416 Dr. G. Bakker on the 
in the same point. The base of this parallelepiped is : 
d$ l = du dv. If we denote the element of the surface opposite 
to d^ by d$i we have, neglecting the infinitely small of 
higher order, 
s (" fe i) — ^ 
on on 
We find therefore 
^ dn = —^ — dk 1 dn+^- ■ (aS/— dS). . (5) 
0?i on o» 
<Zw and dv are the elements of the curves of curvature in 
the considered point of the surface S^ Therefore 
d$ 1 / = dudv(l+ jf )(l+ Jf )=<*Si + (jf + j^V^^00 
If the surface-elements, denoted by dS l5 be specially the 
elements of the equipotential surfaces and if dn denotes the 
differentials of the lines of force, we have 
and the equation (4) gives with the aid of (5) and (6) : 
where ^t denotes the element of volume. 
For the potential energy of the forces of attraction we have 
also the general expression : 
Varying only the density, we have 
SY8pdr=$pdYdT, 
and thus 
W=i$V8pdT + ij p 8VdT=$pSVdT. ... (8) 
The equalization of (7) and (8) gives: 
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