. Elements of astronomy: accompanied with numerous illustrations, a colored representation of the solar, stellar, and nebular spectra, and celestial charts of the northern and the southern hemisphere. rface to centre. 201. Density of the Earths Crust.—The density of theEarths crust is only about half of the mean density of theEarth taken as a whole. This has been accounted for bysupposing that the materials of which it is composed aremade denser at great depths than at the surface, by theenormous pressure of the overlying mass; but there arestrong reasons for believing that the central portion
. Elements of astronomy: accompanied with numerous illustrations, a colored representation of the solar, stellar, and nebular spectra, and celestial charts of the northern and the southern hemisphere. rface to centre. 201. Density of the Earths Crust.—The density of theEarths crust is only about half of the mean density of theEarth taken as a whole. This has been accounted for bysupposing that the materials of which it is composed aremade denser at great depths than at the surface, by theenormous pressure of the overlying mass; but there arestrong reasons for believing that the central portions aremade up of much denser bodies than are common at thesurface,—such as metals and the metallic compounds. 202. The Flattening at the Poles explained.—It wasprior to the solidification of its crust, and while the surfacewas in a soft or fluid condition, that the Earth put on itspresent flattened shape, the flattening being due to abulging out at the equator, caused by the Earths rotation. If wearrange a thin flexible hoop, asshown in Fig. 50, so that theupper part of it may move freelyup and down on an axis, and thenmake it revolve very rapidly,it will assumean oval form,bulging out at. Fig. 50.—Explanation of the Flattening at theEarths Poles. those parts which are farthest from the axis, the motion opposite opinion is held by some? 201. How does the density of the Earthscrust compare with that of the whole planet? How is this accounted for? is the flattening at the Earths poles accounted for? Illustrate this with THE EARTHS ATMOSFHERE. iq7 being there most rapid, just as the Earth does at theequator. 203. The form of the Earth, moreover, is exactly thatwhich any fluid mass would take under the same circum-stances. This has been proved by placing a quantity ofoil in a transparent liquid of exactly the same density asthe oil. As long as the oil was at rest, it took the form ofa perfect sphere floating in the middle of the fluid, exactlyas the Earth floats in s
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Keywords: ., bookcentury1800, bookdecade1870, booksubjectastronomy, bookyear18
