IN a recent communication, S. Chapman1 has calculated, on the basis of the theory of proportionality between magnetic moment and angular momentum developed recently by P. M. S. Blackett2, that the ratio MM/ME between the magnetic dipole moment of the moon and that of the earth is 1/38,400. It may be readily shown that if this result is correct the magnetic field of the moon can only affect particles of primary cosmic radiation of very small energy, which are able to reach the earth only in the immediate vicinity of the geomagnetic poles. In fact, we have3 which on replacing MM/ME by Chapman‘s value, taking for the ratio DM/RE (DM, distance from the earth to the moon; RE, radius of the earth) its known value, and using for rM the limiting value rM = 0·414, gives rE = 0·05 millistormers, or approximately 0·1 MeV. (1 MeV. = 106 eV.) if the primary particles are protons, 0·3 MeV. if electrons.