Messengers from outer space

An account is given of cosmic rays, with particular reference to primary rays arriving at Earth with energies of 1019 eV and above, and the extensive air showers that are produced. The methods of detecting and measuring cosmic rays are explained, with some details of scintillation detectors and the Cerenkov effect in water and air. A diagram shows the density pattern of particles per sq. m. in an air shower produced by a 1020-eV primary cosmic ray, recorded by an array of scintillation detectors at Haverah Park in Yorkshire. A 'Fly's Eye' of light-sensitive photomultipliers, mounted at the foci of a number of spherical mirrors, arranged to cover a complete hemisphere of the sky, is described. The origin of the particles is considered. (U.K.)