216 BELL SYSTEM TECHNICAL JOURNAL 



For this there is some evidence, of the following kinds. First let 

 us compare (in imagination) the number (per unit time per unit area) 

 of penetrating particles flying vertically downward and the number 

 flying obliquely downward. The comparison can be readily made 

 with such an apparatus as that sketched in Fig. 3, the cloud-chamber 

 being superfluous and the lead absorber reduced to the least thickness 

 sufficient to stop electrons; the axis is oriented first at 90° and then 

 at various lesser angles d to the horizontal plane. Even the whole of 

 the atmosphere is insufficient to stop such mesotrons as the cloud- 

 chamber discloses; and yet the observations show a marked decline of 

 the number thereof as d decreases. But the particles which travel 

 obliquely traverse a greater distance from the top of the atmosphere 

 than those which come vertically down, and take a longer time in 

 ■doing so; the decline of number with decrease of may therefore be 

 ascribed to the perishing of the mesotrons en route to the apparatus 

 as the route grows longer and longer. Second: Let us compare the 

 effect of the obliquely-traversed atmosphere with that of a sheet of 

 lead in cutting down the number of particles arriving at the apparatus. 

 One must make a guess as to the thickness of lead which would be 

 required to produce a falling-off^ of the number of particles equivalent 

 to that observed in the atmosphere, if the falling off were due to 

 actual stopping of mesotrons in air and lead respectively, and the 

 impermanence of the mesotron did not enter in at all. It is commonly 

 conjectured that the equivalent thicknesses of lead and air would 

 stand to one another inversely as the densities of these materials. 

 When, however, the effects of such "equivalent" thicknesses are com- 

 pared, it is found that the falling-off beyond the lead is decidedly less 

 than that beyond the air. Now the mesotrons take very much less 

 time for traversing the sheet of lead than the wide expanses of the 

 atmosphere; and the "anomaly," as it has been called, is tentatively 

 explained by assuming that few of them perish in the lead, many in 

 the long journey through the atmosphere. 



Estimates of the mean life of the mesotron thus made yield values 

 of the order of a millionth of a second. It is supposed by many that 

 the mesotrons are born in the upper layers of the atmosphere. Such 

 conjectures, however, lead beyond the scope of this article, which must 

 be confined to these few recent fruits of the seemingly exhaustless 

 cornucopia of the cosmic rays. 



Selections From The Literature 



Energy-losses of particles traversing metals: Anderson and Neddermeyer, 

 London Conference on Nuclear Physics (1934); Phys. Rev. 50, 263 (1936); Nedder- 

 meyer and Anderson, Phys. Rev. 51, 884 (1937); Blackett and Wilson, Proc. Roy. Sac. 



