This follows independently from (14), assuming the special variations
[delta]x = x`dt, &c., and therefore [delta]q1 = q`1dt, [delta]q2 =
q`2dt, ...
Reciprocal theorems.
Again, if the values of the velocities and the momenta in any other
motion of the system through the same configuration be distinguished
by accents, we have the identity
p1q`'1 + p2q`'2 + ... = p'1q`1 + p'2q`2 + ..., (16)
each side being equal to the symmetrical expression