A slip of a girl entered from the library, saw us, paused, and was about to turn back.  Silhouetted against the curtained door, there was health, animation, gracefulness, in every line of her wavy chestnut hair, her soft, sparkling brown eyes, her white dress and hat to match, which contrasted with the healthy glow of tan on her full neck and arms, and her dainty little white shoes, ready for anything from tennis to tango.

“My daughter Gladys, Professor Kennedy and Mr. Jameson,” introduced the captain.  “We are going to try the Z99 again, Gladys.”

A moment later we four were walking to the edge of the cliff where Captain Shirley had a sort of workshop and signal-station.

He lighted the gas, for Lookout Hill was only on the edge of the town and boasted gas, electricity, and all modern improvements, as well as the atmosphere of old New England.

“The Z99 is moored just below us at my private dock,” began the captain.  “I have a shed down there where we usually keep her, but I expected you, and she is waiting, thoroughly overhauled.  I have signalled to my men—­fellows I can trust, too, who used to be with me in the navy—­to cast her off.  There—­now we are ready.”

The captain turned a switch.  Instantly a couple of hundred feet below us, on the dark and rippling water, a light broke forth.  Another signal, and the light changed.

It was moving.

“The principle of the thing,” said Captain Shirley, talking to us but watching the moving light intently, “briefly, is that I use the Hertzian waves to actuate relays on the Z99.  That is, I send a child with a message, the grown man, through the relay, so to speak, does the work.  So, you see, I can sit up here and send my little David out anywhere to strike down a huge Goliath.

“I won’t bore you, yet, with explanations of my radio-combinator, the telecommutator, the aerial coherer relay, and the rest of the technicalities of wireless control of dirigible, self-propelled vessels.  They are well known, beginning with pioneers like Wilson and Gardner in England, Roberts in Australia, Wirth and Lirpa in Germany, Gabet in France, and Tesla, Edison, Sims, and the younger Hammond in our own country.

“The one thing, you may not know, that has kept us back while wireless telegraphy has gone ahead so fast is that in wireless we have been able to discard coherers and relays and use detectors and microphones in their places.  But in telautomatics we have to keep the coherer.  That has been the barrier.  The coherer until recently has been spasmodic, until we had Hammond’s mercury steel-disc coherer and now my own.  Why,” he cried, “we are just on the threshold, now, of this great science which Tesla has named telautomatics—­the electric arm that we can stretch out through space to do our work and fight our battles.”

It was not difficult to feel the enthusiasm of the captain over an invention of such momentous possibilities, especially as the Z99 was well out in the harbour now and we could see her flashing her red and green signal-lights back to us.