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being condensed, a degree of repellency greater than its natural, it turns the spark out of its straight course; the neighbouring air, which must be less dense, and therefore has a smaller degree of repellency, giving it a more ready passage. The spark having taken a new direction must now act on, or most strongly repel, the column of air which lies in that direction, and consequently must condense that column in the same manner as the former, when the spark must again change its course, which course will be repeatedly changed, till the spark reaches the body that attracted it.

To this account one objection occurs; that, as air is very fluid and elastic, and so endeavours to diffuse itself equally, the supposed accumulated air within the column aforesaid would be immediately diffused among the contiguous air, and circulate to fill the space it was driven from, and consequently that the said column, on the greater density of which the phenomenon is supposed to depend, would not repel the spark more strongly than the neighbouring air.

This might be an objection, if the electrical fire was as sluggish and inactive as air. Air takes a sensible time to diffuse itself equally, as is manifest from winds, which often blow for a considerable time together from the same point, and with a velocity, even in the greatest storms, not exceeding, as it is said, sixty miles an hour; but the electrical fire seems propagated instantaneously, taking up no perceptible time in going very great distances. It must be, then, an inconceivably short time in its progress from an electrified to an unelectrified body, which, in the present case, can be but a few inches apart. But this small portion of time is not sufficient for the elasticity of the air to exert itself, and therefore the column aforesaid must be in a denser state than its neighbouring air.




About the velocity of the electrical fire more is said below, which perhaps may more fully obviate this objection. But let us have recourse to experiments. Experiments will obviate all objections, or confound the hypothesis. The electrical spark, if the foregoing be true, will pass through a vacuum in a right line.

To try this, let a wire be fixed perpendicular on the plate of an air-pump, having a leaden ball on its

upper end ; let another wire, passing through the top of a receiver, have on each end a leaden ball; let the leaden balls within the receiver, when put on the air-pump, be within two or three inches of each other; the receiver being exhausted, the spark given from a charged phial to the upper wire will pass through rarefied air, nearly approaching to a vacuum, to the lower wire, and I suppose in a right line, or nearly so; the small

portion of air remaining in the receiver, which cannot be entirely exhausted, may possibly cause it to deviate a little, but perhaps not sensibly, from a right line. The spark also might be made to pass through air greatly condensed, which perhaps would give it a still more crooked direction. I have not had an opportunity to make any experiments of this sort, not knowing of an air-pump nearer than Cambridge ; but you can easily make them at your State-house, where there is one. If these experiments answer, I think the crooked direction of lightning will be also accounted for.

With respect to your letters on electricity, it will be no new thing to you to be told, that they are very curious and entertaining, and by far the best and most rational that have been written on that subject. Your hypothesis, in particular, for explaining the phenomena of lightning is very ingenious. That some clouds are highly charged with electrical fire, and that their communicating it to those that have less, to mountains, and 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 per se 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

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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 Electric Fire.


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


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