In the absence of external forces, the potential energy of a system and hence its Lagrangian function do not vary at the shift of beginning of coordinates. Taking in (22) the transformations of Cartesian coordinates $x_i^*=x_i+\alpha, y^*=y_i, z^*=z_i$, $(i=1, \ldots, N), t^*=t$, prove that (23) has a form $\phi=\sum_{i=1}^N m_i \dot{x}_i=$ const, i.e.for such a system the momentum conservation law in projection on axis $x$ is valid.