  It is well-known that the circulation of the velocity field of a fluid along
a closed material curve is conserved for any solution of the Euler equation. We
offer a slightly more explicit proof of that fact than that generally found in
the literature. We then rewrite that property in terms of the rescaled
variables and functions leading to the Euler-Leray equations and appropriate
for studying self-similar solutions. We finally discuss the implications of the
conservation of circulation on the existence of such solutions.
