  This article employs fluid equations to analyse muon beams in gases subject
to crossed electric and magnetic fields, focussing in particular on a scheme
proposed by D. Taqqu in 2006, whereby transverse compression of the beam is
achieved by creating a density gradient in the gas. A general criterion for
maximising beam compression, derived from first principles, is then applied to
determine optimal experimental conditions for {\mu ^ +} in helium gas. Although
the calculations require input of transport data for ({\mu ^ +}, He), which are
generally unavailable, this issue is circumvented by "aliasing" ({\mu ^ +}, He)
with (H{^ +}, He), for which transport coefficient data are available.
