  The BL Lac object (blazar) Mrk 421 was observed during its outburst in April
2004 with the Whipple 10 m telescope for a total of about 24.5 hours. The
measured gamma-ray rate varied substantially over the range from 4 to 10
gamma's/min and eventually exceeded the steady gamma-ray rate of the Crab
Nebula (standard candle) by a factor of 3. The overall significance of the
gamma-ray signal exceeded 70 sigma and the total number of excess events was
more than 10,000. The signal light curve does not show any particular
variability pattern. This unique Mrk 421 outburst enabled the measurement of a
high quality spectrum of very high-energy gamma rays in a high state of
emission. This spectrum is a power-law and it extends beyond 10 TeV.
