  We present the first measurement of the energy spectrum up to 70 MeV of
electrons from the decay of negative muons after they become bound in $^{27}$Al
atoms. The data were taken with the TWIST apparatus at TRIUMF. We find a muon
lifetime of (864.6 $\pm$ 1.2) ns, in agreement with earlier measurements. The
asymmetry of the decay spectrum is consistent with zero, indicating that the
atomic capture has completely depolarised the muons. The measured momentum
spectrum is in reasonable agreement with theoretical predictions at the higher
energies, but differences around the peak of the spectrum indicate the need for
\oalpha radiative corrections to the calculations. The present measurement is
the most precise measurement of the decay spectrum of muons bound to any
nucleus.
