All at once Rodier descried a cloud of smoke on the horizon far up the strait.  Lifting his binocular, he shouted excitedly—­

“It is a gunboat, mister.  She flies the British flag.”

“We’ve beaten her!” cried Smith.

He was divided between pleasure at his success, and sorrow that the castaways were as yet unrelieved, for he could not doubt that the gunboat was the same that had been dispatched from Brisbane to their assistance.  Before many minutes had elapsed he had overtaken the vessel.  Slowing down and wheeling overhead, he saw that the aeroplane was the object of wondering interest on the crowded deck.

“Ahoy, there!  Who are you?” he shouted through his megaphone.

“Gunboat Frobisher, Captain Warren,” came the reply.  “Who are you?”

“Aeroplane without a name, Lieutenant Smith of H.M.S. Imperturbable, bound for Ysabel Island to relieve Lieutenant Underhill.”

“The dickens!  That’s my job!  Where do you hail from?”

“From London, sir.  I’m afraid I’ve beaten you by a neck.”

“Great Scott!  Is this the Admiralty’s latest?”

“Not official, sir; I’m here in a private capacity.  My father’s among the wrecked party.  I’m on leave.”

“So it seems.  When are you due back?”

“On Friday morning.”

“I’m sorry for you, then.  But, goodness alive! when did you start?  The wreck was only reported four days ago.”

“Started Friday morning, sir.”

“Gammon!”

“Rasher to you, sir.”

“You haven’t lost much time, at any rate.  What’s your speed?”

“About a hundred and ninety.  Whereabouts was the wreck, sir?”

“A hundred miles or so up the coast, according to the men of Underhill’s party with me.”

“Then I’ll bid you good-bye for the present.  I’ll tell him you’re coming.”

“Hope you’ll find him alive.”

Waving a good-bye, Smith flew on at full speed.  For twenty minutes he did not attempt to follow the indentations of the coast, but set a course parallel with its general trend.  Then, however, he steered so that, without actually tracing every curve of the shore, he was able to survey it pretty closely.  By dead reckoning and the assistance of his chart he was able to check from minute to minute his approximate position.

He had passed Mount Gaillard, and saw, some miles to the north, the remarkable saddle shape of Mount Mahaga.  Then he made a bee-line for Fulakora Point.  Rounding this, his course was to the north-west.  The coast was steep and precipitous; here and there were reefs, over which the sea broke in white upward cascades, and he was at no loss to understand how even the most skilfully navigated vessel might easily come to grief.  About forty miles from the extremity of the island he flew over an immense lagoon, extending for several miles between Ysabel Island and a series of islets and reefs lying off the shore.  From