Cosmic Rays have been known for about half a century . The fact that we are only today beginning to understand some of the problems associated with them indicates the order of magnitude of the complexity of the problem, and the degree to which we were dealing with phenomena with which we had little familiarity. Our opinions have gone through a series of changes as we have slowly realized the true nature of the factors involved. For example, in the early 1920’s the general opinion about cosmic rays was that they were of the nature of gamma rays, of some hundreds of Mev’s in energy, and that most of the secondary particles at sea level were electrons. Contrast this with the fact that it has been proved that the majo- rity of primary cosmic rays are protons, with energies tens to thousands of times the previous hundreds of Mev’s, and that the secondary particles reaching sea level are mostly mesons. We see then just how revolutionary has been the change of view which modern detection techniques have made possible.