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rod held in the other? 2dly, Whether that commotion was perceptible, if the observer at E, being in all respects as before, the observer at D, standing on wax, took his rod out of the water? 3dly, Whether that commotion was perceptible to both observers, if the observer at D was placed on wax, and touched the ground with his iron rod in a dry gravelly field at least 300 yards from the water?
To try the first proposition, several explosions were made with the observers at D and E, touching the water, and standing on wax, with their iron rods in the water; when the observers at both stations felt the electrical shock.
To try the second proposition, four explosions were made with the observers at D standing on an originally.electric, and taking his iron rod out of the water, the observer at E as before. In each of these the observer at D felt a small pulsation between his finger and thumb of that hand, which held the wire. The observer at E felt each of these as strong as before. The four other explo. sions were made without any other alteration in the apparatus, than that the observer at D stood on the ground about four yards from the water without any communication with it. The obser ver at E felt the shocks in his arms as before; but the observer at D standing on the ground was shocked in the elbow and wrist of that arm which held the wire, and in both his ancles.
To try the third proposition, eight explosions were made with the observer at D standing on an originally-electric with his rod in the water of the river as before; but the observer at E was placed in a dry gravelly field about three hundred yards nearer the machine than his last station, and about one hundred yards distant from the river. He there stood on the wax, holding the conducting wire in one hand, and touched the ground with an iron rod held in the other. The shock was each time felt by the observer at D, but sensibly weaker than in the former trials; but the observer at E felt them all equally strong with the former; the first four in his arms, when he stood on the wax, and touched the ground with his iron rod; the other four in his arm and ancles, when he stood on the ground without the iron rod.
By the experiments of this day, the gentlemen were satisfied that the dry gravelly ground conducted the electricity as strongly as water, which, though otherwise at first conjectured, they now
found not to be necessary to convey that power to great distances; as well as that, from difference of distance only, the force of the electrical commotion was very little if at all impaired.
In one instance the circuit was formed from the phial by the ob server at D and his wire, a line of ground which reached from the station at D to the broken wire that lay on the ground, and so much of this wire as reached to the short iron rod, which touched the gun-barrel in making the explosions. This induced the gen. tlemen to conclude, as from many experiments it was manifest, that when the intervening substances conduct electricity equally well, the circuit was performed in the shortest way possible; that when the observers holding their iron rods in the river at D and E were both shocked, the electricity was not conveyed by the water of the river, being two miles in length, but by land, where the distance was only one mile; in which space that power must necessarily pass over the New River twice, through several gravel-pits, and a large stubble-field. So that admitting the electricity did not fol low the track of the river, the circuit from D to E was at least two miles, viz. somewhat more than one mile of wire, which conducted the electricity from the house to the stations, and another mile of ground, the shortest distance between those stations. The same inference was now drawn with regard to the experiments at A, B, and C, in the New River before recited, viz. that as in all of them the distance between the observers was much greater by water than by land, the electricity passed by land from one observer to the other, and not by water.
From the shocks which the gentlemen received in their bodies, when the electrical power was conducted by dry sticks, they were of opinion, that from difference of distance simply considered, as far as they had yet experienced, the force was very little if at all impaired. When they stood on originally-electrics, and touched the water or ground with an iron rod, the electrical commotion was always felt in their arms and wrists: when they stood on the ground, and touched either the water or ground with their iron rods, they felt the shock in their elbows, wrists, and ancles; when they stood on the ground without the rod, the shock was always in the elbow and wrist of that hand which held the conducting wire, and in both ancles. The observers here being sensible of the electrical commotion in different parts of their bodies, was ow.
ing in the first instance to the whole of it passing, because the observer stood on wax, through their arms, and through the iron rod; in the second, when they stood on the ground, the electricity passed both through their legs, and through the iron; in the third, when they stood on the ground without either wax or rod, the electricity directed its way through one arm, and through both legs to complete the circuit.
The gentlemen were desirous of closing the present inquiry, by examining not only whether the electrical commotions were perceptible at double the distance of the last experiments in ground perfectly dry, and where no water was near; but also, if possible, to distinguish the respective velocities of electricity and sound. To execute this, required their whole sagacity and address; for they had met with very great difficulties in the last day's operations, where the wire was conducted but little more than a mile; all which could not but be greatly augmented by doubling that dis. tance; because it was necessary, that the house, in which the electrifying machine was placed, should be visible at least at one of the stations; and that the space between that house and the sta tions, through which the wire was conducted, should be very little intersected by hedges, roads, or foot-paths; neither should the wire in this space be subject to be disturbed by the horses or cattle, which were grazing; nor ought to touch in its passage the trees, or any other vegetables, which at this season of the year were every where luxuriant. To find a place within a convenient dis tance of London with these requisites was not very easy; but at last Shooter's Hill was pitched on, as the most convenient. As only one shower of rain had fallen during the preceding five weeks, the ground could not but be very dry; and as no water was near, if the electrical commotion was felt by the observers at the stations, it might be safely concluded, that water had no share in conduct. ing it.
