tial maximum stress-safety
at the hub of something over two-tenths of a gee, and the two finally
settled on one-tenth as well within the limits.

"Now the other limit," said Ishie. "This little piece of plastic will
only stand a pressure approaching the point at which it begins to
distort and run out of the field. This stuff is quoted to have a
compression-yield strength of one hundred ten pounds to the square
inch. We probably shouldn't exceed ... hm-m-m ... ninety pounds. Let's
get the Cow to tell us how big a chunk of surface area that
represents."

The answer was discouraging. Mike rapidly converted the figure in
centimeters to feet, and came up with nearly an eighty-three foot
diameter for a circular surface.

"Looks like we'll have to put it out on the spokes," he muttered in
disgust, but Ishie shook his head quickly.

"No need, Mike. Later on we'll need a few thrust points out on the rim
for good aiming, but we don't have to have all this surface area in
one unit or even in one place. Also, we do not need to consider only
the surface of an homogeneous piece of plastic material.

"This plastic can be cast. Very easily. In it, we can insert
structures that will absorb the strain from many surfaces within,
rather than only on a front surface.

"I expect some of the glass thread with which the hull of the ship was
made could be inserted with no trouble. Each thread, then, would take
up the strain, and a mass of them distributed through the plastic
could deliver a greatly increased amount of thrust from a volume of
plastic rather than from a surface area."

* * * * *

Mike started to object. "To get an absolutely parallel magnetic field,
the gap between the pole faces can't be very wide."

"Perhaps I wasn't considering pole faces," Ishie answered. "Our
investigation has already shown that once initiated the thrust-effect
works best in a very low magnetic field.

"Such a low, parallel magnetic field would quite probably be found
inside of a simple solenoid coil."

"O.K.," Mike answered, "but you have also found that a very high
magnetic field is required to initiate the action. How do you get that
inside a solenoid without an iron core?"

"As you say, a strong field must _initiate_ the action. Let us try
another experiment, Mike."

Ishie turned the Confusor off, selected a piece of wire from Mike's
supplies, and wound a ten-turn coil over the large magnetic coils of
the