Sir: P. M. S. Blackett, the winner of the Nobel prize in physics in 1948, believes that thousands of atomic bombs would be needed to produce decisive results in a war with the Soviet Union; William L. Laurence, New York Times reporter whose articles about the bomb won a Pulitzer prize for him in 1946, believes that fifty bombs would suffice. This is an astonishing range. Blackett's estimate may be the result of more deliberate study than Laurence's, but the newspaperman's figure agrees better with the impressions given by most American physicists in their discussion of the effectiveness of the atomic bomb. Which estimate is closer to the truth? Assuming other factors to be about equal, what would be the order of magnitude of the number of bombs necessary to determine the outcome of a war between two great continental powers?