Accordingly, August 14, 1747, they met at Shooter's Hill for this purpose. It was here determined to make twelve explosions of the coated phial, with an observer placed at the seven-mile stone, and another at the nine-mile stone, both standing on wax, and touching the ground with an iron rod. This number of explo. sions was thought the more necessary, as the observers at these stations were not only to examine whether the electricity would be
propagated to so great a distance, but if it were, the observer at the seven-mile stone was by a second watch to take notice of the time lapsed between feeling the electrical commotion, and hearing the report of a gun fired near the machine, as close as might be to the instant of making the explosion; and therefore, to examine this matter with the requisite exactness, this number of explosions should be made.
To execute this, the electrifying machine was placed up one pair of stairs in a house on the west side of Shooter's Hill, and a wire from a short iron rod, with which the gun-barrel was touched in making the explosions, was conducted on dry sticks as before into a field near the seven-mile stone. The length of this wire, exclusive of its turnings, round the sticks, was a mile, a quarter, and eight poles, or 6732 feet. In great part of this space it was found very difficult to support the wire, on account of our scarcely being able to fix the sticks in the strong gravel there almost without any cover of soil; nor could the wire in some places be prevented from touching the brambles and bushes, nor in one field the ripe barley.
Another wire was likewise conducted on sticks from the coated phial to the nine-mile stone. In this space, the soil being a strong clay, the wire was very well secured, and in its whole length did not touch the bushes. The length of this wire was 3868 feet. As much as the place, where the observers were stationed in a corn. field, was nearer the machine than the seven-mile stone, so much were the other observers placed beyond the nine-mile stone, that their distance from each other might be two miles. The 40 feet of wire in these two measures exceeding two miles, was what connected the short iron rod beforementioned, and the coated phial, with their respective conducting wires.
The observers being placed at their respective stations, the observer at the machine proceeded in making the explosions of the coated phial; by which the observers at the nine-mile stone were very strongly shocked; and they were also felt at the seven-mile stone.
This demonstrated that the circuit here formed by the electricity was four miles, viz. two miles of wire, and two miles of ground, the space between the extremities of that wire. A distance without trial too great to be credited! how much farther the electrical commotion will be perceptible, future observations can only determine.
The electrical commotion by the observers near the seven mile stone was but slightly felt; nor could it be otherwise expected, the wire in many parts of its length touching, as was beforemen. tioned, the moist vegetables; which, in as many places as they were touched, formed subordinate circuits. We find, in all other instances, that the whole quantity of electricity, accumulated in the coated phial, is felt equally through the whole circuit, when every part is in a great degree non-electric; so here the whole quantity, or nearly so, determined that way, was felt by the observers at the nine-mile stone; while those at the other station felt so much of their quantity only, as did not go through the vegetables; that is, that proportion only in which iron is a greater non-electric than the vegetables.
Though the electrical commotions, felt by the observers near the seven-mile stone, were not strong; they were equally conclu. sive in showing the difference between the respective velocities of electricity and sound. The space through which sound is propa. gated in a given time, has been very differently estimated by the authors who have written on this subject. Roberval gives it at the rate of 560 feet in a second; Gassendus at 1473; Mersenne at 1474; Du Hamel, in the History of the Academy of Sciences at Paris, at 1172; the Academy del Cimento at 1185; Boyle at 1200; Roberts at 1300; Walker at 1338; Sir Isaac Newton at 968; Dr. Derham, in whose measure Mr. Flamstead and Dr. Halley acquiesced, at 1142. But by the accounts since published by M.Cassini de Thury in the Memoirs of the Royal Academy of Sciences at Paris for the year 1738, where cannon were fired at various as well as great distances, under great variety of weather, wind, and other circumstances, and where the measures of the different places had been settled with the utmost exactness, sound was propagated at a medium at the rate only of 1038 French feet in a second. The French foot exceeds the English by 7 lines, or is as 107 to 114; and consequently 1038 French feet are equal to 1106 English feet. The difference therefore of the measures of Dr. Derham and M. Cassini is 34 French or 36 English feet in a second. According to the last measure, the velocity of sound,
M. Cassini de Thury afterwards measured the velocity of sound at Aigue mortes in Languedoc, and found the observations there from those made about Paris vary only half a toise in a second. See Mem. de l'Acad. Royale des Sciences, pour l'anné 1739, p. 126.-Orig.