Page:Popular Science Monthly Volume 31.djvu/881

Rh photography. When we come to the other end of the spectrum, the yellow and red, or more refrangible rays, we find that they promote the formation of chlorophyl and so turn vegetation exposed to them green, favoring the growth of green plants. In this we may discover one of the purposes which chlorophyl fills—as that of a special coloring-matter to the plant to filter out the more injurious rays and protect the delicate protoplasmic cell-contents from their destructive action.

Healthy and Unhealthy Occupations.—The English Registrar-General has made a comparison between healthy and unhealthy occupations. Assuming the normal average death-rate of the community as the unit of comparison, and calling it 1,000, particular occupations may be regarded as healthy or unhealthy according as the death-rates among those pursuing them fall above or below that figure. The most healthy occupation appears to be that of ministers of religion, whose rate is 556. Next are gardeners and nurserymen, 599; farmers and graziers, 631; agricultural laborers, 701; schoolmasters, 719; and grocers, coal-merchants, paper, lace, and hosiery manufacturers, wheelwrights, ship-builders and coal-miners, with all of whom the average death-rate is under 775. The most unhealthy occupations are the trades connected with the liquor-traffic and hotel service, with which the death-rate is 2,205; following these are general laborers in London, 2,020; costermongers, bankers, and street sellers, 1,879; innkeepers, etc., 1,521; and brewers, 1,361. After the trades concerned with alcohol, the highest rates are furnished by occupations that involve the breathing of dust other than coal-dust and exposure to lead-poisoning. The death-rate among butchers is also high, 1,170.

Cause of Thunder.—M. Him explains thunder and the explosive noise of meteorites by observing that the air traversed by an electric spark—that is, a flash of lightning—is suddenly raised to a very high temperature, and has its volume considerably increased. The column of gas thus suddenly heated and expanded is sometimes several miles long; as the duration of the flash is not even a millionth of a second, it follows that the noise bursts forth at once from the whole column; but for an observer in any given place, it begins when the lightning is at the least distance. In precise terms, the beginning of the thunder-clap gives us the minimum distance of the lightning, and its duration the length of the column. The author points out that a bullet whistles in traversing the air, so that we can to a certain extent follow its flight; the same thing happens with a falling meteorite just before striking the earth. The noise actually heard has been compared to the flight of wild geese, or to the sound produced when one tears linen; it is due to the fact that the air, rapidly pushed on one side in front of the projectile, whether bullet or meteorite, quickly rushes back to fill the gap left in the rear. The velocity of the meteorite is so great that the matter on its surface will be torn away by the violence of the gaseous friction produced, and will be vaporized at the same time by the heat. This is undoubtedly the origin of the smoke which meteorites leave trailing behind them. With this velocity the sound following the meteorite is vastly deeper and more like thunder than that which attends the passage of the relatively slow-going bullet.

Prehistoric Chronology of America.—Dr. D. G. Brinton, Vice-President of the Anthropological Section of the American Association, gave there a "Review of the Data for the Study of the Prehistoric Chronology of America." The resemblances between American legends and Oriental myths were considered accidental. The annals of the Mexicans, the Mayas of Yucatan, and the Quichas of Peru, carry us back hardly more than five hundred years. The recollections of the more savage tribes did not extend back more than two centuries. A calm weighing of the testimony respecting the stone buildings of Mexico, Yucatan, and Peru, places them all well within our own era, and most of them within a few centuries of the discovery. The much more ancient artificial shell-heaps along the coasts furnish data to prove that the land was inhabited several thousand years ago. The industrial activity of man in America may be traced by the remains of his weapons, ornaments, and tools,