  Using micron-sized Hall sensor arrays to obtain time-resolved measurements of
the local magnetization, we report a systematic study in the molecular magnet
Mn$_{12}$-acetate of magnetic avalanches controllably triggered in different
fixed external magnetic fields and for different values of the initial
magnetization. The speeds of propagation of the spin-reversal fronts are in
good overall agreement with the theory of magnetic deflagration of Garanin and
Chudnovsky \cite{Garanin}.
