on
in place of that by which the girl called it) is like a great safety
valve, which, by permitting the waters to flow northward, saves the
continents from inundation.

"But when mid-Summer arrives, the snows around the pole, having been
completely melted away, the flood ceases and the water begins to recede.
At this time, but for a device which the Martians have employed, the
canals connected with the oceans would run dry, and the vegetation left
without moisture under the Summer sun, would quickly perish.

"To prevent this they have built a series of enormous gates extending
completely across the Syrtis Major at its narrowest point (latitude 25
degrees south). These gates are all controlled by machinery collected at
a single point on the shore of the strait. As soon as the flood in the
Syrtis Major begins to recede, the gates are closed, and, the water
being thus restrained, the irrigating canals are kept full long enough
to mature the harvests."

"The clue! The clue at last!" exclaimed Mr. Edison. "That is the place
where we shall nip them. If we can close those gates now at the moment
of high tide we shall flood the country. Did you say," he continued,
turning to Aina, "that the movement of the gates was all controlled from
a single point?"

"Yes," said the girl. "There is a great building (power house) full of
tremendous machinery which I once entered when my father was taken there
by his master, and where I saw one Martian, by turning a little handle,
cause the great line of gates, stretching a hundred miles across the
sea, to slowly shut in, edge to edge, until the flow of the water toward
the north had been stopped."

"How is the building protected?"

"So completely," said Aina, "that my only fear is that you may not be
able to reach it. On account of the danger from their enemies on Ceres,
the Martians have fortified it strongly on all sides, and have even
surrounded it and covered it overhead with a great electrical network,
to touch which would be instant death."

"Ah," said Mr. Edison, "they have got an electric shield, have they?
Well, I think we shall be able to manage that."

"Anyhow," he continued, "we have got to get into that power house, and
we have got to close those gates, and we must not lose much time in
making up our minds how it is to be done. Evidently this is our only
chance. We have not force enough to contend in open battle with the
Martians, but if we can flood them out, and there