Don't miss the next Galaxy Magazine! THE BIG ENGINE by Fritz Leiber CRITICAL MASS by Pohl & Kornbluth THE RAG AND BONE MEN by Algis Budrys And many more, including Willy Ley science column and the great conclusion of Poul Anderson's THE DAY AFTER DOOMSDAY! February _Galaxy_ on sale December 10th--ask your newsdealer to reserve it for you! Transcriber's Note This etext was produced from _Worlds of If_ November 1961 and January 1962. Extensive research did not uncover any evidence that the U.S. copyright on these publications was renewed. The chapter headers of the second instalment, originally starting from X, have been correctly numbered. Minor spelling and typographical errors have been corrected without note.