Editing SSC/FreeEnergy/EquilibriumSequenceInstabilities (section)

====Jeans (1919)====
From &sect; 50 (p. 46) of  [http://adsabs.harvard.edu/abs/1919pcsd.book.....J J. H. Jeans (1919)] &#8212; ''verbatum'' text in green:  <font color="green">
Let the two bodies be spoken of as primary and secondary, and let their masses be <math>~M</math>, <math>~M^'</math> respectively; let the distance apart of their centres of gravity be <math>~R</math>, and let the angular velocity of rotation of the line joining them be <math>~\omega</math>.  It will be sufficient to fix our attention on the conditions of equilibrium of one of the two masses, say the primary.  Let its centre of gravity be taken as origin, let the line joining it to the centre of the secondary be axis of <math>~x</math>, and let the plane in which the rotation takes place be that of <math>~xy</math>.  Then the equation of the axis of rotation is
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<math>~x = \frac{M^'}{M + M^'} ~ R</math>
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&nbsp; &nbsp; &nbsp; and &nbsp; &nbsp; &nbsp; 
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<math>~y = 0 \, .</math>
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The problem may be reduced to a statical one (cf. &sect; 31) by supposing the masses acted on by a field of force of</font> [the centrifugal] <font color="green">potential
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<math>~\frac{1}{2}\omega^2\biggl[ \biggl( x - \frac{M^' }{M + M^'} ~R \biggr)^2 + y^2\biggr] \, .</math>
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