electricity are thereby established in the latter direction. These give rise to the magnetism of the earth, the directive power of which is of necessity at right angles with the former. 240. The earth may therefore be viewed, first, as one huge electrical machine, of which the direction of the currents is from east to west; and, secondly, as a vast magnet, of which the poles are beneath the surface, and of which the directive force is nearly parallel with its axis, or from north to south. 241. Electricity of the atmosphere.—The atmosphere abounds with electricity, which it derives from the thermo-electric agency of the earth. 242. Electricity, from the universality of its action, doubtless plays an all-important part in the sustentation of health, the germination of plants, the production of ozone, the formation of clouds and rain, snow and hail, in the thunder-storm, the tempest, and the whirlwind. 243. The sources of electricity in the atmosphere are to be found in the chemical changes which often accompany the evaporation of water and the respiration of plants, and in combustion. 244. Dr. Prout is of opinion that one source of electricity of the atmosphere is derived from a combination of water and oxygen, a compound analogous to the deutoxide of hydrogen (ozone), which he supposes to be a frequent, if not a constant, ingredient in the atmosphere, and the cause of numerous atmospheric phenomena which are at present but little understood. 245. The electricity of the air increases continually in proportion as we get further from the surface of the earth; but at a certain height it becomes uniformly constant. 246. The effects of electricity are much more striking, as connected with the water in the atmosphere, than with the constituents of the atmosphere itself. 244. Bridgewater Treatise, p. 342, and Appendix, p. 569. 245. De Saussure, op. cit. tom. ii. p. 413, § 1035. 247. Dry and pure air is one of the most complete non-conductors of electricity. 248. In clear, calm, and unclouded weather, the electricity is generally of the positive kind, but upon the first appearance of rain, snow, or hail, it usually changes to negative. That of the earth is invariably negative. 249. The usual positive electricity is weakest during the night, and at its minimum at 3 A.M.; it increases with sun-rise, decreases about noon, increases again towards sun-set, and diminishes and remains feeble during the night. 250. The atmosphere is least electrical during the prevalence of N.N.W. and N. winds, and N.N.E. and N.E. winds; and most electrical when the wind blows from E.S.E. to S.E., and from W.N.W. to N.W., facts which have been recorded by M. Quételet, Astronomer Royal of Belgium. 251. The same learned professor, in a communication to the Société Royale de Belge, 8 Sept. 1849, stated, that atmospheric electricity is in its highest intensity during the month of January, and attains to its minimum in June. The value of the months is in the relation of 32 to 1 when the sky is perfectly serene, and of 8 to 1 when it is clouded. 252. The distribution of electricity decreases from the equator to the poles. 253. Electrical phenomena are most energetic and of most frequent occurrence in countries and in seasons in which the solar influence is the greatest. 254. The electrical powers follow the law of gravitation in being in the inverse ratio of the squares of the distances of the acting bodies. 255. The prevalence of epidemic or pestilential disease has been associated with the absence or deficiency of positive elec 247. Prout, op. cit. p. 339. 249. Ibid. p. 190. 252. Prout, op. cit. p. 340. tricity in the atmosphere; and the mortality has been found to be in the inverse ratio of the amount of positive electricity with which the air is charged. 256. This was especially remarked during the last visitation of the Asiatic cholera. 257. In the "non-electric" states of the air, or when the electricity is "weak" or "nothing," diseases of a low type prevail, and the mortality increases. On the contrary, when the electricity is "positive," strongly positive," and "active throughout the day," the number of deaths decreases. 258. Thunder and lightning. The electrical equilibrium of the atmosphere is disturbed by thunder-storms. 259. Lightning is caused by the passage of electricity between one cloud and another, or between a cloud and the earth. Thunder is the noise produced by such passage. 