  We propose a dynamical explanation of the Meissner effect in superconductors
and predict the existence of a spin Meissner effect: that a macroscopic spin
current flows within a London penetration depth $\lambda_L$ of the surface of
superconductors in the absence of applied external fields, with carrier density
= the superfluid density and carrier speed $v=\hbar/(4m_e\lambda_L$)
($m_e=$bare electron mass). The two members of a Cooper pair circulate in
orbits of radius $2\lambda_L$ in opposite direction and the spin current in a
Cooper pair has orbital angular momentum $\hbar$. Our description also provides
a 'geometric' interpretation of the difference between type I and type II
superconductors.
