DENSITY OF THE EARTH. 



3 



lorial diameter of the earth exceeds its polar about twenty-seven 

 miles; the length of the equatorial diameter being 7927, that of the 

 polar 7900 miles. 



The relative density of the sun, the earth, and of the other planets, 

 is estimated by the attractive force which they exert on each other, 

 as they move round their common center of gravity. The absolute 

 density or the quantity of matter contained in the earth, compared 

 with an equal bulk of any known substance, may be nearly deter- 

 mined by the attractive force which any given mass of matter exerts 

 upon a plummet (when suspended in its vicinity) to draw it from a 

 vertical line. This will be proportional to the absolute quantity of 

 matter in that mass compared with that of the earth. By this meth- 

 od, it has been found that the mean density of the earth is about five 

 times greater than that of water, or nearly twice the average density 

 of the rocks and stones on the surface. 



Hence it has been inferred that the interior part of the earth is 

 solid ; or, if it be cavernous, that the solid matter must possess great 

 density. It is not improbable that iron, nearly in a metallic state, 

 may be one of the constituent parts of the central mass, and to this 

 it may owe its magnetic polarity. 



Dr. Halley supposed that the earth is a hollow sphere, containing 

 within it a central magnetic globe, and that the revolutions of this 

 globe on its axis, occasioned the variations of the magnetic needle. 

 Laplace, the celebrated French astronomer, asserts, that the nutation 

 of the earth's axis, and experiments on the vibration of the pendu- 

 lum, indicate an increase of density of the mineral beds, as they 

 approach nearer to its centre, at least to a certain depth from the sur- 

 face. The rapid transition of motion to very distant parts of the 

 earth during violent earthquakes, renders it probable that there are 

 cavities filled with fluid or gaseous matter, which extend to different 

 parts of the globe, at great depths under the surface. 



An opinion has long been entertained, that our planet contains 

 within it a mass of igneous matter, the source of central heat, which 

 is supposed to be an important agent in maintaining the present tem- 

 perature of the globe, nor are facts wanting to lend support to this 

 opinion. The occurrence of numerous active volcanoes in both 

 hemispheres, and in every degree of latitude ; the existence of ex- 

 tinct ancient volcanoes, and of rocks of igneous origin in almost 

 every country ; and the numerous hot and warm springs that pre- 

 serve an unvarying temperature for centuries^ — all indicate the ex- 

 istence of a source of heat deeply seated beneath the surface. It 

 seems also to be proved by observations made for the purpose in deep 

 mines, that the temperature of the earth increases as we descend ; 

 though at a small distance from the surface, the temperature of the 

 ground and of w^ells is the same in every season, but it varies in different 

 latitudes. The animals and vegetables whose remains in a fossil state 

 are found in northern climates, are, generally, analogous in structure 



