THE INTERIOR OF THE EARTH.
No. IY. By Rev. B. J. Westbiiooke. The most conclusive evidence of the rigid state of the earth’s interior is that of the earth’s movements, known as precession and nutation—the former a kind of rolling motion, the latter an oscillatory swinging. These movements are caused by the force of gravitation. The laws of this mighty and wide spread force are now pretty well understood—thanks to the acumen and patience of such men as Kepler, Newton, and Cavendish. Three of these recognised principles it will be to my purpose to give. They are : Firstly—The laws of gravitation are universal. Secondly—These laws are common to all kinds of matter. And thirdly ' —every particle of matter attracts every other particle with a force proportional directly to the product of the numbers representing their masses, ami inversely to the square of the distance separating one from the other. It must not be supposed that these are tho only laws, or, indeed, the most ynporfcant laws of gravitation. They are simply given here as apropos to what follows.
The more the laws of gravity are studied, in their relation to physiography, the more clearly do we see the truth of the statement, that “In the whole realm of Physics there is no such thing as rest.” The ether of space is continually quivering with countless undulatory rays of light; the molecules of matter ceaselessly whirling and dancing with the vibrations of heat ; planets ever rolling in the majesty of subdued light; and suns cease not to swing in orbits, to the human mind, quite infinite ; while comets come leaping up, with increasing swiftness, from the profound depth of space, salute their suns, and then plunge hack with decreasing velocity to the unexplored regions whence they came. And yet the laws of gravity demonstrate that from the smallest meteor that for a moment lights our sky, to the most distant star, all are so pulling and swaying each other that the student of astronomy is almost overwhelmed with the mighty signs of perpetual unrest; and his feelings find the relief of sympathy as he rehearses the following noble translation of Jean Paul’s words, by De Quincy ; “ God called up from dreams a man into heaven, saying, “Come up hither and observe the piety of my house.” And to the servant standing round His throne, He said, “Take and undress him from his robes of flesh ; cleanse his vision, and put a new breath into his nostrils, only touch not with any change hiS human heart—the heart that weeps and trembles.” It was done ; and with a nvrditv angel for his guide the man stood ready for his infinite voyage; and. from the terraces of heaven, without sound or farewell, at once thev wheeled away into endless space. Sometimes with the solemn flight of angel-wing they fled through Zaarrahs of darkness—through deserts of death that divided the worlds of life ; sometimes they swept oyer frontiers where new worlds were being formed from chaotic matter. Then, from a distance which could be counted only in heaven, light dawned fora time through a sleepy film ; by unutterable pace the light swept to them, they by unutterable pace to the light. In a moment the rushing of worlds was upon them ; in a moment the blazingof suns was around them.
“Then came eternities of twilight that revealed but were not revealed. On the right hand and on the left towered mighty constellations. . . • Without measure were the architraves,, past number were the archways, beyond memory the gates. Within were stairs that scaled the eternities below: above was below, below 'was above, to the man snipped of gravitating body; depth was swallowed up in height'insurmountable, height was Swallowed up in depth unfathomable. Suddenly as they rode from infinite to infinite suddenly, as they tilted over abyssmal worlds —a- mighty cry arose that systems more mysterious, that worlds more billowy, other heights and other depths were coming, were nearing, were at hand. “ Then the man sighed, stopped, shuddered, and wept. ‘Angel,’ said he, as his overladen heart uttered itself in tears, ‘ I will go no farther, for the spirit of man acheth with this infinity. Insufferable is the glory of God Let me lie down in the narrow grave, and hide from the persecution of the infinite ; for end 1 see there is none.’ And from all the listeningstars that shone around issued a choral voice, ‘The man speaks truly; end there is none that ever yet we heard of.’ ‘ End is there none ?’ the angel solemnly demanded ; ‘is there indeed no end !—and is this the sorrow that ki’ls you ? ’ But no voice answered, that he himself might answer. Then the angel threw up his glorious hands to the heaven of heavens, saying, ‘ End there is none to the universe of God. Lo ! also there is no b’egining.’” Those who have even only a slight, acquaintance with these studies will be will mg to forgive this digression.
But to return to the argument from precession and nutation. As the sun, moon, and planets, in their apparent movements round the earth, do not circle the earth in the plain of her equator, they obtain a kind of purchase upon the equatorial protuberance—already described at length in No. ll.—the effect of which is, by their sideway, or angularat:rac ti on. to tilt it slightly, and so cause the axis of the earth to rotate about the pole of the ecliptic -with a circular motion. This isvery simply described by Norman Lockver in his elementary lessons in astronomy. I will give his words. “Let the equatorial protuberance of the earth bo represented by a ring, supported by two points at the extremities of a diameter, and inclined to its support as tho earth’s equator k is inclined to the eliptic.
Let a long string bo attached to the highest portion of the ring, and let the string be pulled horizontally, at right angles to the two points of suspension, and away from the centre of the ring. This pull will represent the sun’s attraction on the protuberance. The effect on the ring will be that it will at once take up a horizontal position ; tho highest part of the ring will fall as if it were pulled from below, the lowest part will rise as if pulled from above. “ The sun’s attraction on the equatorial protuberance in certain parts of the orbit is exactly similar to the action of the string on the ring, but the problem is complicated by the two motions of the earth.” The north end of the earth’s axis is at present directed very nearly towards the polo star ; the smith end, of course, to that point of the heavens known as the southern point. If the earth’s axis were stationary, the above points would continue to lie in the direction of a continuation of her axis. But, owing to the force of gravity exerted by the other members of this system, tho pole of the heavens describes a circle which it would take 25,308 years to complete. Besides this circular motion of the earth’s axis, caused by the sun, moon, and planets, as a whole there is superadded another movement, the effect of the attraction of the moon alone, and making the line, which describes the rotation of the earth’s axis about the pole of the eliptic, an undulatory one. This undulatory oscillation is called nutation.
So long ago as 1839, Hopkins showed that a shell of 100 miles in thickness would not permit these motions to take place as they do. And Sir William Thomson has so developed the theory of Hopkins, that it is now thought by many to be absolutely set at rest. Thomson’s argument is, that if the earth were not rigid it would modify its shape, instead of showing these variations in its axial revolutions. He points out that if the crust were of solid steel, 500 miles thick, it would yield to the attractions of sun and moon almost as readily as if it were indiarubber. The earth would thus be distorted in figure. It would heap up under the moon like the waters of the sea, which it would carry with it, and thus nullify the tides by preventing any relative change of level between land, and and water.
He also shows that the “ equilibrium of the crust would be unstable. The secular cooling causes upheavals and depressions, and any portion of the crust rent from its continuity with the rest would behave like a ship that has been rammed, one portion would rise up, another would sink down, and then all would go down tor gather. Thus, the crust would not maintain its integrity, but would inevitably shatter and sink piecemeal through the lighter lava,to build up a solid framework from below.”
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