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```530                                ON  THE THEORY  OF  LUBRICATION                                [428
the integrations being always over the length.    Eliminating 8H, we get
/Q0,
x ............. (32)
The evanescence of £P for all possible variations 8h would demand that over the whole range either
\$lrsxdx            j        „-„.                              XOQ\
"- TJFvJT   or  h"^H' ..................... (33)
But this is not the requirement postulated. It suffices that the coefficient of Bli on the righfc of (32) vanish over that part of the range where h>hl, and that it be negative when h=-hif so that a positive Bh in this region involves a decrease in P, a negative 8/1 here being excluded a priori. These conditions may be satisfied if we make h = hl from x = 0 at the edge where the layer is thin to x = cl, where Cj is finite, and k = %H over the remainder of the range from cx to GI + c2, where GI + cz = c, the whole length concerned (Fig. 2). For the moment we regard Cj. and c2 as prescribed.
In,
0
Fig. 2. For the first condition we have by (8)
,,   i           -r-r       C-l/fil       T"
so that
0^ = ^(2^-3),   ........................... (34)
determining k, where as before k = h2/Jil~ The fulfilment of (34) secures that h = §H over that part of the range where h = h2. When h = hl} h — \$H is negative ; and the second condition requires that over the range from 0 to Ci
be positive, or since c^ is the greatest value of as involved, that
/ A-8 xdx ~ cj h~3 dx = + ...................... (35)
The integrals can be written down at once, and the condition becomes
J<*<c/(c12, ........................... , ..... (36)
whence on substitution of the value of GS/CJ from (34),
&(2&-3)2>l ............................... (37)*
If k be such as to satisfy (37) and Ca/Cj be then chosen in accordance with (34) and regarded as fixed, every admissible variation of h diminishes P. But the ratio c.2/Ci is still at disposal within certain limits, while c-i + C2 (= c) is prescribed.
[* This inequality may be written (A; -1) {4 (fc-l)2-3}>0; showing that, since /c>l and the conditions are satisfied when (37) becomes an equality, w'e must have k -f: 1 + -5- -j: 1 "866.   W. F. S.]lowly (P = Q'Ulfj.Ue*/hi*, when k*i).    W. F. 8.]
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