where Xn Yx .....Xz, Tt are real magnitudes.
For a virtual displacement in a space-time sichel (with the previously applied designation) the value of the integral
(17) W + δW = ∫∫∫∫ (∑Sh k (∂(xk + δxk))/∂xh dx dy dz dt
extended over the whole range of the sichel, may be called the tensional work of the virtual displacement.
The sum which comes forth here, written in real magnitudes, is
Xx + Yy + Zz + Tt + Xx (∂δx)/∂x + Xy (∂δx)/∂y + ... Zz (∂δz)/∂z
- Xt (∂δx/∂t - ... + Tx (∂δt)/∂x + ... Tt (∂δt)/∂t
we can now postulate the following minimum principle in mechanics.
If any space-time Sichel be bounded, then for each virtual displacement in the Sichel, the sum of the mass-works, and tension works shall always be an extremum for that process of the space-time line in the Sichel which actually occurs.
The meaning is, that for each virtual displacement,