EARTH CURRENTS 



1905 



EARTHQUAKE 



length of a degree of latitude at 



equator (miles) , 68.704 



Cubic contents of earth 



(cubic miles) 259,911,035,515. 



J.R. 



Consult Reade's The Evolution of Earth Struc- 

 ture; Bonney's The Story of Our Planet. 



Related Subjects. For age of the earth, see 

 GEOLOGY ; for manner of reckoning time, LONGI- 

 TUDE AND TIME ; for relation to other planets, 

 SOLAR SYSTEM and ASTRONOMY. The article 

 GEOGRAPHY with its list of related subjects may 

 also be consulted, and the following topics will 

 be found especially helpful : 



Latitude 



Longitude 



Meridian 



Moon 



Nebular Hypothesis 



Ocean 



Oceanography 



Perigee 



Physical Geography 



Pole 



Sky 



Solstice 



Tides 



Tropics 



Zodiac 



Zone 



Air 



Antarctic Circle 



Apsides 



Arctic Circle 



Axis 



Chart 



Climate 



Degree 



Earth Currents 



Earthquake 



Ecliptic 



Equator 



Equinox 



Globe 



Gravitation 



Hemisphere 



Hydrography 



EARTH CUR 'RENTS. It has been found 

 that telegraph lines in which the circuit is 

 completed by the earth are traversed by natu- 

 ral electric currents which sometimes become 

 strong enough to interfere with the proper 

 working of the line. These currents have been 

 named earth currents. The earth currents are 

 always connected with magnetic disturbances 

 in the earth, and when they are strong they 

 are accompanied by brilliant appearances of 

 the aurora borealis. The view which is now 

 generally accepted by men of science is that 

 both the earth currents and the magnetic dis- 

 turbances are caused by electric currents in 

 the upper regions of the atmosphere, and that 

 these currents are also responsible for the 

 appearance of the aurora borealis. See MAG- 

 NETISM ; AURORA BOREALIS. 



EARTH 'QUAKE, a shaking of the ground 

 caused by disturbances within the earth, is a 

 phenomenon about which there is little scien- 

 tific knowledge. Only within the last three- 

 quarters of a century has there been systematic 

 study of the causes, characteristics and effects 

 of earthquakes. (More about this study, called 

 seismology, will be found in the article SEIS- 

 MOGRAPH.) 



Earthquakes are in some places more fre- 

 quent than storms. In Japan, between 1885 

 and 1892, about three shocks a day were re- 

 120 



corded. Most earthquakes are perceived only 

 with the aid of a detecting and recording instru- 

 ment, the seismograph. When they are violent, 

 the damage is never great unless densely-popu- 

 lated regions are affected ; even then it is often 

 due more to resulting fire or waves from the 

 sea than to the shaking of the earth itself. At 

 San Francisco, in 1906, fire was the chief de- 

 stroyer; at Lisbon, in 1755, the sea. 



An earthquake travels through the ground in 

 waves usually but a very small fraction of 

 an inch in width. Waves with a motion up and 

 down of half an inch, or of an inch to and 

 fro, are destructive. In soft land made by 

 rivers or by man, they grow larger; thus at 

 Valparaiso, Chile, in 1906, steel frame buildings 

 on made land collapsed, while ordinary houses 

 built on rock suffered but slightly. Following 

 the example of Japanese seismologists, engi- 

 neers are learning to construct buildings and 

 bridges capable of withstanding earthquake 

 waves of the usual intensity. One quake in 

 Japan, however, is claimed to have had a wave 

 movement of nearly a foot, which no practical 

 type of building could withstand. Quakes 

 under the sea, of which there are many, pro- 

 duce waves of water too small to wreak dam- 

 age unless they reach shallow water, where 

 they sometimes roll to a height of thirty or 

 forty feet. 



Causes. Most earthquakes are believed the 

 result of changes in the earth's crust, usually 

 the extension of old or the formation of new 

 faults (see FAULT). After the California earth- 

 quake of 1906 it was found that along a pre- 

 viously-known fault nearly 300 miles long the 

 strata had shifted horizontally from seven 

 to twenty-one feet, and vertically from one 

 to three feet. The usual points of origin for 

 the disturbance are estimated to be from ten 

 to twenty miles beneath the surface. A sec- 

 ond class of shocks is ascribed to volcanoes. 

 Its waves travel only a few miles, while those 

 from the first variety sometimes extend 

 around the world. A few minor earthquakes 

 are thought, to be due to the collapse of the 

 underground caves. 



The accompanying map shows that nearly 

 all the great earthquake regions are in volcanic 

 areas. Some scientists infer from this a com- 

 mon origin for the two, namely, the explosion 

 of steam generated in the interior of the earth 

 through the overheating of rocks containing 

 water. But because the majority of the earth- 

 quake regions are those particularly liable to 

 shifting strata lofty ranges, as the Himalayas, 



