Its rising again shows that the prime conductor lost little or none of its electric charge, as it had done through the point: the falling of the ball while the large body was under the conductor therefore shows, that a quantity of its atmosphere was drawn from the end where the electrometer is placed to the part immediately over the large body, and there accumulated ready to strike into it with its whole undiminished force, as soon as within the striking distance; and, were the prime conductor moveable like a cloud, it would approach the body by attraction till within that distance. The swift motion of clouds, as driven by the winds, probably prevents this happening so often as otherwise it might do: for, though parts of the cloud may stoop towards a building as they pass, in consequence of such attraction, yet they are carried forward beyond the striking distance before they could by their descending come within it.
EXPERIMENT V.
Attach a small light lock of cotton to the underside of the prime conductor, so that it may hang down towards the pointed wire mentioned in the first experiment. Cover the point with your finger, and the globe being turned, the cotton will extend itself, stretching down towards the finger, as at a; but on uncovering the point, it instantly flies up to the prime conductor, as at b, and continues there as long as the point is uncovered. The moment you cover it again, the cotton flies down again, extending itself towards the finger; and the same happens in degree, if (instead of the finger) you use, uncovered, the blunt end of the wire uppermost.
OBSERVATION.
To explain this, it is supposed that the cotton, by its connection with the prime conductor, receives from it a quantity of its electricity; which occasions its being attracted by the finger that remains still in nearly its natural state. But when a point is opposed to the cotton, its electricity is thereby taken from it, faster than it can at a distance be supplied with a fresh quantity from the conductor. Therefore being reduced nearer to the natural state, it is attracted up to the electrified prime conductor; rather than down, as before, to the finger.
Supposing farther that the prime conductor represents a cloud charged with the electric fluid; the cotton, a ragged fragment of cloud (of which the underside of great thunder-clouds are seen to have many) the finger, a chimney or highest part of a building.—We then may conceive that when such a cloud passes over a building, some one of its ragged under-hanging fragments may be drawn down by the chimney or other high part of the edifice; creating thereby a more easy communication between it and the great cloud.—But a long pointed rod being presented to this fragment, may occasion its receding, like the cotton, up to the great cloud; and thereby increase, instead of lessening the distance, so as often to make it greater than the striking distance. Turning the blunt end of a wire uppermost (which represents the unpointed bar) it appears that the same good effect is not from that to be expected. A long pointed rod it is therefore imagined, may prevent some strokes; as well as conduct others that fall upon it, when a great body of cloud comes on so heavily that the above repelling operation on fragments cannot take place.
EXPERIMENT VI.
Opposite the side of the prime conductor place separately, isolated by wax stems, Mr. Canton's two boxes with pith balls suspended by fine linen threads. On each box, lay a wire six inches long and one-fifth of an inch thick, tapering to a sharp point; but so laid, as that four inches of the pointed end of one wire, and an equal length of the blunt end of the other, may project beyond the ends of the boxes; and both at eighteen inches distance from the prime conductor. Then charging the prime conductor by a turn or two of the globe, the balls of each pair will separate; those of the box, whence the point projects most, considerably; the others less. Touch the prime conductor, and those of the box with the blunt point will collapse, and join. Those connected with the point will at the same time approach each other, till within about an inch, and there remain.
OBSERVATION.
This seems a proof, that though the small sharpened part of the wire must have had a less natural quantity in it before the operation, than the thick blunt part; yet a greater quantity was driven down from it to the balls. Thence it is again inferred, that the pointed rod is rendered more negative: and farther, that if a stroke must fall from the cloud over a building, furnished with such a rod, it is more likely to be drawn to that pointed rod, than to a blunt one; as being more strongly negative, and of course its attraction stronger. And it seems more eligible, that the lightning should fall on the point of the conductor (provided to convey it into the earth) than on any other part of the building, thence to proceed to such conductor. Which end is also more likely to be obtained by the length and loftiness of the rod; as protecting more extensively the building under it.