Edinburgh New Philosophical Journal (1855) Edinburgh New Philosophical Journal edinburghnewphil11855 Year: 1855 322 James Elliot on certain what we commonly call the centre of gravity is truly the centre of momentum. The same reasoning applies to the earth in reference to the sun's attraction : its centre of mo- mentum—the centre round which it revolves in its diurnal motion—is the centre of the sphere (or spheroid) ; while the varying centre of gravity is always within the hemisphere nearest the sun. Here, then, is the very desideratum sup- plied, to complete our analogy between the earth's
Edinburgh New Philosophical Journal (1855) Edinburgh New Philosophical Journal edinburghnewphil11855 Year: 1855 322 James Elliot on certain what we commonly call the centre of gravity is truly the centre of momentum. The same reasoning applies to the earth in reference to the sun's attraction : its centre of mo- mentum—the centre round which it revolves in its diurnal motion—is the centre of the sphere (or spheroid) ; while the varying centre of gravity is always within the hemisphere nearest the sun. Here, then, is the very desideratum sup- plied, to complete our analogy between the earth's motions and those of the top : here are our two centres,—the one the centre of the mass, the centre of momentum,—the other the proper centre of gravity. The next difficulty is this. Sir Isaac Newton, as is well known, has demonstrated that the conical revolution of the axis would not belong to the earth were it a perfect sphere, but that it is indebted for it to its spheroidal form ; whereas the same motion in the top is independent of its form. The reply to that is, that there is no tendency to such a motion in the top while in a vertical position,—that is, when its centre of gravity is directly above or directly below its centre of motion, because then there is no tendency either to fall or to rise, and that the same thing precisely would be the case with the earth if it were a perfect sphere: the centre of gravity would then be directly between the sun and the centre of momentum. But, in the case of a spheroid, the centre of gravity will be a little out of that line, pro- ducing a tendency to fall into it, and this tendency is con- verted into the motion in question. Thus, let the point 0 be the centre of the spheroid AB, and consequently its cen- tre of momentum : let the line AB be the transverse axis, and S the attracting s body; and let the spheroid be divided into two half spheroids by a plane, CD, coincident with the line OS, and perpendicular to the plane BO
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