  Magneto-Optical Traps have been used for several decades. Among fundamental
mechanisms occuring in such traps, the magnitude of the multiple scattering is
still unclear. Indeed, many experimental situations cannot be modeled easily,
different models predict different values of the re-absortion cross section,
and no simple experimental measurement of this cross section are available. We
propose in this paper a simple measurement of this cross section through the
size and the shape of the cloud of cold atoms. We apply this method to traps
with a configuration where theoretical values are available, and show that the
measured values are compatible with some models. We also apply the method to
configurations where models are not relevant, and show that the re-absorption
is sometimes much larger than the usually assumed value.
