  In this paper, proceeding from the recently developed way of deriving the
quantum-mechanical equations from the classical ones, the complete system of
hydrodynamical equations, including the quantum Euler equation, is derived for
a perfect fluid and an imperfect fluid with pairwise interaction between the
particles. For the Bose-Einstein condensate of the latter one the Bogolyubov
spectrum of elementary excitations is easily reproduced in the acoustic
approximation.
