That the effects of this fluid bore a great resemblance to those of lightning, had been several times remarked by philosophers and especially by the Abbè Nollet; but that they should be found to be effects of the same cause, and that the phenomena of electricity could be imitated by lightning, or those of lightning by electricity, was not suspected, till our countryman Dr. Franklin made the assertion in 1750, and afterwards demonstrated its truth by undeniable experiment in 1752.
This discovery is almost the only one in the whole science which has not been the result of accident.
The Doctor had for a long time observed the effects of pointed bodies in drawing off the electric matter more powerfully than could be done by others.—Improving upon this, he supposed that pointed iron rods, raised to a considerable height in the air, when the atmosphere was loaded with lightning, might “draw off the matter of the thunder-bolt, without noise or danger.” As he was waiting for the erection of a spire in Philadelphia, that he might have an opportunity of ascertaining the correctness of his hypothesis, it occurred to him, that, by means of a common kite, he could have a readier access to the higher regions of the atmosphere than in any other way. Preparing therefore a large silk handkerchief, and two cross sticks upon which he might easily extend it, he took the opportunity of the first approaching thunder storm to walk into a field, where there was a shed convenient for his purpose; but, dreading the ridicule which too commonly attends unsuccessful attempts in science, he communicated his design to no one but his son, who assisted him in preparing and raising the kite.
A considerable time elapsed before there was any appearance of success: one very considerable cloud had passed over the kite without any effect; when, just as he was beginning to despair, he observed some loose threads of the hempen string to stand erect, and avoid one another just as if they had been suspended from the prime conductor of an electrical machine. On this he presented his knuckle to a key which was fastened to the string, and received a spark. Others succeeded even before the string was wet; but when the rain began to fall he collected the electrical fire very copiously.
He afterwards had an insulated iron rod, to draw lightning into his house, and performed almost every experiment with real lightning, that he had before made with electricity collected by a machine. Thus a new field was opened for the philosophy of electricity.
CHAP. XIV.
Of the structure and use of the electrical kite.
In the structure of an electrical kite, the circumstances to be principally attended to are those near, and on the ground. Silk being a non-conductor, the end of the string which is held in the hand is to be of that substance—a silk handkerchief tied to the hempen twine of the kite will answer very well. An iron key is to be tied on the hempen string, an inch or two above its junction with the silk, and from this key, when the kite is electrified, the sparks are to be received into a Leyden phial, to be used in the same manner as if it had been charged from the electrical machine. As curiosity may prompt many to repeat the experiments made with this kite, and as no experiments with atmospheric electricity can be made without some danger,[[16]] we shall give the substance of Mr. Cavallo’s directions (the best we are acquainted with) relative to the forming and using of this instrument.—He observes that the whole power of the machine lies in the string: and that in other respects a common school boy’s kite, will answer the purpose as well as any other. The string is made by twisting two threads of twine with one of brass wire or copper, such as is commonly used for trimmings. When a kite constructed in this manner was raised, the string always gave signs of electricity except once, when the weather was warm, and the wind so weak that the kite could not be kept up for a few minutes; afterwards, however, when the wind increased, he obtained as usual a considerable quantity of electricity.
Concerning the management of this kite he gives the following directions.—
In raising the kite when the weather is very cloudy and rainy, at which time there is much danger of meeting a great quantity of electricity, I usually hang upon the string a chain with one extremity touching the ground; and sometimes I use another caution besides, which is, to stand upon an insulated stool; in which situation, I think that if any quantity of electricity, suddenly discharged by the clouds, strikes the kite, it cannot much affect my person. Although I have raised my electrical kite a hundred times without any caution whatever, I have very seldom received a few exceedingly slight shocks in my arms. In time of a thunder storm, if the kite has not been raised before, I would not advise a person to attempt it while the stormy clouds are over head, the danger at such time being very great, even when every caution is used. At that time the electricity of the clouds may be observed by means of a cork ball electrometer, placed in an open situation.
But Mr. Cavallo with all his caution could not avoid danger in making experiments on atmospheric electricity, as appears from the following account of his observations on the 13th of October 1773. “After having rained a great deal in the morning and the night before, the weather became a little clear in the afternoon, the clouds appearing separated and pretty well defined; the wind was west and pretty strong; the atmosphere was in a temperate degree of heat. In these circumstances, at three o’clock P. M. I raised my electrical kite, with 360 feet of string. After the end of the string was insulated, and a leather ball coated with tin foil, hung to it, I tried the power and quality of the electricity, which appeared to be positive and pretty strong; in a short time a small cloud passing over, the electricity increased a little; but the cloud being gone it returned pretty soon to its former degree.