226 B. ROSSI AND D. B. HALL According to the discussion in the foregoing transferable energy, and I=13.5 ev (this ex- section, the counting rates [aBCD(E+F)] with pression differs by a factor 1/B from the expres 115 g/cm2 of lead in 2 can be taken as a measure sion for the energy loss). a correction has to be of the number N of mesotron entering the applied to account for the polarization effect apparatus with a residual range larger than pointed out by Fermi. The correction, however Rb=311 g/cm of lead, while the figures listed is very small for the mesotron momenta in which under A can be taken as a measure of the number we are interested According to some recent n of mesotron entering the apparatus with a calculations of Halpern and Hall, it is of the residual range between Ro=196 and R=311 order of 2 percent for iron and of 3 percent for g/cm2 of lead lead. Numerical integration of the equation for Let Nl, NI and Ne be the values of N at echo the momentum loss yields the range as a function Lake under 200 g/cm of iron, at Echo Lake of the momentum. The ranges Ra=196 g/cm2 of without the iron absorber and at denver without lead and R,=311 g/cm of lead, which define the the iron absorber, respectively. Let ni, n and n2 mesotron groups considered in the present ex- be the corresponding values of n. Considering periments, are thus found to correspond to the first the measurements taken without the iron momenta Pa=3.1x108 and Pb=4.5X10 ev/c, respectively. Mesotrons reaching Denver with n1/N1=0.082+0.005, n3/N2=0.058+0.002. momenta equal to pa and Po had momenta equal It appears that the fractional number of slow altitude of Echo Lake. For mesotron with mesotron increases rapidly with altitude, in momentum between 3. 1x10 and 7. 3 X108 ev/c agreement with the results of the absorption the ratio between momentum losses per g/cm of measurement nts in carbon by Rossi, Hilberry and air and of iron is very nearly a constant and Hoag. Because of a possible effect of scattering equal to 1. 23. Thus, as far as collision losses are on the determination of n, the above figures concerned, 200 g/cm2 of iron is approximately cannot be trusted to represent accurately the equivalent to 147 g/cm of air. 10 Consequently, if n2/N,. However, the ratios between values of the same mesotron intensity at Echo Lake under LABCD(E+F)] or A at different depths should 709 g/cm of air plus 200 g/cm? of iron as at ot be appreciably affected by scattering or by Denver under 856 g/cm2 of air alone. This other disturbing effects. Thus we have applies to the mesotron band between 3. 1 x105 M2/N1=0.883±0.00772/71=0.698±0.031 and45×10°ev/ cas well as to the whole mesotron N2/N=0.738±0.009n/m=0.520+0.035 pectrum above45×10°e/c Our experimental results show that both N and here the actual errors should not exceed the n are larger at Echo Lake under the iron absorber statistical errors indicated than at Denver without this absorber The difference is accounted for by the decay of mesotron on their way down from 3240 m to In order to discuss our experimental results, we 1616 m. Let us define the probability of survival and momenta for mesotron. The momentum loss due to collision is See O Halpern and H Hall, Phys. Rev. 57, 459(1940) given by the Bethe-Bloch formula 2roNZuelo gmy如H31hw where ro is the classical radius of the electron, N bad. iHewevergebofthr tht cofncideisces ect becepreterere he number of atoms per cm, Z the atomic and the an number,He the rest energy of the electron, B the slowly with the thickness of the velocity of the mesotron, Wm the maximum mental resu