The only possible solution of the mystery of the deviation had suddenly dawned upon him, and a very serious solution it was.

He remembered there were such things as dead suns--the derelicts of the Ocean of Space--vast, invisible orbs, lightless and lifeless, too distant from any living sun to be illumined by its rays, and yet exercising the only force left to them--the force of attraction.

Might not one of these have wandered near enough to the confines of the Solar System to exert this force, a force of absolutely unknown magnitude, upon the _Astronef_?
He went to the desk beside the instrument-table and plunged into a maze of mathematics, of masses and weights, angles and distances.

Half an hour later he stood looking at the last symbol on the last sheet of paper with something like fear.

It was the fatal _x_ which remained to satisfy the last equation, the unknown quantity which represented the unseen force that was dragging them into the outer wilderness of insterstellar space, into far-off regions from which, with the remaining force at his disposal, no return would be possible.
He signalled to Murgatroyd to increase the development of the R.Force from a tenth to a fifth.