“Where to, first?” queried Frank. “I vote for the Cliff House and Seal Rocks. Here in the guide book it says ‘the seals play sportively in the restless tide.’ And Sutro Baths are nearby, too, I gather—the largest indoor salt water pool in the world.”

All three chums stood on the Market Street sidewalk before the Palace Hotel. The hour was near eleven. The usual early morning fog which had hung over the city, as it does practically every day of the year, had been dissipated for an hour or more. The sky was cloudless and blue, the sunshine brilliant. A brisk breeze blew along the tremendously wide thoroughfare, which is the widest of all the great city streets of the land, so wide, in fact, that it accommodates four street car lines with the width of an ordinary street left over on each side between the outer tracks and the curbs.

“How delightfully cool and exhilarating!” commented big Bob, drawing in and expelling great lungfuls of the crisp air. “I haven’t felt so peppy in days.”

“The guide book says that’s the San Francisco climate,” said Frank. “Cool, snappy days all the year round.”

“Your car, sir,” said a uniformed doorman to Jack.

They looked up to find a handsome limousine drawn to the curb. This was the car they had ordered for the day. The boys moved toward it.

“We ought to decide right now where we want to go,” declared Frank.

Jack had an inspiration.

“I’ll tell you what, fellows,” he said. “Father gave me the name and address of a man who invented some new radio equipment, and advised me to look him up. Suppose we do that, first. Then we can go sightseeing. It just occurred to me. Wonder where that address is.”

He began leafing over the pages of a small memorandum book.

“Here it is. Bender, Silas Bender. 1453 Mission Street. Let’s ask the chauffeur how far away that is.”

After a little discussion, it developed the address given—on the first street paralleling Market to the south—lay on the route to Golden Gate Park, the Cliff House and Seal Rocks, whither the boys wanted to go. Accordingly, all piled into the car and sped away.

Mr. Bender maintained a little equipment store supplying radio apparatus. The shop was empty of customers when the boys arrived, and, at the ringing of the bell on their entrance, a medium-sized man, brisk and alert, came from the rear room outfitted as workshop. His thinning hair was rumpled. He was in his shirt sleeves.

“What can I do for you, gentlemen?” he asked inquiringly.

Jack stepped forward.

“Are you Mr. Bender?”

“I am.”

“Well, I’m Jack Hampton,” said Jack, extending his hand. “Here’s a note from my father. I believe you have met him.”

“Mr. Hampton the engineer?”

Jack nodded.

“Say, I am glad to meet you,” said Mr. Bender enthusiastically. “Yes. I know your father. When he was on the Coast some years ago on his way to Alaska I met him. He’s enthusiastic about radio telephony. We had a number of very pleasant talks. I remember him very well. But here, I’m keeping you standing. Won’t you come back into my workshop and sit down. Bring your friends.”

Jack accomplished the necessary introductions, and they followed Mr. Bender into the room in the rear.

For a time the boys were kept busy examining various radio appliances, which the energetic Mr. Bender kept thrusting at them. All the time he kept up a running fire of comment.

“Now this,” he said, taking up a small device of unusual shape, “is a sound detector. The only similar device in the field so far is the radio compass, but it is clumsy and unreliable. With this device, however, I am quite certain I have solved the problem of locating the point of origin of any strange or unusual sounds in the air.”

Jack gave an exclamation.

“What say?” asked Mr. Bender, turning toward him.

Jack could hardly conceal his impatience.

“How does it work?” he asked eagerly.

“Well, suppose we wanted to locate the point of origin of some strange message heard at the radio station out at Golden Gate Park. First, we would use a sound detector there, and find out along what line the strange sound came to the station. It might be up the coast or down, or east, southeast or northeast. Suppose it came from down the coast, or south. Then, at a point southeast of this city, we would again apply the sound detector and again at a third point south of the second. When at all three stations, the strange sound was loudest, we would have three bearings upon the point of its origin. Where they intersected, the——”

“The smuggler’s cove would be located,” said Frank quick-tongued.

The next moment he was covered with confusion as Mr. Bender regarded him blankly. So intent had the inventor been upon the description of his device and the method of its operation that he was aware only of an interruption but did not realize the nature of it.

Jack and Bob glared at Frank.

“Eh?” said Mr. Bender. “What say?”

“I just said something about the point of origin being where the lines intersected,” declared Frank, considering it wise to withhold the whole truth, inasmuch as the matter of the smugglers was not his to divulge.

“Yes, certainly,” said Mr. Bender, abstractedly. “Yes, project imaginary lines from each station and where they intersect will be the station you are hunting.”

Abruptly he put aside the sound detector as if, now that he had explained its operation, it were of no more value.

“Here,” he said, taking up a suitcase, and swinging it around, “is a radio receiving device that can be carried easily in this small suitcase. And here”—putting down the suitcase before the boys could examine it and taking up a finger ring from a workbench—“is the smallest receiving set I have yet devised. It is, as you see, in the shape of a ring and can be worn without the presence of the device being suspected.”

“Mr. Bender,” said Frank, “will you excuse my friends and me for a few moments while we step aside and have a little confab. I believe we will have a proposal to make that will interest you.”

“I know what you mean,” said Frank, as Mr. Bender withdrew, leaving them alone. “That sound detector, hey? If the Secret Service man had that he would be able to locate the smuggler’s cove.”

“That’s it, exactly,” said Jack. “Inspector Burton said he would not be leaving for Los Angeles until tonight. I believe we ought to get hold of him at once and tell him about this possibility.”

“I’m with you,” said Bob. “But we don’t know how to reach him. Suppose I call Father at the office of his business representative, and ask him to get Inspector Burton.”

“Good idea,” said Jack. “I didn’t know just how to work it. But if your father gets Inspector Burton to come up here, we will not be revealing anything to Mr. Bender, and the inspector can tell as much or little as he wants.”

“Then I’ll telephone father,” said Bob. “I saw a telephone in the store when we came in. I suppose Mr. Bender will let me use it.”

“And I’ll explain as much as necessary to Mr. Bender,” said Jack.

Accordingly, he called the inventor back to the workroom while Bob telephoned Mr. Temple, and explained they were inviting a man to come up and talk to him about the sound detector.

“I can’t tell you any more than that now, Mr. Bender,” said Jack. “But I promise you, of course, that your invention is not in any danger of being stolen. On the contrary, the man we have asked to come here may put you in the way of making your fortune.”