Shortly afterwards, Max, having delivered his message to the Arabian chief, was standing beside Carl under the shadow of a cluster of trees on the very margin of the pool. That wonderful instrument, the vibrative drum, which is fashioned somewhat on the principle of the human ear, but with a large horn-shaped receptacle for receiving the very minutest sound waves, and focussing them on to a very sensitive drum, was engaging their attention.
Every now and then, when they fancied they heard a sound that broke the stillness of the desert, they would listen acutely, turning the horn this way and that way to discover whence came the sound.
"They are due about mid-day, the chief says," remarked Carl, after a brief pause in their conversation. "What time do you make it now?"
"A quarter of an hour yet," responded Max, consulting his chronometer, and making a rapid calculation to allow for the difference in longitude, for he still carried Central European time.
"And they're sure to follow the 30th parallel?"
"Yes, it's their shortest route," replied the wireless expert.
"Then they should pass within three or four miles from here," observed Carl.
"Yes, unless they've drifted a little out of their course."
"But we should hear them on the vibrator even if they were fifty miles away in a silent land like this."
"Undoubtedly."