anything"; which meant that he was to be liable for neither admission fee nor annual dues, and was even to receive his copy of the Transactions of the Society free of charge. Nor did it stop here. It also awarded to Franklin, for the year 1753, the Copley gold medal, accompanied by an address, in which Lord Macclesfield, its President, endeavored to make full amends to him for its belated recognition of the value of his discoveries.

The suggestion by Franklin, which led to the experiments of D'Alibard and De Lor, is as matter-of-fact as a cooking recipe.

To determine the question [he said in a letter to Peter Collinson] whether the clouds that contain lightning are electrified or not, I would propose an experiment to be try'd where it may be done conveniently. On the top of some high tower or steeple, place a kind of centry box,... big enough to contain a man and an electrical stand. From the middle of the stand let an iron rod rise and pa.s.s bending out of the door, and then upright 20 or 30 feet, pointed very sharp at the end. If the electrical stand be kept clean and dry, a man standing on it when such clouds are pa.s.sing low, might be electrified and afford sparks, the rod drawing fire to him from a cloud. If any danger to the man should be apprehended (though I think there would be none), let him stand on the floor of his box, and now and then bring near to the rod the loop of a wire that has one end fastened to the leads, he holding it by a wax handle; so the sparks, if the rod is electrified, will strike from the rod to the wire, and not affect him.

Before the news of the success achieved by D'Alibard and De Lor reached Franklin, he himself had conducted a similar experiment "though made in a different and more easy manner." This experiment has become one of the veriest commonplaces of physical science. It was performed, when a thunder gust was coming on, in a field near Philadelphia, with such simple materials as a silk kite, topped off with a foot or more of sharp pointed wire, and controlled by a twine string, equipped with a key for casting off the electric sparks, and ending in a silk ribbon to secure the safety of the hand that held it. The whole construction is set out in a letter written to Collinson by Franklin shortly after the incident, in which, with his usual modesty, the latter describes the kite as if he had had nothing to do with it. Something like the feelings of Sir Isaac Newton, when the falling apple brought to his ear the real music of the spheres, must have been those of Franklin, when the loose filaments of twine bristled up stiffly, as if stirred by some violated instinct of wild freedom, and the stream of sparks from the key told him that he was right in supposing that the mysterious and appalling agency, which had for centuries been a.s.sociated in the human mind with the resistless wrath of Omnipotence, was but the same subtle fluid that had so often lit up his electrical apparatus with its playful corruscations.

The letters to Collinson contained another suggestion almost equally pregnant. Speaking of the power of pointed conductors to draw off electricity noiselessly and harmlessly, Franklin asked,

May not the knowledge of this power of points be of use to mankind, in preserving houses, churches, ships, &c.

from the stroke of lightning, by directing us to fix on the highest parts of those edifices, upright rods of iron made sharp as a needle, and gilt to prevent rusting, and from the foot of those rods a wire down the outside of the building into the ground, or down round one of the shrouds of a ship, and down her side till it reaches the water? Would not these pointed rods probably draw the electrical fire silently out of a cloud before it came nigh enough to strike, and thereby secure us from that most sudden and terrible mischief?

The suggestion was but slowly adopted, not in Europe, indeed, at all, until the efficacy of the lightning rod in protecting buildings had been generally recognized in America. In time, however, the device came into use both in Great Britain and on the Continent; Voltaire being one of the first persons in Geneva to erect one, and, wherever it was erected, it helped to confirm the fame of Franklin by its silent effect upon the human imagination. In recent years, the lightning rod, once in almost universal use in America, has fallen into neglect, but the explanation of this fact is to be found not in any just doubts about its utility, when properly constructed, affixed and grounded, but in the growth of fire insurance, and the inutility, or danger, of such rods, if carelessly set in place.[51]

