Since the joint discovery by Carmichael (1948) and Williams (1948), that the majority of the bursts occurring in their shielded ionization chambers had been produced not by air showers but by nuclear disintegrations, and following the segregation of thes disintegrations from the true showers – many experiments have been carried out to determine the later structure of air showers, both at sea level and at higher altitudes. Unfortunately, much of the work done prior to this discovery must be viewed with caution because of the possibility of a significant contribution from nuclear disintegrations. (From Introduction)