In this thesis the firsts two moments of the maximum depth of the air showers are measured. The fluorescence detector of the Pierre Auger Observatory are used. E(Xmax) and sigma(Xmax) are presented for almost three order of magnitude of the energy thanks to the new telescopes HEAT. The results at energies above 10^18 eV confirm the previous measurements of the Pierre Auger Collaboration. Below this energy, an unexpected feature at 10^17.5 eV is found in the analysis of the distribution.