Cosmic rays from the knee to the second knee: 10¹4 to 10¹8 eV

The energies of cosmic rays, fully ionized charged nuclei, extend over a wide range up to 10^20 eV. A particularly interesting energy region spans from 10^14 to 10^18 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 of 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.