  Well-known Bloch equations describe the spin systems (electronic and nuclear)
for any scale of time, from transient processes to steady states. Usually in
solids T_2 << T_1. The question arises: what are the approximations that should
be made in order the "roughen" Bloch equations to describe the processes at the
time T_2 < t < T_1? The answer to this question is given in this article. As an
example, the saturation of magnetic resonance is considered under the
conditions of harmonic modulation of a constant magnetic field.
