120 Mr. Ivory on the figure requisite to maintain the equilibrium 
Again ; if we put, 
P = (§)'+ 
then, because the line & is perpendicular to the outer surface, 
it follows, from the known properties of curve surfaces, that 
the cosines of the angles which k makes with the co-ordi- 
nates, are respectively equal to, 
i dp i dp i dp 
p * dx ’ p * dy ’ p * dz 
and hence, 
p dx ^ p dp p dz 
and by substituting these values in the preceding formula, 
we obtain, 
7M(n)'+ (?,)*+(£)*} = = 
Now p is the resultant, of the accelerating forces at the sur- 
face * and the line k, or the thickness, is proportional to the 
quantity of matter in the stratum at the same point ; where- 
fore kxp is the pressure ; and the formula, 
k x p = $ C, 
shows that the uppermost stratum presses upon the fluid 
below it equally at all points. 
As the attraction of the whole fluid mass is one of the 
component forces of the gravitation^), the attraction of the 
stratum must enter as a part of the same force. But it is 
evidently only an infinitely small part of it ; and conse- 
quently produces only an infinitely small part of the pressure 
p % k. We may therefore consider the gravitation at the 
