Contemporary Advances in Physics, XXIII 

 Data and Nature of Cosmic Rays 



By KARL K. DARROW 



"Cosmic rays" is the name of the ultimate cause which maintains that 

 part of the ionization of the air which cannot be ascribed to the rays of radio- 

 active substances on earth nor to any other known agency. The measure- 

 ment of this residue, the discrimination between it and that part of the 

 ionization which is due to familiar rays, is the first problem of cosmic-ray 

 research. Second comes the problem of learning, from measurements made 

 at as many places and under as many conditions as possible, the nature of 

 the mysterious ionizing agent. One naturally begins by assuming it to be 

 like in kind to one or another of the known types of ionizing rays, but 

 different in quality; e.g. to consist of electrons faster than any known 

 electrons, or photons of greater energy and lesser wave-length than any 

 known photons. It is not yet certain whether one of these hypotheses will 

 fit, or rays of some new type must be imagined. The direct measurements 

 of ionization are supplemented by observations of material particles of 

 evidently enormous energy which dart across the atmosphere in straight 

 paths leaving trains of ions behind. On the whole it seems highly probable 

 that these particles, or those (if such there be) from which they receive their 

 energy, come to the earth from outer space; and the energy which they bear 

 is so great, that its source must be some process not yet known. 



THE subject of this article is unique in modern physics for the 

 minuteness of the phenomena, the deUcacy of the observations, 

 the adventurous excursions of the observers, the subtlety of the 

 analysis, and the grandeur of the inferences. The effect which is 

 studied, which may be described as the liberation of electrons from the 

 molecules of the air by agents otherwise unknown, amounts at sea-level 

 to the liberation of only about 1500 of these per cubic decimetre of air 

 per second. It is not the whole of the observed effect; these 1500 

 electrons are those which are left over out of a quantity often much 

 greater, after allowance is made for the actions of all known ion- 

 izing agents. The methods employed are ranked among the most 

 ingenious and sensitive of science; yet the apparatus is not invariably 

 set up in the calm seclusion of the laboratory. Physicists with their 

 frail machines have gone to high mountain ponds in the Sierras and 

 the Andes, to the distant wildernesses about the earth's magnetic 

 poles; they have scooped out cavities in Alpine glaciers, they have lifted 

 hundredweights of lead to the tops of peaks above the snow-line, they 

 have cruised the arctic and the tropical oceans, they have descended 

 into tunnels and deep mines, they have ascended into the sky. in 

 aeroplanes and balloons. As for the analysis of the precious data 



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