"Four Miles Within" was a good story, but I am unable to understand why they did not find a lot of stagnant air. Air that had lain stagnant for the time that cavern must have been closed would have killed the person who breathed it. Also, I would imagine that it wouldn't be safe to handle a chunk of radium like the characters in the story did; it's liable to burn. However, it probably wasn't pure radium, just pitchblende-bearing rocks. The rest of the stories were fine. I especially like the stories of the Special Patrol Service which S. P. Wright has created. Let's have some more stories of Commander John Hanson and his crew. "The Exile of Time" has started off quite well and I look forward to the next installments. Cummings is always good for a batch of thrills and some swell adventure, to say nothing of the enjoyable way he introduces science into the story. Wish you would publish this, as I would like to get in touch with some other Science-Fiction fans. By the way, some of the readers seem to want the mag changed, but don't you do a thing to it. All the suggestions, if followed, would make "our" mag like the other S.-F. mags on the market, and I read Astounding Stories because it is DIFFERENT, and I mean every one of those capitals!--Ben Smith, Box 444, Billings, Mont. Mr. Ernst's Answers: (a) No, it has not been actually proved. It has been indicated mathematically (by formulae based on conjecture), but never actually solved--for the very good reason that it is impossible to reproduce spacial conditions in earthly laboratories. Know how an explosive force would react in space? We don't even know positively what space is, let alone how our chemicals and instruments would behave in it. The majority theory is that explosive charges would propel a rocket or space ship more effectively in the (theoretical) emptiness of space, than in our atmosphere. But to my mind it is quite possible that an explosion--a violent expansion of gases causing rapid increase of pressures--would be ineffectual where there are no pressures to be increased. Might not the violently expanding gases fly forth from an exhaust vent to expand instantly, frictionlessly and impotently to the ends of the universe? In my story, "The World Behind the Moon," I assumed that would occur. And no man living is in a position positively to disprove it. And, as a corollary, if a propulsion explosion cannot have effect in empty space, as presumed in the story, the space ship must enter atmosphere before it can stop by firing its bow tubes. Otherwise, with the bow tubes shooting their expanding gases futilely into nothingness, you could go into "reverse" till the cows came home and the ship would hurtle forward just the same.