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when the wind is still, is settled at the rate of a mile, or 5280 English feet in 4777".
To return to our purpose: the length of the conducting wire from the machine to the observers near the seven-mile stone was 6732 feet; the length of that to the nine-mile stone 3868 feet. The first of these measures only was made use of in the present operations concerning the velocity of electricity. In twelve discharges of the coated phial, which were felt by the observers near the seven-mile stone, and who, by a second watch of Mr. Graham's, measured the time between feeling the electrical commotion and hearing the report of the gun, with the utmost attention and exactness, was at a medium 5 seconds. And as the gun was distant from these observers 6732 feet, it follows, from the experiments, which have been made on the velocity of sound, that the real instant of the discharge of the gun preceded that of the observers hearing its report, at this time, when the strength of the wind was not so great as to enter into the computation, 670"; or preceded the instant when the electrical commotion was felt only ". But this instant was, from the nature of the experiment, necessarily prior to that of the electrical explosion, which was not made till the fire of the gun was actually seen; and therefore the time between the making of that explosion and its being actually felt by the observer, which must have been less than 0.837", was really so small, as not to fall under any certain observation, when it is to be distinguished from that which must of necessity be lost, between the firing of the gun and the electrical explosion itself.
In all the experiments where the circuit was formed to any considerable length, though the coated phial was very well charged, the snap at the gun-barrel, on this explosion, was not near so loud as when the circuit is formed in a room; so that a bystander, though versed in these operations, from seeing the flash and hearing the report, would imagine the stroke at the ends of the conducting wire to be very slight; the contrary of which, when the wire has been properly conducted, has always happened.
* Dr. Derham found, that when sound was carried against the wind, not only its distance but its velocity was lessened; and in M. Cassini's memoir, there is an experiment, where sound being carried against the wind, which then blew very strong, was retarded near a twelfth part of the usual time in its progress.-Orig.
From a review of these experiments, the following observations may be deduced.
1. That in all the preceding operations, when the wires have been properly conducted, the electrical commotions from the charged phial have only been very considerable, when the observers at the extremities of the wire have touched some sub. stance readily conducting electricity with some part of their bo dies.
2. That the electrical commotion is always felt most sensibly in those parts of the bodies of the observers, which are between the conducting wires, and the nearest and the most non-electric substance; or in other words, so much of their bodies as comes within the electric circuit.
3. That on these considerations we infer, that the electrical power is conducted between these observers by any non-electric substances, which happen to be situated between them, and contribute to form the electric circuit.
4. That the electrical commotion has been perceptible to two or more observers at considerable distances from each other, even as far as two miles.
5. That when the observers have been shocked at the end of two miles of wire, we infer that the electrical circuit is four miles, viz. two miles of wire, and the space of two miles of the non-electric matter between the observers, whether it be water, earth, or both. 6. That the electrical commotion is equally strong, whether it is conducted by water or dry ground.
7. That if the wires, between the electrifying machine and the observers, are conducted on dry sticks, or other substances nonelectric in a slight degree only, the effects of the electrical power are much greater than when the wires in their progress touch the ground, or moist vegetables, or other substances in a great degree non-electric.
8. That by comparing the respective velocities of electricity and sound; that of electricity, in any of the distances yet expe rienced, is nearly instantaneous.
The gentlemen concerned were still desirous, if possible, of ascertaining the absolute velocity of electricity at a certain distance; because, though last year in measuring the respective velocities of electricity and sound, the time of its progress was found to be very little, yet they were desirous of knowing, small as
that time was, whether it was measureable; and Mr. W. had thought of another method for this purpose.
Accordingly, August 5, 1748, there met at Shooter's Hill for this purpose the president of the Royal Society, and several other gentlemen: when it was agreed to make the electrical circuit of two miles, in the middle of which an observer was to take in each hand one of the extremities of a wire, which was a mile in length. These wires were to be so disposed that, this observer being placed on the floor of the room near the electrifying machine, the other observers might be able in the same view to see the explosion of the charged phial and the observer holding the wires, and might take notice of the time lapsed between the discharging the phial and the convulsive motions of the arms of the observer in conse. quence of it; as this time would show the velocity of electricity, through a space equal to the length of the wire between the coated phial and this observer.
