Cosmic Rays from the Knee to the Second Knee: 10 [sup] 14 to 10 [sup] 18 eV

The energies of cosmic rays, fully ionized charged nuclei, extend over a wide range up to 1020 eV. A particularly interesting energy region spans from 1014 to 1018 eV, where the all-particle energy spectrum exhibits two interesting structures, the "knee" and the "second knee". An explanation of these features is thought to be an important step in understanding the origin of the high-energy particles. Recent results of air shower experiments in this region are discussed. Special attention is drawn to explain the principle of air shower measurements --- a simple Heitler model of (hadronic) air showers is developed