Encyclopaedia Britannica, 11th Edition, Volume 8, Slice 9

mx` = X', my` = Y', mz` = Z', (2)

where X', Y', Z' are the components of the impulse on m. Now let [delta]x, [delta]y, [delta]z be any infinitesimal variations of x, y, z which are consistent with the connexions of the system, and let us form the equation

[Sigma]m(x`[delta]x + y`[delta]y + z`[delta]z) = [Sigma](X'[delta]x + Y'[delta]y + Z'[delta]z), (3)

where the sign [Sigma] indicates (as throughout this article) a summation extending over all the particles of the system. To transform (3) into an equation involving the variations [delta]q1, [delta]q