THE EARTH AS IT IS. 79 



CHAPTER IX. 



THE EARTH AS IT IS. 



156. The Earth as a "Whole. The earth is a ball near- 

 ly round, composed of solid, liquid, and gaseous substan- 

 ces, the gaseous occupying the interstices of both solids 

 and liquids, there being also a gaseous envelope around 

 the earth of about fifty miles in thickness. The most 

 abundant liquid is water, which fills up all the cavities 

 on the earth's surface. If the solid part of the earth 

 were uniform instead of having its present diversified 

 condition, it would every where be covered with water, 

 the atmosphere remaining, as now, outside of the liquid 

 envelope. The earth, as a whole, has been found, by vari- 

 ous observations and calculations, to be between five and 

 six times the weight of water, and about two and a half 

 times as heavy as the average of common rocks. This 

 great weight of the earth is owing to the pressure to 

 which its internal parts are subjected from the influence 

 of gravitation. Every thing is attracted toward the 

 centre, and therefore any internal portion of the globe is 

 pressed toward this centre by the weight of all which is 

 outside of it. It is just as the lower portions of the air 

 that is, those which are close to the earth are con- 

 densed by the pressure of those portipns outside of them 

 through gravitation, as illustrated in 152, Part I. It 

 has been calculated that at the depth of 34 miles in the 

 earth air would be so condensed that it would be as 

 heavy as water, and at the depth of 362 miles water 

 would be so condensed that it would be as heavy as 

 mercury, although here upon the surface of the earth 

 water is so little compressed by the strongest pressure 

 man can bring to bear upon it that it is regarded practi- 



