Doc Deduce Explosive

"What dizzies me," Renny muttered, "is how they fixed their bomb to explode in the air, and not on the ground."

Doc banked his plane, set a course directly for the city of Washington, using not only the gyroscopic compass with which the craft was fitted, but calculating wind drift expertly.

"How they made the bomb explode in the air can be simply explained," he told Renny at last. "They probably put an altimeter or barometer in the bomb. The altimeter would register a change in height. All they had to do was fix an electrical contact to be closed at a given height, and--bang!"

"Bang, is right!" Monk put in, grinning.

- The Man or Bronze (1933) Chapter 7