The domestication of lightning and the invention of the lightning rod were the two things to which Franklin was princ.i.p.ally indebted for his brilliant reputation as a philosopher. At this day, the application of electricity to common uses is so familiar to us that it is hard, without a little reflection, to realize how well calculated his electrical achievements were to send a thrill of astonishment and awe through the human mind. Of all the manifestations of the physical world, lightning with its inscrutable, swift, and all but irresistible, stroke, followed by the sublime detonations of thunder, is the one most suggestive of supernatural influence exerted by an all-powerful deity. The mythological dreams of the Greeks, the visions of the Old Testament, the simple emotions of the savage had all paid their homage of dread to the fearful force--like a madman pitilessly destructive, and yet like a madman diverted from its rage by the barest trifle--which had clothed Jove with the greater part of his grandeur, licked up even the water that was in the trench about the altar, built by Elijah in the name of the Lord, and filled the breast of the Indian with superst.i.tious terror. Discovery, that laid bare the real nature and destructive limits of this force, could not fail to excite an extraordinary degree of attention everywhere. It was the singular fortune of Franklin, though a practical, sober-minded denizen of the earth, if ever man was, to have enjoyed in his day a reputation not unlike that of a divinity of the upper ether.[52] It so happens that the atmosphere was, in one way or another, the home of all the scientific problems which engaged his interest most deeply. His philosophical Pegasus, so little akin to the humble brute bestrid by Poor Richard, was "a beast for Perseus--pure air and fire"; and especially, it is needless to say, was this true of his relations to the lightning. When the fact became known throughout the civilized world that human ingenuity had succeeded in even snaring it, Franklin was exalted for a time to a seat on Olympus. All the literature of the period, as well as that of a much later period, bears out the statement that rarely has any single, peaceful incident ever so fired the human imagination.[53] For many years, the natural background for a portrait of Franklin might have been a bank of cloud lit up by the incessant play of summer lightning. _Eripuit coelo fulmen sceptrumque tyrannis_, was but the mightiest of the electrical discharges that flattery poured upon him. Turn where we may to the poetry of the latter half of the eighteenth century, and of the earlier part of the nineteenth, whether epigram or otherwise, we are likely to come upon some imprint left upon the thought of those periods by the subjugation of lightning.

The interest of Franklin in electrical science was but another sequel of the world-wide avidity with which learned men had recently turned to the study of that subject. One of them, Grey, had pursued a series of experiments for the purpose of determining the relative conductivity of various substances, another, Du Fay, had erroneously cla.s.sified electricity as resinous and vitreous, and the perfected Leyden Jar particularly had given a new momentum to the progress of electrical investigation. Into this movement, after witnessing Dr. Spence's awkward experiments at Boston, Franklin threw himself with the utmost enthusiasm, and his discovery of the ident.i.ty of lightning and electricity and his lightning-rod conception were but the chief fruits of this enthusiasm. Between the _Autobiography_ and his letters, we are at no loss to follow closely the steps by which he reached all the results which have given him such a high position as an electrical investigator. "I purchased all Dr. Spence's apparatus ..." he tells us in the _Autobiography_, "and I proceeded in my electrical experiments with great alacrity." How keen this alacrity became, after he had been rubbing for a time the gla.s.s tube, sent over to Philadelphia by Collinson, may be seen in what he wrote to Collinson himself on March 28, 1747:

For my own part, I never was before engaged in any study that so totally engrossed my attention and my time as this has lately done; for what with making experiments when I can be alone, and repeating them to my Friends and Acquaintance, who, from the novelty of the thing, come continually in crouds to see them, I have, during some months past, had little leisure for anything else.

The result of this experimentation was the various letters to Collinson and others that const.i.tute Franklin's highest claim to distinction as a man of science. By following them in their chronological order, the reader can trace with little difficulty the genesis of each of his more valuable conclusions touching electricity. They are distinguished by remarkable simplicity and force of reasoning and by a clearness of statement as transparent as crystal. Moreover, they are even enlivened at times by gleams of fancy or humor. In a word they indisputably merit the judgment that Sir Humphry Davy, no mean judge of style as well as scientific truth, pa.s.ses upon them:

The style and manner of his publication on electricity are almost as worthy of admiration as the doctrine it contains. He has endeavoured to remove all mystery and obscurity from the subject. He has written equally for the uninitiated and the philosopher; and he has rendered his details amusing as well as perspicuous, elegant as well as simple. Science appears in his language in a dress wonderfully decorous, the best adapted to display her native loveliness. He has in no instance exhibited that false dignity, by which philosophy is kept aloof from common applications; and he has sought rather to make her a useful inmate and servant in the common habitations of man, than to preserve her merely as an object of admiration in temples and palaces.

