EARTH 



1903 



EARTH 



paratively slight, the deepest mine in the 

 world not exceeding 6,000 feet, a little over a 

 mile. This depth, however, is sufficient to 

 raise the temperature almost beyond the point 

 of human endurance. Scientific ventilation 

 counteracts the increase in heat and renders it 

 safe for men to work even at such a depth. 



The Oceans, or Hydrosphere. About three- 

 fourths of the earth's surface is covered by 

 water oceans and seas. The average depth of 

 the oceans is about two miles; thus the vol- 

 ume of the oceans is 315,000,000 cubic miles. 

 This is about l /soo part of the entire volume 

 of the earth. The earth's crust, covering the 

 earth to a depth of several miles, and of greater 

 density, has a mass of about fourteen times 

 that of the oceans. 



The Oceans, or Hydrosphere. About three- 

 what the center, or nucleus, of the earth is com- 

 posed of, opinions vary ; scientists have brought 

 forth many theories. One thing clearly estab- 

 lished is that the density of the interior, or 

 core, of the earth is greater than that of the 

 earth's crust which rests on it. Some scientists 

 maintain that the center of the earth is a 

 molten mass, while others claim that this can- 

 not be true, as the pressure of the crust on the 

 nucleus, or center, is so great that heat far 

 above the ordinary melting point can be 

 resisted under such pressure. Increase the 

 pressure on iron and the temperature of the 

 melting point rises; thus, it is argued, the in- 

 terior of the earth may hold sufficient heat to 

 melt any metal at the surface, but not suffi- 

 cient to melt it under the pressure of countless 

 millions of tons of the earth's crust. An emi- 

 nent English scientist maintains that the earth 

 is solid throughout, rigid, but containing gas- 

 eous outlets or flaws here and there which, by 

 reducing pressure, permit volcanic escapes and 

 melting of matter which under pressure has re- 

 mained solid. From the amount of pressure 

 it is probable that such flaws cannot exist at a 

 depth of more than a few miles; so the theory 

 of an entirely solid nucleus finds wide accept- 

 ance among scientists. 



The heat from the center of the earth con- 

 tinually escapes through the crust in appre- 

 ciable quantities. It is estimated that the 

 escaping heat would be sufficient to melt a 

 coating of one-fourth inch of ice all over the 

 world, or 800 cubic miles of ice every year. 

 That the earth is shrinking as this heat escapes 

 is a theory advanced by many scientists, but 

 it would take thousands ef millions of years 

 to shrink to any appreciable extent. 



Weight of the Earth. Archimedes, the fa- 

 mous Greek mathematician, was so proud of 

 his invention of the lever that he exclaimed, 

 "Give me a place to stand on, and I will lift the 

 earth." At first thought, the task of the man 

 who would undertake to weigh the earth 

 would appear to be hopeless. Yet the earth 

 has been weighed, not by putting it in the 

 scales, but by scientific calculations which are 

 equally certain in their results. The first cal- 

 culations of the earth's weight were made by 

 observing how much the attraction of a moun- 

 tain caused a plummet to vary. The mountain 

 could be measured and the weight calculated. 

 From the proportion of the mass of the moun- 

 tain to the earth, the weight of the earth was 

 calculated, the result being expressed in. figures 

 absolutely beyond human comprehension. The 

 figures in the table at the end of this article 

 accord with scientific belief. 



Motions of the Earth. The Annual Revolu- 

 tion. In common with all planets belonging 

 to the solar system the earth moves round the 

 sun and rotates on its o\vn axis at the same 

 time. Its revolution round the sun is coin- 



Orbit of the earth and of the three other "Inner 

 planets." 



pleted in about 365V4 days, our common year; 

 the precise time is 365 days, 9 hours, 9 min- 

 utes, 9.3 seconds, this time being known as the 

 sidereal year. The path of the earth being an 

 ellipse, with the sun as one of its foci (see 

 Focus), the globe is not equally distant from 

 the sun at all times of the year. Its least dis- 

 tance from the sun is 89397,000 miles ; its great- 

 est distance, 92,963,000, generally expressed in 

 round numbers as about 93 millions of miles. 

 The earth travels around the sun at a speed 