260. In England, France, and Germany, the mean annual number of days of thunder and lightning rarely amounts to twenty. At Rio Janeiro, and at places in India, it is above fifty. In the equatorial regions on all days of the year, and at all hours of the day, electrical discharges are continually taking place in the atmosphere. 261. In Norway it rarely thunders in Iceland, once only in two years: in Peru, never. 262. Above the 70° N. lat. we are beyond the regions of thunder-storms. 263. Lightning is of three kinds, zig-zag or forked; sheet lightning, which illuminates whole clouds, and that in the form of fire-balls. The duration of the two first is scarcely 1000 part of a second; of the globular kind several seconds. 255. Glaisher, Meteorology of London. 257. Hingeston, op. cit. 259. Glaisher, op. cit. p. 45. 260. Arago, op. cit. p. 111. Boussingault. 261, 262, 263. Kosmos. 264. The vertical height of storm-clouds varies considerably (193). The greatest elevation, with one exception, which has been recorded is 26,510 ft. = 5.02 miles. 265. At the temperature of 62° Fah. the velocity of sound has been determined by Sir John Herschel to be 1125 feet per second = 12.784 miles per minute. Every increase or decrease of temperature of 1° of Fahrenheit's scale occasions a corresponding increase or decrease of 1.14 foot per second in the velocity of sound. At 32° Fah. sound travels at the rate of 1090.8 feet per second. 266. De l'Isle once counted 72 seconds between the lightning and the thunder. Assuming the temperature of the air to have been only 45° Fah., the velocity of sound would be 1105 62 feet in every second of time. Seventy-two seconds multiplied by 1105.62 feet, gives 14.07 miles as the distance of the cloud in which the lightning appeared. 267. Lightning sometimes destroys altogether the magnetism of the needle of a compass, and occasionally inverts its magnetic poles. 268. Lightning is never more dangerous than in winter. 269. Thunder-storms occur less frequently in winter than in summer, and at sea than on land. 270. Mr. Lewis Weston Dillwyn maintains that thunderstorms are more frequent and severe in limestone countries than others. 271. It is said that lightning never strikes the northern face of buildings; and that the south-east is the aspect most exposed to danger. 264. Arago, op. cit. 265. Encyclopædia Metropolitana, article "Sound." 267. Arago, op. cit. p. 88. 268. Ibid. p. 137. 269. Ibid. p. 126. 270. Howard, op. cit. 271. Arago, p. 198. Each ray of light consists of three separate rays, a ray of heat, a ray of light, and a ray of actinism (from ảκтìv, a ray). Mr. R. Hunt, to whom we are indebted for the discovery of the actinic ray, has shown that this is necessary to germination and vegetation. In spring, during germination, and the budding of trees and plants, the solar beams abound with these rays; in summer, during the formation of the woody portions of plants, the light-giving rays prevail; and in autumn, the calorific, heat-giving, or ripening principle of the solar ray is increased. Yellow glass is impervious to the actinic ray. 285. It has been calculated that a vertical ray of light in its passage through the clearest air loses at least a fifth part of its intensity before it reaches the earth's surface; and that, of a thousand rays emanating from the sun, only 378, on an average, can penetrate to the surface of the earth at the equator; 228 at the latitude of 45°; and 110 at the poles. 286. Euler has calculated the light emitted by the sun to be equal to that of 6560 candles at one foot distance. Leslie considers it 12,000 times more powerful than that of a wax candle; in other words, if a portion of the luminous solar matter rather less than half an inch in diameter, were transferred to our planet, it would throw forth a blaze of light equal to the effect of twelve thousand wax candles. Humboldt, on the other hand, defines the sun to be a dark, black mass. It is now generally admitted that the centre of our system is an opaque body surrounded by a luminous envelope. Arago has proved beyond all doubt that the light of the sun emanates, not from an incandescent solid body, but from a gaseous atmosphere; the light of the sun, like that emitted by gaseous 284. Kosmos, vol. i. p. 163. 285. Encyclopædia Britannica, article "Climate." Leslie, op. cit. p. 61. |