While recalling these words, it is not amiss to recall, too, what Lord Brougham had to say about the agencies with which Franklin conducted his experiments.

He could make an experiment [said Brougham] with less apparatus and conduct his experimental inquiry to a discovery with more ordinary materials than any other philosopher we ever saw. With an old key, a silk thread, some sealing wax and a sheet of paper he discovered the ident.i.ty of lightning and electricity.

The truth of these observations is strikingly instanced in a story told of Franklin in Pettigrew's _Life of Lettsom_. When Henry Smeathman was insisting that the flight of birds is on inclined planes, and that they could not fly at all, but would simply float with the wind, if they were not heavier than the air, Franklin launched half a sheet of paper obliquely into the air, observing, as he watched its course, that that was an evident proof of the propriety of Smeathman's doctrines.

In a letter to Collinson, dated July 11, 1747, Franklin communicated to him the earliest results of his experimental use of the gla.s.s tube that Collinson had sent over to Philadelphia. The first phenomenon, which fixed his attention, was the wonderful effect of pointed bodies in drawing off the electrical fire. This was the lightning rod in its protoplasmal stage.

The manner in which he described the experiment, by which this particular truth was demonstrated, is a good specimen of his remarkable faculty for simple and clear statement:

Place an iron shot of three or four inches diameter on the mouth of a clean dry gla.s.s bottle. By a fine silken thread from the ceiling, right over the mouth of the bottle, suspend a small cork ball, about the bigness of a marble; the thread of such a length, as that the cork ball may rest against the side of the shot. Electrify the shot, and the ball will be repelled to the distance of four or five inches, more or less, according to the quant.i.ty of Electricity. When in this state, if you present to the shot the point of a long slender sharp bodkin, at six or eight inches distance, the repellency is instantly destroy'd, and the cork flies to the shot.

A blunt body must be brought within an inch, and draw a spark, to produce the same effect. To prove that the electrical fire is _drawn off_ by the point, if you take the blade of the bodkin out of the wooden handle, and fix it in a stick of sealing wax, and then present it at the distance aforesaid, or if you bring it very near, no such effect follows; but sliding one finger along the wax till you touch the blade, and the ball flies to the shot immediately. If you present the point in the dark, you will see, sometimes at a foot distance, and more, a light gather upon it, like that of a firefly, or glowworm; the less sharp the point, the nearer you must bring it to observe the light; and, at whatever distance you see the light, you may draw off the electrical fire, and destroy the repellency. If a cork ball so suspended be repelled by the tube, and a point be presented quick to it, tho' at a considerable distance, 'tis surprizing to see how suddenly it flies back to the tube. Points of wood will do near as well as those of iron, provided the wood is not dry; but perfectly dry wood will no more conduct electricity than sealing-wax.

The repellency between the ball and the shot was likewise destroyed, Franklin stated, 1, by sifting fine sand on it; this did it gradually, 2, by breathing on it, 3, by making a smoke about it from burning wood, and 4, by candlelight, even though the candle was at a foot distance; these did it suddenly.

The same result was also produced, he found, by the light of a bright coal from a wood fire, or the light of red-hot iron; but not at so great a distance. Such was not the effect, however, he said, of smoke from dry resin dropped on hot iron. It was merely attracted by both shot and cork ball, forming proportionable atmospheres round them, making them look beautifully, somewhat like some of the figures in Burnet's or Whiston's _Theory of the Earth_.

Franklin also noted the fact that, unlike fire-light, sunlight, when thrown on both cork and shot, did not impair the repellency between them in the least.

In the same letter, guided by the belief that he had formed that electricity is not created by friction but, except when acc.u.mulated or depleted by special causes, is equally diffused through material substances generally, he also reached the conclusion that electrical discharges are due to circuits set up by substances that offer little resistance to the transit of the electrical current between bodies charged with more than the ordinary quant.i.ty of electrical energy and bodies not in that condition. In other words, electricity is always alert to restore its equilibrium when lost, and, if acc.u.mulated beyond its normal measure in one body, seeks with violent eagerness, as soon as a favorable medium of transmission is presented to it, to pa.s.s on its surplus of electrical energy to another body less amply supplied.

