Licence: Public Domain Mark
Credit: The atmosphere in relation to human life and health. Source: Wellcome Collection.
Provider: This material has been provided by the Augustus C. Long Health Sciences Library at Columbia University and Columbia University Libraries/Information Services, through the Medical Heritage Library. The original may be consulted at the the Augustus C. Long Health Sciences Library at Columbia University and Columbia University.
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![stoppage is nonliomogeneity of atmosphere, or aerial reflection by a number of currents, columns, or laminae of clilierent density. On one day guns and sirens were heard at 10^ miles; two days later were inaudible at 3 miles. Water in the state of vapor mixed with air, in nonhomogeneous parcels, acts powerfully in wasting sounds. ]Sot only clouds, but layers of transparent air, may produce echoes both intense and long. The power of the particles of cloud to produce aubible echoes has been doubted by Tyndall; but we may observe that a grove of trees in leaf, even of larches and pines, has a very strong eflect in reflecting sound and in heightening its pitch. Let any passen- ger by railway note the marked rise of pitch as the train i)asses between woods of beech or oak. The sound resembles that of a small cascade, or of wind among rustling leaves. The blasts of the fog siren have hitherto been found to be most effectual of all sounds tried for prolongation, x^enetration, and small cost. Its audibility is good at a range of 2 miles under all conditions. Experiments are still needed in order to attain a higher eiflciency in sound propagation for maritime and other purposes, and to ascertain the effect of air in various conditions. The transmission and collection of sound through a few miles by means of suitable exciters, polished funnels, and acoustic mirrors of large size has not been developed as it might be. ,' AURORA BOREALIS AND AUSTEALIS. The aurora borealis or australis is very far from being understood. The height of the luminous arch has been variously estimated and calculated as between 33 and 281 miles, and no doubt greatly varies in different latitudes and in different displays. The greatest height estimated was 500 miles. But in high latitudes the aurora has been observed to emerge from the tops of hills and even as a rule from the ocean, but not from ice floes. Loomis has given much information concerning the distribution of the aurora over the globe in the Smith- sonian Eeport for 1865. Near latitude 40 in the United States only 10 aurorse, on an average, are seen annually. Near latitude 42, about 20; near 45, about 40; and near 50, about 80 are seen. Between lati- tude 50 and 62 aurorae are seen almost every night, as often to the south as to the north. Farther north they are seldom seen except in the south, and from this point northward they diminish in brilliancy and fre<(uency. Near latitude 78 the number is reduced to 10 annu- ally. In the meridian of St. Petersburg the region of 80 auroras is found between 66° and 75°. The region of greatest auroral action is a zone of oval form encircling the North Pole. This zone resembles a line everywhere perpendicular to a magnetic meridian. In Europe aurorse are much rarer than in North America. Some auroral dis- plays, such as the remarkable one of March 30, 1894, are visible both in Europe and America. It seems that an exhibition around one mag- netic pole is often simultaneous with a similar exhibition around the other magnetic pole of the earth.](https://iiif.wellcomecollection.org/image/b21208724_0122.jp2/full/800%2C/0/default.jpg)
