She reached out one slim hand, and gently patted his knee. "About the Beckerley electrical fields. What are they? How do they operate?"

HE FROWNED thoughtfully, trying to phrase his answer in words that a girl would understand.

"It's an application of Morelle's experiment with electrical eddy currents," he said. "I'm a botanist, not an electrical engineer, but I know that the idea involves the refraction of lines of magnetic force.

"It works like this : Over each army on the Alaekan front the scientists have created a dome of electrical tension — a magnetic field. Any shell or drop bomb passing through this magnetic field is instantly heated to white heat by the electrical eddy currents induced in it by the field, and is thus caused to explode in mid-air.

"Every city is likewise protected by a Beckerlev field. You know how each auto-copter, on leaving the city limits of San Francisco, has to stop and be pushed through an iron-shielded subway until it's oeyond the Beckerley field?