These conceptions, too, which lie at the very foundations of modern electrical science, are ill.u.s.trated by Franklin with extraordinary simplicity and clearness as follows:

1. A person standing on wax, and rubbing the tube, and another person on wax drawing the fire, they will both of them, (provided they do not stand so as to touch one another) appear to be electrised, to a person standing on the floor; that is, he will perceive a spark on approaching each of them with his knuckle.

2. But, if the persons on wax touch one another during the exciting of the tube, neither of them will appear to be electrised.

3. If they touch one another after exciting the tube, and drawing the fire as aforesaid, there will be a stronger spark between them, than was between either of them and the person on the floor.

4. After such strong spark, neither of them discover any electricity.

These appearances we attempt to account for thus: We suppose, as aforesaid, that electrical fire is a common element, of which every one of the three persons above mentioned has his equal share, before any operation is begun with the tube. A, who stands on wax and rubs the tube, collects the electrical fire from himself into the gla.s.s; and his communication with the common stock being cut off by the wax, his body is not again immediately supply'd. B, (who stands on wax likewise) pa.s.sing his knuckle along near the tube, receives the fire which was collected by the gla.s.s from A; and his communication with the common stock being likewise cut off, he retains the additional quant.i.ty received. To C, standing on the floor, both appear to be electrised: for he having only the middle quant.i.ty of electrical fire, receives a spark upon approaching B, who has an over quant.i.ty; but gives one to A, who has an under quant.i.ty. If A and B approach to touch each other, the spark is stronger, because the difference between them is greater: After such touch there is no spark between either of them and C, because the electrical fire in all is reduced to the original equality. If they touch while electrising, the equality is never destroy'd, the fire only circulating. Hence have arisen some new terms among us: We say, B, (and bodies like circ.u.mstanced) is electrised _positively_; A, _negatively_. Or rather, B is electrised _plus_; A, _minus_. And we daily in our experiments electrise bodies _plus_ or _minus_, as we think proper. To electrise _plus_ or _minus_, no more needs to be known than this, that the parts of the tube or sphere that are rubbed, do, in the instant of the friction, attract the electrical fire, and therefore take it from the thing rubbing: The same parts immediately, as the friction upon them ceases, are disposed to give the fire they have received, to anybody that has less. Thus you may circulate it, as Mr. _Watson_ has shown; you may also acc.u.mulate or subtract it upon, or from anybody, as you connect that body with the rubber or with the receiver, the communication with the common stock being cut off.

The same letter recounts some of the tricks that Franklin and his fellow-experimenters were in the habit of making their new plaything perform. They fired spirits, lit candles just blown out, mimicked lightning, produced sparks with the touch of the finger, on the human hand or face, and gave electrical kisses. Other feats consisted in animating an artificial spider in such a way as to keep him oscillating in a very lifelike and entertaining manner between two wires, and lighting up the gilding on the covers of a book with a brilliant flash. This letter also shows that the provincial philosophers had already made improvements in the usual electrical methods. They had found that it was better to fill the phial with granulated lead than with water because of the superior facility with which the former could be warmed, and kept warm and dry in a damp place. They rubbed their tubes with buckskin, and, by observing certain precautions, such as never sullying the tubes by handling them, and keeping them in tight, close-fitting cases of pasteboard, lined with flannel, increased their efficiency. Their spheres for charging phials with electricity were mounted on iron axes with a small handle on one end, with which they could be set revolving like a common grindstone. It was in this same letter that Franklin with his usual generosity was careful to state that the power of pointed bodies to throw off as well as draw off the electrical fire was a discovery of his friend Hopkinson, and that the revolving sphere used by them was the invention of his friend Syng. About a month later, Franklin wrote to Collinson that, in the course of further experiments, he had observed several phenomena which made him distrust some of his former conclusions. "If there is no other use discover'd of Electricity," he said, "this however is something considerable, that it may _help to make a vain man humble_."

Another letter from Franklin to Collinson, written about two weeks later, communicated to him some valuable observations upon "M. Muschenbroeck's wonderful bottle"--the Leyden Jar. This bottle was a mere ordinary bottle, with a common cork in its neck, into which a common wire had been inserted.

