I think you can find the text online - which I would not normally recommend, but I am not sure if it is in print these days.
It's a wonderful story. Better yet, considering that Clarke was an RAF boffin in WW2, the villain of the piece - the scientist responsible for the astounding but impracticable weapons - is Professor-General Norden.
I'm sure the fact that he has the same name as the USAAF's precision bombsight, which arrived in Europe heralded as the supersecret war-winning weapon, is a complete coincidence.