  Let $X$ be a smooth projective geometrically connected curve over a finite
field with function field $K$. Let $\G$ be a connected semisimple group scheme
over $X$. Under certain hypothesis we prove the equality of two numbers
associated with $\G$. The first is an arithmetic invariant, its Tamagawa
number. The second, is a geometric invariant, the number of connected
components of the moduli stack of $\G$-torsors on $X$.
