310 ATMOSPHERE IN RELATION TO HUMAN LIFE AND HEALTH. 



stoppage is nonhomogeneity of atmosphere, or aerial reflection by a 

 number of currents, columns, or laininse of different density. On one 

 day guns and sirens were beard at 10^ miles; two days later were 

 inaudible at 3 miles. Water in the state of vapor mixed with air, 

 in nonbomogeneous parcels, acts powerfully in wasting sounds. Not 

 only clouds, but layers of transparent air, may produce echoes both 

 intense and long. Tbe 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 

 effect in reflecting sound and in heightening its pitch. Let any passen- 

 ger by railway note the marked rise of pitch as the train passes 

 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, penetration, 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 efficiency 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 AUSTRALIS. 



Tbe 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. Tbe 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 aurorse 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 frequency. 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. 



