"And what was the experiment?" Dan demanded eagerly.
"Have you ever looked at Mars through a good telescope?" she countered. "Then you must have seen the canals--straight dark lines running from the white polar caps to the equatorial zone. All scientists did not agree as to what they were, but nobody could suggest a natural origin for them.
"My father was one of those who thought that the canals were fertile, cultivated strips, irrigated with water brought down from the melting ice-caps. Irrigation systems meant intelligent life upon the planet, and his experiment was an attempt to communicate with that intelligence."
"Yes. The means was simple enough: other men had suggested it years before, in fact. Any fairly bright light on Mars--such as the beam of a searchlight directed toward earth--would be visible in a good telescope, when the planet is favorably situated: it follows that such a light on earth should be visible to an observer with a similar instrument on Mars.
"It was possible, of course, but unlikely, that Mars would have intelligent inhabitants still ignorant of the telescope. It was also possible that their senses would be different from ours--that, if they saw at all, it would be with a different part of the spectrum. Father took the chance. And he succeeded.
"The call was simple: merely three flashes of light, repeated again and again. We used a portable searchlight, mounted on a motor-truck, such as is used in the army. The three flashes meant that we were on the third planet of the solar system. The answering call, from the fourth planet, should be four flashes, of course.
"For three nights we kept signaling. One of the men watched the motor-generator, and I operated the searchlight, swinging it on Mars and off again, to make the flashes. Dad kept his eye screwed to the telescope. Nothing happened and he got discouraged. I persuaded him to keep on for another night, in case they hadn't seen us at first; or needed more time to get their searchlight ready.
"And on the fourth night poor Dad came out of the observatory, shouting that he had seen four flashes."
Dan gasped, speechless with astonishment. "Then that machine, with the needle pointing at Mars, and the green flashes, and the thing that jumped at me--"
