EARTH 



1902 



EARTH 



explained by this hypothesis, which, its spon- 

 sors say, overcomes various difficulties of the 

 Laplace system. 



Form of the Earth. Though in popular lan- 

 guage the earth is said to be round, like a ball, 

 it is really an irregular sphere, slightly flattened 

 at the poles. For lack of a better word, geog- 

 raphers call it a geoid; this word is of Greek 

 derivation and means earthlike. The slight 

 departure from rotundity is accounted for by 

 the rapid motion of the earth while in a more 

 plastic state. Because of the flattening at the 

 poles the diameter of the earth from pole to 

 pole through the equator is shorter than that 

 at the equator. The meter, the unit of linear 

 measure in the French, or metric, system, is 

 exactly, one-ten-millionth of the distance from 

 equator to pole. 



The belief of the ancients concerning the 

 shape of the earth is one that prevailed for 

 many centuries before Columbus made his 

 epoch-making voyage in 1492. Ancient people, 

 as do primitive races of to-day, believed that 

 the earth was a flat disk of land, floating on 

 water beneath a huge inverted bowl of sky 

 which met the water at a distance beyond the 

 measurement of man. So persistently was this 

 belief held that the first expedition of Colum- 

 bus was considered by large numbers of people 

 as not only ridiculous but highly dangerous; 

 his vessels, they said, were sure to fall off the 

 earth if they sailed far enough. 



It is not true, however, that no one before 

 the days of Columbus knew that the earth is 

 round. That fact had been proved before the 

 time of Aristotle (d. 322 B.C.), and Aristotle 

 himself figured the circumference of the earth 

 at 40,000 miles. A more accurate estimate was 

 made by Eratosthenes, a Greek of Alexandria, 

 in 200 B.C.; he figured it to be 25,000 miles, 

 which is only 101 miles more than the exact 

 distance. During the Middle Ages, however, 

 the knowledge of the earth's rotundity seems to 

 have been lost altogether ; it was during the ex- 

 ploration era, which began in the closing years 

 of the fifteenth century, that the fact was 

 proved beyond all doubt and became a perma- 

 nently-established principle of geography. The 

 arguments that are advanced the disappear- 

 ance first of the hull of a vessel as it sails away 

 from harbor, the appearance of the top mast 

 first as it approaches harbor, the circular shadow 

 thrown on the moon by the earth in times of 

 eclipse have their culmination in the greatest 

 argument of all, that men have actually sailed 

 around the earth. 



Parts of the Earth. Returning to the divi- 

 sion of the earth into its component parts, the 

 first to be considered is atmosphere, or air. By 

 means of atmosphere all plant and animal life 

 is sustained, for both plants and animals 

 breathe. The atmosphere envelops the globe 

 in the form of an intangible, invisible outer 

 covering. Its depth is not known, but we 

 know that it is at least 100 miles, although at 

 that height it is not at all dense. When peo- 

 ple were first told that the earth is traveling 

 through space at the enormous pace of 66,000 

 miles an hour in its journey round the sun, 

 they believed that if that were true the world 

 would run away from its air. They did not 

 realize that the atmosphere was an integral 

 part of the earth's system, enveloping it com- 

 pletely and traveling with it on its endless 

 journey. 



The atmosphere bears on the surface of the 

 earth with a pressure equal to 14.7 pounds per 

 square inch; that is, a column of air with a 

 base of one square inch reaching as high as 

 the atmosphere extends weighs 14.7 pounds. 

 Its average thickness is believed to be about 

 as stated above, but it is so much denser near 

 the earth that half its bulk is within two and 

 one-half miles of the earth's surface. This is 

 readily proved by those who ascend high 

 mountains; the higher the climber goes the 

 more rarefied and the lighter the air grows, 

 until breathing becomes a matter of difficulty, 

 as there is not enough density or weight in the 

 atmosphere to satisfy the lungs. Aviators who 

 fly at extreme heights suffer greatly from this 

 cause. 



The Solid Crust, or Lithosphere. Under the 

 title lithosphere is to be considered the outer 

 crust of the earth, the shell which encloses the 

 nucleus of the earth. It is from this shell that 

 we derive our means of sustenance, on which 

 we grow crops, from which we extract gold, 

 coal, iron, and other valuable commodities. It 

 is estimated that the crust of the earth con- 

 sists of a mass of solid matter whose volume 

 is approximately 1,677,000,000 cubic miles. 

 Such a mass is entirely beyond human compre- 

 hension, and worked out into tons would be 

 represented by 210 followed by eighteen 

 noughts. The average volume of the earth's 

 crust has a density of 5.5 times that of water. 

 The earth contains a great store of heat which, 

 even in the outer crust, causes an increase in 

 temperature in mines and wells of 1 F. for 

 every sixty feet of descent. The depth to 

 which the crust has been penetrated is c6m- 



