at his friends. Finally he broke into their thoughts which had
been too cryptically abbreviated for him to follow, like the work of a
professor solving some problem, his steps taken so swiftly and so
abbreviated that their following was impossible to his students.

"But how is it that the machine is not moved when exerting such force on
some other body?" he asked at last.

"Oh, the ray concentrates the gravitational force, and projects it. The
actual strain is in space. It is space that takes the strain, but in
normal cases, unless the masses are very large, no considerable
acceleration is produced over any great distance. That law operates in
the case of the pulled body; it pulls the gravitational field as a
normal field, the inverse-square law applying.

"But on the other hand, the gravity-beam pulls with a constant force.

"It might be likened to the light-pressure effects of a spotlight and a
star. The spotlight would push the sun with a force that was constant;
no matter what the distance, while the light pressure of the sun would
vary as the inverse square of the distance.

"But remember, it is not a body that pulls another body, but a
gravitational field that pulls another. The field is in space. A normal
field is necessarily attached to the matter that it represents, or that
represents it as you prefer, but this artificial field has no connection
in the form of matter. It is a product of a machine, and exists only as
a strain in space. To move it you must move all space, since it, like
artificial matter, exists only where it is created in space.

"Do you see now why the law of action and reaction is apparently
flouted? Actually the reaction is taken up by space."

Arcot rose, and stretched. Morey and Wade had been looking at him, and
now they asked when he intended leaving for the intergalactic spaces.

"Now, I think. We have a lot of work to do. At present we have the
mathematics of the artificial matter to carry on, and the math of the
artificial gravity to develop. We gave the Sirians all we had on
artificial matter and on moleculars.

"They gave us all they had--which wasn't much beyond the artificial
gravity, and a lot of work. At any rate, let's go!"

Chapter XIII

ATTACKED

The _Ancient Mariner_ stirred, and rose lightly from its place beside
the city. Visible over the horizon now, and coming at terrific speed,
was a fleet of seven Thessian ships.

They must do their best to protect