He wrote that, at the same time that the wire and the top of the bottle were electrised positively or plus, the bottom of the bottle was electrised negatively or minus, in exact proportion; the consequence was that, whatever quant.i.ty of electrical fire was thrown in at the top, an equal quant.i.ty went out at the bottom until, if the process was kept up long enough, the point was reached in the operation, when no more could be thrown into the upper part of the bottle, because no more could be drawn out of the lower part. If the attempt was made to throw more in, the fire was spewed back through the wire, or flew out in loud cracks through the sides of the bottle.

He also noted that an equilibrium could not be restored in the bottle by inward communication or contact of the parts, but only by a communication, formed without the bottle between its top and bottom.

He also noted that no electrical fire could be thrown into the top of the bottle, when none could get out at its bottom, either because the bottom was too thick, or because it stood on some non-conducting material, and likewise that, when the bottle was electrified, but little of the electrical fire could be drawn from the top by touching the wire, unless an equal quant.i.ty could at the same time get in at the bottom.

So wonderfully [he adds] are these two states of electricity, the _plus_ and _minus_, combined and balanced in this miraculous bottle! situated and related to each in a manner that I can by no means comprehend! If it were possible that a bottle should in one part contain a quant.i.ty of air strongly comprest, and in another part a perfect vacuum, we know the equilibrium would be instantly restored _within_. But here we have a bottle containing at the same time a _plenum_ of electrical fire, and a _vacuum_ of the same fire; and yet the equilibrium cannot be restored between them but by a communication without! though the _plenum_ presses violently to expand, and the hungry vacuum seems to attract as violently in order to be filled.

The letter concludes with an elaborate statement of the experiments by which the correctness of its conclusions could be established.

Franklin's next discovery communicated to Collinson in a letter dated the succeeding year was that, when the bottle was electrified, the electric fluid resided in the gla.s.s itself of the bottle. The manner in which he proved this fact is a good example of his inductive thoroughness.

Purposing [he said] to a.n.a.lyze the electrified bottle, in order to find wherein its strength lay, we placed it on gla.s.s, and drew out the cork and wire, which for that purpose had been loosely put in. Then taking the bottle in one hand, and bringing a finger of the other near its mouth, a strong spark came from the water, and the shock was as violent as if the wire had remained in it, which shewed that the force did not lie in the wire. Then, to find if it resided in the water, being crouded into and condensed in it, as confin'd by the gla.s.s, which had been our former opinion, we electrified the bottle again, and, placing it on gla.s.s, drew out the wire and cork as before; then, taking up the bottle, we decanted all its water into an empty bottle, which likewise stood on gla.s.s; and taking up that other bottle, we expected, if the force resided in the water, to find a shock from it; but there was none. We judged then, that it must either be lost in decanting, or remain in the first bottle. The latter we found to be true; for that bottle on trial gave the shock, though filled up as it stood with fresh unelectrified water from a teapot.

By a similar course of experimentation with sash gla.s.s and lead plates, he also demonstrated that the form of the gla.s.s in the bottle was immaterial, that the power resided in the gla.s.s as gla.s.s, and that the non-electrics in contact served only like the armature of a loadstone to unite the force of the several parts, and to bring them at once to any point desired; it being the property of a non-electric that the whole body instantly receives or gives what electric fire is given to, or taken from, anyone of its parts.

These experiments suggested the idea of intensifying the application of electrical forces by grouping numerous electrical centres.

We made [he said] what we called an _electrical battery_, consisting of eleven panes of large sash-gla.s.s, arm'd with thin leaden plates, pasted on each side, placed vertically, and supported at two inches distance on silk cords, with thick hooks of leaden wire, one from each side, standing upright, distant from each other, and convenient communications of wire and chain, from the giving side of one pane, to the receiving side of the other; that so the whole might be charged together, and with the same labour as one single pane; and another contrivance to bring the giving sides, after charging, in contact with one long wire, and the receivers with another, which two long wires would give the force of all the plates of gla.s.s at once through the body of any animal forming the circle with them. The plates may also be discharged separately, or any number together that is required.

When the idea of the electrical battery was formed by him, Franklin was not aware that Smeaton and Bains had previously a.s.sembled panes of gla.s.s for the purpose of giving an electrical shock.

At the time that this letter was written, Franklin had added to his electrical exploits that of electrifying a mezzotint of the King in such a manner that, if anyone attempted to take the crown off his head, he would receive a "terrible blow."