of a homogeneous fluid mass that revolves upon an axis. 103 
Now this expression is derived from the equation, 
<P = C, 
on the one hand, by changing C into C + $ C ; and, on the 
other, by substituting, in the function <p, the co-ordinates 
x + y + $y, z + $ % of the upper surface of the stratum, 
in place of x, y, z, the co-ordinates of the surface H K I. 
Thus it appears, that the equation of the new fluid body is 
derived from that of the first one, merely by varying the 
constant introduced in the integration. 
Before proceeding farther, it is requisite to distinguish 
carefully two separate cases. The first is, when the particles 
of the fluid do not attract one another ; and the second, when 
they are endowed with attractive powers. These are plainly 
two cases essentially different from one another : for, in the 
first, a stratum added induces no other change than an in- 
crease of pressure ; but, in the second, besides the pressure 
a new force is introduced, arising from the attraction which 
the matter of the stratum exerts upon the fluid body to 
which it is added. 
In the first case, when there are no new forces introduced 
by attraction, it is manifest from what has been said, that 
the fluid body of which the equation is, 
(p=C-i-^C 
is in equilibrio ; because the stratum presses equally upon all 
parts of the surface H K I. If we suppose a second stratum 
to be laid upon the first, and compute its thickness by the 
gravity at the surface N O L, in the same manner that the 
thickness of the first was determined by means of the gravity 
at the surface H K I, we shall have another fluid body in 
equilibrio, of which the equation will be, 
