"There is another thing about inter-planar travel that you ought to remember," said Bristol. "When a ship returns to our universe, it causes a wide area disturbance; you have probably heard it called space shiver or the bong wave. The beta universe is so much smaller than our own alpha that you can imagine a spaceship when shifted toward it as being several beta light-years long. Now, if you think of a ship, moving between the alpha and beta lines on this envelope, as getting tangled in the dotted lines that connect the points on the two lines, that would mean that it would affect an area smaller than its own size on beta--a vastly larger area on alpha. "So when a ship returns to alpha, it 'twangs' those connecting lines, setting up a sort of shock in our universe covering a volume of space nearly a parsec in diameter. It makes a sort of 'bong' sound on your T.V. set. Naturally, this effect occurs simultaneously over the whole volume of space affected. As a result, when an invader arrives, using inter-planar ships, we know instantaneously he is in the vicinity. Unfortunately, his sudden appearance and the ease with which he can disappear makes it impossible, even with this knowledge, to make adequate preparations to receive him. Even if he is in serious trouble, he has gone again long before we can detect the bong." * * * * * "Well, dear," said Anne. "As usual, I'm sure you have made me understand perfectly. This time you did so well that I may still remember what stitching is by tomorrow. If the Oracle means anything at all by his statement, I suppose it means that we can use stitching to help defend ourselves, just as the invaders are using it to attack us. But the whole thing sounds completely silly to me. The Oracle, I mean." Anne Bristol stood up, put her hands on her shapely hips and shook her head at her husband. "Honestly," she said, "you men are all alike. Paying so much attention to a toy you built yourself, and only last week you made fun of my going to a fortune teller. And the fuss you made about the ten dollars when you know it was worth every cent of it. She really told me the most amazing things. If you'd only let me tell you some of...." "Darling!" interrupted John with the hopeless patience of a harassed husband. "It isn't the same thing at all. Buster isn't a fortune teller or the ghost of somebody's great aunt wobbling tables and blowing through horns. And Buster isn't just a toy, either. It is a very elaborate calculating machine designed to think logically when fed a vast mass of data. Unfortunately, it has a sense of humor and a sense of responsibility."