four o’clock in the
morning
His heart stood still . `` Great God ! What could have caused that bell to ring ? '' It could not ring ; it was a physical impossibility unless somebody were handling the sister instrument , and at four o'clock in the morning , who could be there , and except one , who would know its working ? With a bound he was by the aerophone and had given the answering signal . Then instantly , as though she were standing at his side in the room , for this machine does not blur the voice or heighten its tone , he heard Stella speaking . `` Is it you who answer me ? ''