Apparently the new device worked fine, for all through the afternoon, at various heights and distances, they kept in perfect touch with Mr. Chadwick.  Every intonation of his voice was borne plainly to their ears, Tom at times taking the wheel and the receivers while Jack relieved him at the engines.

The storm which had threatened the night before, still was hovering about, as was evidenced by the white thunderheads piled on the horizon.  But the electricity in the air did not, as is sometimes the case, interfere with the powerful impulses sent out from workshop and airship.  Although the air felt heavy, the instruments worked perfectly.

The boys flew over hill and dale for more than seventy miles prior to any perceptible weakening in the current.  But once it began to fail it reduced rapidly until the messages were scarcely audible.  But the experiments were kept up till almost dusk, when Mr. Chadwick told the boys to come back.

As they returned the radio ’phones were kept working and as the distance decreased the impulses grew stronger.

“If only I had some of that Z.2.X.,” said Mr. Chadwick, “I believe it would be possible to send a message across the ocean or the continent.”

Not long after this Jack heard again from his father.  It was a commonplace message enough.  Sent merely to keep the air-line in operation.

“Here is Jupe with the afternoon mail,” he said.

“Anything for us?” asked Jack, enjoying the novel sensation of talking through the air concerning such everyday matters.

“Yes, there’s one from Ned Nevins,” was the rejoinder, “and here is one for me from my New York brokers.  Let me see—­ah-h-h-h!”

The last was a sharp exclamation, as if Mr. Chadwick had received a sudden shock.  It was followed by silence.  Again and again Jack flashed the red signaling lamp but there was no reply.

He was seriously worried.  The sudden sharp intake of breath, almost like an outcry, that he had heard, oppressed him with a sense of apprehension.  What could have happened?  Turning to Tom he called for full speed ahead for the trip back.

Tom was not slow in responding.  He speeded the motors up to their top capacity.  In the air there were no speed laws to look out for, or other motorists or pedestrians to avoid.  It was a clear road.  The steel stays and stanchions of the stanch Wonder ship fairly hummed as she shot forward, while an indefinable fear clutched at Jack’s heart.

He knew that his father was subject to fainting spells and he had been overworking recently.  Fast as the Wondership was cutting through the air it felt like an eternity to Jack before the gray walls and the well-laid-out grounds of High Towers came into view.

The boys lost no time in landing, and not waiting to place the Wondership in her shed, set out to look for Mr. Chadwick.  Jupe came shuffling by on his way from the cornpatch.