CONTEMPORARY ADVANCES IN PHYSICS 



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of this simplest idea is needed. It was supplied when the cloud- 

 chamber, "Ch" in the figure, was inserted. The chamber was com- 

 pelled by mechanism to expand, always when and only when a three- 

 fold coincidence happened ; and at the great majority of its expansions 

 it showed a vertical track. Figure 3 exhibits the arrangement of 

 Street, Woodward and Stevenson at Harvard, who found the track 

 of the traversing particle at 202 expansions out of 219. Auger and 

 Ehrenfest at Paris had already set up four counters and a cloud- 

 chamber and a block of lead in a vertical line, and found the track of 

 the single traversing particle at fifty-five expansions out of sixty-nine. 

 Another test is made by displacing one of the counters out of line 

 with the others, whereupon it is found that the coincidences fall off 

 in number sharply. And now to come to the point which most con- 

 cerns us: there were 45 cm of lead between the counters in the experi- 

 ment of Fig. 3, and 50 cm in the experiment by Auger and Ehrenfest, 

 and no fewer than 101 cm in an early experiment of Rossi's with 

 counters though without the chamber! Such is the power of pene- 

 tration of some of the charged corpuscles of the cosmic rays. 



Fig. 4 — Three showers, two evoked by charged particles and one presumably by a 

 photon. (Street and Stevenson, Harvard University) 



