104 Mr. Ivory on the figure requisite to maintain the equilibrium 
<p = C + £C + 2'C 
S' C being equal to the pressure caused by the new stratum. 
And, in like manner, any number of strata may be added 
composing a fluid body in equilibrio. 
But as strata have been added without disturbing the equi- 
librium, in like manner any number of strata below the sur- 
face H K I, may be successively taken away, so as to leave 
the remaining fluid in equilibrio. The original body H K I 
may be thus exhausted, or reduced to an infinitesimal quan- 
tity that may be neglected ; and then the whole mass, both 
above and below the surface H K I, will consist of level strata 
separated by surfaces having a common equation, in which 
the constant quantity introduced in the integration alone varies 
in passing from one surface to another. We may therefore 
conclude that, when the particles of the fluid do not attract 
one another, the only conditions necessary for the equilibrium 
are, first, that the force resulting from X, Y, Z, be directed 
into the interior of the mass ; and, secondly, that Xdoc 
Y dy -f- Z d z be an exact differential. 
But this first case can have no application in the theory of 
the figure of the planets, the leading principle of which is, 
that every particle of matter attracts every other particle. 
We must therefore proceed to consider what new conditions 
are required in the second case, when the particles are pos- 
sessed of attractive powers. 
All the forces, whether attractive or not, that urge the 
particles of the fluid body H K I, are supposed to be included 
in the expressions X, Y, Z ; and it has been shown that the 
gravity arising from these forces produces, by its action upon 
the stratum of which the thickness is K S, equal pressures 
