er, advancing in intensity until about two hours after sunset; when it once more begins to abate, growing more and more feeble until sunrise. Thus, during the day, there is a regular fluctuation in the strength of the atmospheric electricity; two periods occurring when its intensity is greatest, and two when it is least. 322. VARIATION IN ALTITude. The electricity of the air increases in strength with the altitude. This is shown by the following experiment, made by Bequerel and Breschet, at the monastery upon the Great St. Bernard. Having extended upon the ground a piece of gummed silk, ten feet long and seven wide, the experimenters placed upon it an electrometer; to this they attached one end of a silk cord, into which was twisted a fine wire, the other end of the cord being fastened to an iron arrow. By means of a bow, the arrow was shot upwards to the height of 250 feet; and as in its ascent the electricity of the air was gradually collected and conveyed along the wire to the electrometer, the straws of the latter were seen to diverge more and more, and at length to strike the sides of the glass jar. When the cord was detached, the electricity of the straws was discovered to be positive. 323. In order to determine whether this increased divergence was really caused by the superior energy of the electricity residing in the higher regions of the atmosphere, the arrow was discharged horizontally to the same distance as before; but, as it speeded on its course, no increased electric action was manifested by the electrometer. 324. Experiments for the same purpose were made by two celebrated French philosophers, Gay Lussac and Biot, during their aërial voyage in 1804. From the car What is said of variation in altitude? Relate the experiment of Bequerel and Breschet. What inference is drawn from both these experiments ? of their balloon was suspended a wire 170 feet long, to the lower end of which a metallic ball was attached; the upper end being connected with an electrometer in the car. By means of this apparatus, these observers were enabled to note the electrical changes occurring in the atmosphere at different heights; and, from their various observations, arrived also at the conclusion, that the electricity of the atmosphere was positive, and increased in strength with the altitude. ORIGIN. 325. EVAPORATION. One of the most abundant sources of atmospherical electricity is evaporation. It was shown by Volta, whose experiments were confirmed by those of Saussure, that electricity was produced when water was evaporated. But it appears from the late researches of Pouillet, that this is only the case when the water is not pure, and chemical decompositions occur. If distilled water is evaporated, no electricity is developed; but if a little chalk, lime, salt, or other foreign matter is dissolved in the water, the rising vapor is then positively electrified, and the vessel containing the fluid negatively. 326. Now the waters of the earth are generally in this latter condition, being seldom pure, and the vapors, which are constantly ascending from the ground, will therefore possess positive electricity, and the earth negative. The briny waves of the ocean also contribute their share, and supply the air with a great amount of positive electricity. 327. The process of evaporation advances invisibly and in silence; and, for this reason, we might easily undervalue its agency in accumulating those vast stores What is the first source of atmospheric electricity? What is said in regard to the experiments of Volta, Saussure and Pouillet? In consequence of evaporation, does the air become positively or negatively electrified? Is the amount of electricity thus developed supposed to be great? of electric matter which arm the storm with such terrific power. But when we reflect, that more than two hundred millions of hogsheads of water are computed to rise daily in vapor from the Mediterranean, we shall find no difficulty in believing, that this influence is one of the most energetic causes of atmospherical electricity. 328. CONDENSATION. Condensation, or the change which vapor undergoes when returning to a fluid state by a decrease of temperature, is another fruitful source of electricity. This is shown from the great amount of electricity occasioned by the condensation of steam, as it issues from the boiler of an engine. In one instance, the steam which rushed from the safety-valve of an insulated locomotive, was found to develop seven times the amount of electricity produced by an electrical machine, having a plate of glass three feet in diameter, and making seventy revolutions in a minute. Machines in which the electricity was generated by steam, have been constructed of such power, that a spark twenty-two inches long has been obtained from the prime conductor, (C. 964,) of sufficient energy to inflame shavings. 329. VEGETATION. The vegetable kingdom also supplies the air with a great amount of electricity. Plants during the day exhale oxygen gas; in the night, carbonic acid gas-and from the experiments of Pouillet it appears that positive electricity rises with the latter when the seeds first sprout, leaving the earth in which they are placed negatively electrified. The same results probably occur during the life of the plant. 330. COMBUSTION. Combustion is still another source of electricity. When any substance is burning, positive electricity escapes from it, while the substance What calculation would lead us to this conclusion? What is the second source? What is condensation ? How is it shown that condensation produces electricity? Illustrate. What is the fourth? During combustion, does the air receive positive electricity or negative? itself is negatively electrified; the atmosphere is therefore the reservoir of all the positive electricity originating in this manner. 331. FRICTION. In accounting for the electricity of the atmosphere, the effect of friction is not to be disregarded. If a piece of silk is shaken in the air, it becomes electrified; and it is highly probable, that when masses of air, moving in contrary directions, encounter each other, electricity is developed by the friction of their surfaces. Such will be the effect, according to Kaemtz, when the masses differ in respect to moisture and temperature; the warmer then becomes positively electrified, and the colder negatively. The action of the wind upon terrestrial objects, as rocks, buildings, trees, and hills, may possibly in like manner produce electricity. CHAPTER II. OF THUNDER-STORMS. 332. GENERAL DISTRIBUTION. Thunder-storms prevail most in the torrid zone, and decrease in frequency towards either pole. During a residence of six years in Greenland, 70° N. Lat., Gisecke heard the rolling of thunder but once; and, according to the testimony of the arctic navigators, Scoresby, Parry and others, thunder-storms rarely occur between the 70th and 75th degree of north latitude; and never beyond the latter parallel. As respects time, they are more frequent during the summer months. What is the effect of friction? If two bodies of air differ in temperature, in what manner will the electricity, developed by their friction, be distributed? What may possibly be the effect of the friction caused by wind? Of what does chapter second treat? How are they distributed in regard to latitude and time? The prevalence of these laws will be seen from the observations contained in the following table. 333. Thunder-storms are most violent within the torrid zone. Here the play of the lightning is incessant, and the crashing bursts of thunder most terrific ; and none but those who have actually witnessed a tropical tempest, can form an idea of its awful power. Occasionally, in the higher latitudes, fierce storms occur, like that which was seen by Simpson in the Straits of Boothia. (Art. 294.) 334. ORIGIN. The thunder-storm is produced in the same manner as the common rain-storm; namely, by the condensation of atmospheric vapor; but it differs in two respects; first, in the rapidity of this condensation, and secondly, in the accumulation of electricity resulting therefrom. 335. We have seen that when vapor is condensed, electricity is developed (Art 328): the cloud then in the very process of formation becomes electrified, and to its own electricity is added that which collects upon its surface from the aimiosphere; whether derived from evaporation, combustion, vegetation, friction, or any other source. This condensation must be copious, or the electricity would be weak; it must also be rapid, else it will es Repeat the table. Where are thunder-storms most violent? How does the thunder-storm differ from the common rain-storm? Why must the condensation be both copious and rapid? |