Page:The American Cyclopædia (1879) Volume X.djvu/470

 464 LIGHTNING the earth, such vapors are sent out by stacks of hay, corn, or other vegetables which heat and reek. . . . Now if the fire of electricity and that of lightning be the same, as I have endeavored to show in a former paper, and a tube of only 10 ft. long will discharge its fire at two or three inches distance, an electrified cloud of perhaps 10,000 acres may strike and discharge on the earth at a proportionally greater distance. ... I say if these things are so [speaking of the discharging power of points], may not the knowledge of this power of points be of use to mankind in preserving houses, churches, ships, &c., from the stroke of lightning, by directing us to fix on the highest parts of those edifices upright rods of iron made sharp as a needle, and gilt to pre- vent rusting, and from the foot of the rods a wire down the outside of the building into the ground, or down round one of the shrouds of a ship, and down her side till it reaches the water ? Would not the pointed rods probably draw the electric fire silently out of the cloud before it came near enough to strike, and thereby secure us from the most sudden and terrible mis- chief ? " It was not till three years afterward that Franklin actually made the experiment of drawing electricity from the clouds, and de- monstrating the identity of atmospheric light- ning and frictional electricity. He had pro- posed various experiments, such as the erection of tall rods on the tops of spires. Dalibard in France, acting according to the instructions of Franklin, on May 10, 1752, obtained electrical sparks from an iron rod 40 ft. high in the garden at Marly, and charged Leyden jars from the same source. Franklin did not make his experiment with the kite till the 15th of June of the same year. These experiments were regarded with the highest interest by scien- tific men, and were repeated with various modi- fications in different parts of Europe. Prof. Richman of St. Petersburg, July 26 (Aug. 6), 1753, while explaining to a companion the construction of an electrometer attached to his conductor, was struck and instantly killed by what appeared to be a ball of blue fire as large as a man's fist, that was seen to leap from the insulated conductor to his head, a space of about a foot. A red mark was left on his forehead, his shoe was burst open, and his clothing slightly singed. His com- panion was benumbed and rendered senseless, and the door case and door were torn apart by the shock. M. Romas, to whom the French academy of sciences awarded the merit of inventing the electrical kite more than a year before it was employed by Franklin, construct- ed a kite 7 ft. 5 in. high, and 3 ft. in its great- est width, with a surface of 18 sq. ft. A cop- per wire was wrapped around the string to in- crease its conducting power, and this was made to terminate in an insulating silk cord, near which an iron tube was placed to receive the electricity. The kite being raised to a height of 550 ft. on the approach of a storm, the iron conductor became so highly charged that elec- trical sparks were obtained, and shocks of great violence. As the storm increased, flashes of fire darted to the earth accompanied with explosions, and straws that happened to be on the ground were attracted alternately by the string and the ground, their movements being accompanied by electrical flashes and constant explosions. Such were the experiments by which the electrical nature of lightning was established, and the thunder proved to be the noise which accompanies the electrical dis- charge. This sound may be prolonged as it is reflected in echoes by the clouds ; or, as suggest- ed by Sir John Herschel, it may come in succes- sive impulses to the ear, as brought from an instantaneous discharge that extends for miles along a line directed away from the observer. So the terrific sudden crash may be the result of a flash occurring all round the observer with no great difference of distance from him in the points of the discharge. Not only was the electrical condition of the atmosphere du- ring thunder storms thus established, but in 1753 the abb6 Mazeas, by means of a wire 370 ft. long attached to a steeple at Maintenon, proved that electrical action is excited in clear, dry, and especially hot weather, at all hours between sunrise and sunset. From a multitude of observations made by Cavallo, Read, De Saussure, and others, it appears that the atmos- phere is almost always positively electrified in relation to the surface of the earth, and the higher the stratum of air the more decidedly positive is its electrical condition. The source of atmospheric electricity is traced by Lavoi- sier, Laplace, Volta, and De Saussure to evap- oration from the surface of the earth, the effect of which is to convey one kind of electricity upward with the vapor, leaving the other with the fluid. But, as shown by Pouillet in 1823, this effect does not take place unless the evap- oration is accompanied with chemical decom- position, as when it occurs from saline mix- tures, from the surface of heated iron, which becomes oxidized, and more especially when the vapor proceeds from the leaves of growing plants. Combustion also is a source of atmos- pheric electricity, as is seen upon a large scale in the constant flashes of lightning that some- times play around the summits of volcanoes during their eruptions. The rushing of cur- rents of wind past each other, or against op- posing objects, also generates electricity by the friction it occasions. The descent of the rain drops develops negative electricity in the air, and the same effect is observed in the vi- cinity of waterfalls, the air for several hun- dred feet distant being filled with negative electricity. To this cause is probably to be at- tributed the highly excited condition of the atmosphere during thunder storms, and the fre- quent alternations then observed of positive and negative indications. However the elec- trical condition of the clouds is produced, the surface beneath assumes the opposite electrical