When all parts of the apparatus were properly disposed, several explosions of the charged phial were made; and it was invariably seen, that the observer, holding in each hand one of the extremi. ties of these wires, was convulsed in both his arms in the instant of making the explosions. Instead of one, four men were then placed holding each other by the hand near the machine, the first of which held in his right hand one extremity of the wire, and the last man the other in his left. They were all seen convulsed in the instant of the explosion. Every one who felt it, complained of the severity of the shock. It was then tried whether, as the ground was wet, if the explosion was made with the observer holding the extremity of each wire standing on the ground near the window of the house, any difference would arise in the success of the experiment: no difference was found, the observer being shocked in the instant of the explosion as before in both his arms, and across his breast. On these considerations they were fully satisfied, that through the whole length of this wire, being 12276 feet, the velo city of electricity was instantaneous.
Mr. W. took notice, in a sequel to the experiments relating to electricity*, of an observation of professor Bose of Wittenberg, viz."that if the electrifying machine is placed on originally
* Printed for C. Davis, London, 1746. 8vo. p. 32.-Orig. VOL. IV.
electrics, the man who rubs the globe with his hands, even under these apparently favourable circumstances, gives no sign of being electrised when touched by an unexcited non-electric. But if another person, standing on the floor, does but touch the globe in motion with the end of one of his fingers, or any other non-electric, the person rubbing is instantly electrised, and that very strongly." This experiment, almost a year since, Dr. Bevis carried further, by placing whatever non-electric touched the globe as a conductor, whether it were a man or a gun-barrel, on originally-electrics. If then, either the man who rubbed the globe, or he who only held his finger near its equator, were touched by any person standing on the floor, a snapping from either of them was perceptible on that touch.
Mr. W. offers a solution of this phænomenon, and then gives another, to the same purport, from Mr. Franklin of America,
At this time, says Mr. W. I am the more particular concerning the solution of this singular appearance, as Mr. Collinson, a worthy member of this society, has received a paper concerning electricity from an ingenious gentleman, Mr. Franklin, a friend of his in Pennsylvania. This paper, dated June 1, 1747, I very lately perused, by favour of our most worthy president. Among other curious remarks, there is a like solution of this fact; for though this gentleman's experiment was made with a tube instead of a globe, the difference is no-ways material. As this experiment was made, and the solution given on the other side of the Atlantic Ocean, before this gentleman could possibly be acquainted with our having observed the same fact here, and as he seems very conversant in this part of Natural Philosophy, I take the liberty of laying before you his own words.
"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 as to touch one another, appear to be electrised to a person standing on the floor; that is, he will per. ceive 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 a strong spark neither of them discover any electricity.
"These appearances we attempt to account for thus: we sup." pose that electrical fire is a common element, of which every one of these three persons 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 glass; and his communication with the common stock being cut off by the wax, his body is not again immediately supplied. B, who stands on wax likewise, passing his knuckle along near the tube, receives the fire which was collected by the glass from A; and his communication with the common stock being cut off, he retains the addi. tional quantity received. To C, standing on the floor, both appear to be electrised: for he, having only the middle quantity of electrical fire, receives a spark on approaching B, who has an over quantity, but gives one to A, who has an under quantity. If A and B approach to touch each other, the spark is stronger; be cause the difference between them is greater. After such touch, there is no spark between either of them and C, because the elec. trical fire in all is reduced to the original equality. If they touch while electrising the equality is never destroyed, the fire only circulating. Hence have arisen some new terms among us. say, B (and bodies alike circumstanced) is electrised positively; A, negatively; or rather, B is electrised plus, A minus. And we daily in our experiments electrise plus or minus, as we think proper. To electrise plus or minus, no more needs 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 on them ceases, are disposed to give the fire, they have received, to any body that has less. Thus you may circulate it, as Mr. Watson has shown; you may also accumulate or substract it, on or from any body, as you connect that body with the rub. ber, or with the receiver, the communication with the common stock being cut off."
The solution of this gentleman, in relation to this phænomenon, so actually corresponds with that which I offered very early last spring, that I could not help communicating it.
In bodies having the power of readily conducting electricity,
* See my Sequel, p. 64.-Orig.