  We report a measurement of the energy spectrum of cosmic rays for energies
above $2.5 {\times} 10^{18}~$eV based on 215,030 events recorded with zenith
angles below $60^\circ$. A key feature of the work is that the estimates of the
energies are independent of assumptions about the unknown hadronic physics or
of the primary mass composition. The measurement is the most precise made
hitherto with the accumulated exposure being so large that the measurements of
the flux are dominated by systematic uncertainties except at energies above $5
{\times} 10^{19}~$eV. The principal conclusions are: (1) The flattening of the
spectrum near $5 {\times} 10^{18}~$eV, the so-called "ankle", is confirmed. (2)
The steepening of the spectrum at around $5 {\times} 10^{19}~$eV is confirmed.
(3) A new feature has been identified in the spectrum: in the region above the
ankle the spectral index $\gamma$ of the particle flux ($\propto E^{-\gamma}$)
changes from $2.51 \pm 0.03~{\rm (stat.)} \pm 0.05~{\rm (sys.)}$ to $3.05 \pm
0.05~{\rm (stat.)} \pm 0.10~{\rm (sys.)}$ before changing sharply to $5.1 \pm
0.3~{\rm (stat.)} \pm 0.1~{\rm (sys.)}$ above $5 {\times} 10^{19}~$eV. (4) No
evidence for any dependence of the spectrum on declination has been found other
than a mild excess from the Southern Hemisphere that is consistent with the
anisotropy observed above $8 {\times} 10^{18}~$eV.
