The components of acceleration of a particle of fluid are consequently
Du du du du du
“,2;'='37+ua;+1J@+wa;-» (3)
Dr dv dv dv dv
7=3;+ 1454-v3;+1v, ;;. (4)
Dw dw dw dw dw
F=ji7+uE+r1;5+wE;» (5)
leading to the equations of motion above.
If F (x, y, z, t) =o represents the equation of a surface containing
always the same particles of fluid,
D F F
Q? =o, or%-+u%E+1/gg-;+w¢%=o, (6)
which is called the differential equation of the bounding surface.
A bounding surface is such that there is no flow°of fluid across it,
as expressed by equation (6). The surface always contains the same
fluid inside it, and condition (6) is satisfied over the complete surface,
as well as an art of it.
Y P
But turbulence in the motion will vitiate the principle that a
bounding surface will always consist of the same fluid particles,
as we sec on the suiface of turbulent water.
24. To integrate the equations of motion, suppose the impressed
force is due to a potential V, such that theiforce in any direction is the
rate of diminution of V, or its downward gradient; and then
X=-dV/dx, Y=-dV/dy, Z=-dV/dz; (1)
and putting
dw dv du dw dv du
E, -&=2E» 5-;, =211, E-5-2r. (2)
dE d dy
Tx " or
the equations of motion may be written
du dH
E; - 211§ '+2wn+Q; =0» (4)
%-2105-f-2u§ '+%I;=0, (5)
%-2uq+2v£+l-(gl-=o, (6)
where
H =fdP/p+V+i9', (7)
9” = u'+v' +w', (8)
and the three terms in may be called the pressure head, potential
head, and head of velocity, when the gravitation unit is employed
and sq' is replaced by %q'/e-Eliminating
H between (5) and (6) .
% ¢§ ¢ @@: du dr/dw
dt idx "7dx'f°dx+f — (9)
and combining this with the equation of continuity
LPQ a+a, a»
p dt +dx dy dz "0' (Io)
1 2 5 'EQ nd” Cdw
we have dt (P)-pdx-Zi;-5- Ec~ 0, (U)
with two similar equations.
Putting
w' 9+ +
= rr' § '. (12)
a vortex line is defined to be such that the tangent is in the direction
Of w. th€ resultant of E, vp, § ', called the components of molecular
rotaton. A small sphere of the fluid, if frozen suddenly, would
retain this angular velocity.
If w vanishes throughout the fluid at any instant, equation (11)
shows that it will always be zero, and the fluid motion is then called
1rro ia1z onal; and a function ¢ exists, called the velocity function,
suc t at
udx+vdy+wdz==-do, (13)
and then the velocity in any direction is the space-decrease or
downward gradient of gb.
25. But in the most general case it is possible to have three
functions ¢, ip, m of x, y, z, such that
udx+vdy+wdz = -d4>-md¢, (1)
as A. Clebsch has shown, from purely analytical considerations
(Crelle, lvi.); and then
d(~P~m) d(¢.m) d(¢, m)
f'*?d<y, z>' "'i3l"z, x)' "'i'”'F'd<x.y (2)
and
d d d
e£+»3§ +:-, }§ =<>, sff§ +»'§§ +r%';'f=<>. (3)
so that, at any instant, the surfaces over which up and m are constant
intersect in the vortex lines.
Putting
the equations of motion (4), (5), (6) § 24 can be written
and therefore
H-§ 4§ -m'§ '§ =K, (4)
%-21¢§ '+2wq- =O, . . ., .; (5)
e'§ , }§ +»'-§ }§ +r§ =0. <6>
Equation (5) becomes, by a rearrangement, .
dK dilf dm dm dm @
EW 71T+“1i;+"@+“¢Z)
dm .za 11./, iz./, 11
+35 '5+14Z§ +1/;§ +w£ =0, —. —, (7)
dK diff Dm dm Di#
Ia;-EC-(E'+E;'W=0, ..., ..., (S)
and as we prove subsequently (§ 37) that the vortex lines are composed
of the same fluid particles throughout the motion, the surface m and
(P satisfies the condition of (6) § 23; so that K is uniform throughout
the fluid at any instant, and changes with the time only, and so
may be replaced by F (t).
26. When the motion is steady, that is, when the velocity at any
point of space does not change with the time,
gi-2'U§ "i'2'£U11=O, ..., . . (1)
£%Lf+n?1§ +I'?§ =0, 1¢'?§ +v?;§ +w'?§ =0. (2)
and I
2 K='f<lP/D-l-V+%q'=H (3)
is constant along a vortex line, and a stream line, the path of a fluid
particle, so that the fluid is traversed by a series of H surfaces, each
covered by a network of stream lines and vortex lines; and if the
motion is irrotational H is a constant throughout the fluid.
Taking the axis of ix for an instant in the normal through a point
on the surface H=constant, this makes u=o, £=o; and in steady
motion the equations reduce to
~ dl-I/dv =2vg'-2w1, =2qw sin6, (4)
where 0 is the angle between the stream line and vortex line; and
this holds for'the1r projection on any plane to which dv is drawn
perpendicular.
In plane motion (4) reduces to
dH d

- =29§ '=9(;%+§)» (5)

if r denotes the radius of curvature of the stream line, so that 1 <1 if! 45- £93; sl.” p£+dv'dv dv ri (6) the normal acceleration. The osculatmg plane of a streain line in steady motion contains the resultant acceleration, the direction ratios of which are Q du du @' dlq2 @ udx+va;+wE- ix -21/5+2w1;—5; dx, . ., (7) and when q is stationary, the acceleration is normal to the surface H =constant, and the stream line is a geodesic. Calling the sum of the pressure and potential head the statical head, surfaces of constant statical and dynamical head intersect in lines on H, and the three surfaces touch where the velocity is stationary. Equation (3) is called Bernoulli's equation, and may be interpreted as the balance-sheet of the energy which enters and leaves a given tube of flow. If homogeneous liquid is drawn off from a vessel so large that the mption at the free surface at a distance maybe neglected, then Bernoulli's equation may be written ~ H=P/n+2+9'/2£=P/P+h, (5) where P denotes the atmospheric pressure and h the height of the free surface, a fundamental equation in hydraulics; a return has been made here to the gravitation unit of hydrostatics, and Oz is taken vertically upward. ' V, In particular, for a jet issuing into the atmosphere, where p=P, Q'/2g=h-2, ' (9) or the velocity of the jet is due to the head k-z of the still free surface above 'the orifice; this is Torricellifs theorem (1643), the foundation of the science of hydrodynamics. 27. Uniplanar M ation.-In the uni planar motion of a homogeneous liquid the equation of continuity reduces to %+%=o, (1) so that we can put

u= -dw/dy, v=d/f/dx, (2)