It was shortly after midnight when he was awakened by hearing a rattling on the window of his room.  The reason he was able to fix the time so accurately was because as soon as he awakened he pressed a little electric button, and it illuminated the face of a small clock on his bureau.  The hands pointed to five minutes past twelve.

“Humph!  That sounds like hail!” exclaimed Tom, as he arose, and looked out of the casement.  “I wonder if any of the skylights of the airship shed are open?  There might be some damage.  Guess I’d better go out and take a look.”

He had mentally reasoned this far before he had looked out, and when he saw that the moon was brightly shining in a clear sky, he was a bit surprised.

“Why—­that wasn’t hail,” he murmured.  “It isn’t even raining.  I wonder what it was?”

He was answered a moment later, for a shower of fine gravel from the walk flew up and clattered against the glass.  With a start, Tom looked down, and saw a dark figure standing under an apple tree.

“Hello!  Who’s there?” called the lad, after he had raised the sash.

“It’s I—­Mr. Jenks,” was the surprising answer.

“Mr. Jenks?” repeated Tom.

“Yes—­Barcoe Jenks, of Earthquake Island.”

“You here?  What do you want?”

“Can you come down?”

“What for?”

“Tom Swift, I’ve something very important to tell you,” was the answer in a low voice, yet which carried to Tom’s ears perfectly.  “Do you want to make a fortune for yourself—­and for me?”

“How?” Tom was beginning to think more and more that Mr. Jenks was crazy.

“How?  By helping me to discover the secret of Phantom Mountain, where the diamonds are made!  Will you?”

“Wait a minute—­I’ll come down,” answered Tom, and he began to grope for his clothes in the dim light of the little electric lamp.

What was the secret of Phantom Mountain?  What did Mr. Jenks really want?  Could he make diamonds?  Tom asked himself these questions as he hastily dressed to go down to his midnight visitor.

CHAPTER III—­A STRANGE STORY

“Well, Mr. Jenks,” began Tom, when he had descended to the garden, and greeted the man who had acted so strangely on Earthquake Island, “this is rather an odd time for a visit.”

“I realize that, Tom Swift,” was the answer, and the lad noticed that the man spoke much more calmly than he had that evening at the jewelry shop.  “I realize that, but I have to be cautious in my movements.”

“Why?”

“Because there are enemies on my track.  If they thought I was seeking aid to discover the secret of Phantom Mountain, my life might pay the forfeit.”

“Are you in earnest, Mr. Jenks?”

“I certainly am, and, while I must apologize for awakening you at this unseemly hour, and for the mysterious nature of my visit, if you will let me tell my story, you will see the need of secrecy.”