Abbildungen der Seite

other eminences, makes it visible and audible, when it is denominated lightning and thunder, is highly probable; but that the sea, which you suppose the grand source of it, can collect it, I think admits of a doubt; for, though the sea be composed of salt and water, an electric perse and a non-electric; and though the friction of electrics per se and non-electrics will collect that fire; yet it is only under certain circumstances, which water will not admit; for it seems necessary, that the electrics per se and non-electrics, rubbing one another, should be of such substances as will not adhere to, or incorporate with, each other. Thus a glass or sulphur sphere turned in water, and so a friction between them, will not collect any fire; nor, I suppose, would a sphere of salt revolving in water; the water adhering to, or incorporating with, those electrics per se. But, granting that the friction between the salt and water would collect the electric fire, that fire, being so extremely subtile and active, would be immediately communicated either to those lower parts of the sea, from which it was drawn, and so only perform quick revolutions, or be communicated to the adjacent islands or continent, and so be diffused instantaneously through the general mass of the earth. I say instantaneously; for the greatest distances we can conceive within the limits of our globe, even that of the two most opposite points, it will take no sensible time in passing through; and therefore it seems a little difficult to conceive how there can be any accumulation of the electric fire upon the surface of the sea, or how the vapors arising from the sea should have a greater share of that fire than other vapors. That the progress of the electrical fire is so amazingly swift, seems evident from an experiment you yourself (not out of choice) made, when two or three large glass jars were discharged through your body. You neither heard the crack, were sensible of the stroke, nor, which is more extraordinary, saw the light; which gave you just reason to conclude, that it was swifter than sound, than animal sensation, and even light itself. Now light, as astronomers have demonstrated, is about six minutes passing from the sun to the earth; a distance, they say, of more than eighty millions of miles. The greatest rectilinear distance within the compass of the earth is about eight thousand miles, equal to its diameter. Supposing, then, that the velocity of the electrical fire be the same as that of light, it will go through a space equal to the earth's diameter in about two sixtieths of one second of a minute. It seems inconceivable, then, that it should be accumulated upon the sea in its present state, which, as it is a non-electric, must give the fire an instantaneous passage to the neighbouring shores, and they convey it to the general mass of the earth. But such accumulation seems still more inconceivable, when the electrical fire has but a few feet depth of water to penetrate, to return to the place from whence it is supposed to be collected. Your thoughts on these remarks I shall receive with a great deal of pleasure. I take notice, that in the printed copies of your letters several things are wanting, which are in the manuscript you sent me, particularly what relates to Mr. Watson.

I understand by your son, that you had written, or was writing a paper on the effects of the electrical fire on loadstones, needles, &c., which I would ask the favor of a copy of, as well as of any other papers on electricity written since I had the manuscript; for which I repeat my obligations to you. I am, with great esteem, Sir, your most obedient humble servant,



Observations on the Subjects of the preceding Letter.

Reasons for supposing the Sea to be the grand Source of Lightning. Reasons for doubting this Hypothesis. Improvement in a Globe for raising the Elec

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

Philadelphia, 24 January, 1752. SIR, I am glad to learn, by your favor of the 21st past, that Mr. Kinnersley's lectures have been acceptable to the gentlemen of Boston, and are like to prove serviceable to himself.

I thank you for the countenance and encouragement you have so kindly afforded my fellow-citizen. .

I send you enclosed an extract of a letter containing the substance of what I observed concerning the communication of magnetism to needles by electricity. The minutes I took at the time of the experiments are mislaid. I am very little acquainted with the nature of magnetism. Dr. Gawin Knight, inventor of the steel magnets, has wrote largely on that subject; but I have not yet had leisure to peruse his writings with the attention necessary to become master of his doctrine.

Your explication of the crooked direction of lightning appears to me both ingenious and solid. When we can account as satisfactorily for the electrification of clouds, I think that branch of natural philosophy will be nearly complete.

The air, undoubtedly, obstructs the motion of the electric fluid. Dry air prevents the dissipation of an electric atmosphere, the denser the more, as in cold weather. I question whether such an atmosphere can

[ocr errors]

be retained by a body in vacuo. A common electrical phial requires a non-electric communication from the wire to every part of the charged glass; otherwise, being dry and clean, and filled with air only, it charges slowly, and discharges gradually by sparks, without a shock; but, exhausted of air, the communication is so open and free between the inserted wire and surface of the glass, that it charges as readily, and shocks as smartly as if filled with water; and I doubt not, but that, in the experiment you propose, the sparks would not only be near straight in vacuo, but strike at a greater distance than in the open 'air, though perhaps there would not be a loud explosion. As soon as I have a little leisure, I will make the experiment, and send you the result.

My supposition, that the sea might possibly be the grand source of lightning, arose from the common observation of its luminous appearance in the night, on the least motion ; an appearance never observed in fresh water. Then I knew, that the electric fluid may be pumped up out of the earth, by the friction of a glass globe, on a non-electric cushion; and that, notwithstanding the surprising activity and swiftness of that fluid, and the non-electric communication between all parts of the cushion and the earth, yet quantities would be snatched up by the revolving surface of the globe, thrown on the prime conductor, and dissipated in air. How this was done, and why that subtile, active spirit did not immediately return again from the globe into some part or other of the cushion, and so into the earth, was difficult to conceive; but, whether from its being opposed by a current setting upwards to the cushion, or from whatever other cause, that it did not so return was an evident fact. Then I considered the separate particles of water as so many hard spherules, capable of touching the salt only in points, and imagined a particle of salt could therefore no more be wet by a particle of water, than a globe by a cushion; that there might therefore be such a friction between these originally constituent particles of salt and water, as in a sea of globes and cushions; that each particle of water on the surface might obtain, from the common mass, some particles of the universally diffused, much finer, and more subtile electric fluid, and, forming to itself an atmosphere of those particles, be repelled from the then generally electrified surface of the sea, and fly away with them into the air. I thought, too, that possibly the great mixture of particles electric per se, in the ocean water, might, in some degree, impede the swift motion and dissipation of the electric fluid through it to the shores, &c. But, having since found, that salt in the water of an electric phial does not lessen the shock; and having endeavoured in vain to produce that luminous appearance from a mixture of salt and water agitated ; and observed, that even the sea-water will not produce it after some hours standing in a bottle; I suspect it to proceed from some principle yet unknown to us (which I would gladly make some experiments to discover, if I lived near the sea), and I grow more doubtful. of my former supposition, and more ready to allow weight to that objection (drawn from the activity of the electric fluid, and the readiness of water to conduct), which you have indeed stated with great strength and clearness.

In the mean time, before we part with this hypothesis, let us think what to substitute in its place. I have sometimes queried, whether the friction of the air, an electric per se, in violent winds, among trees, and against the surface of the earth, might not pump up, as so many glass globes, quantities of the electric fluid, which

« ZurückWeiter »