But we did not hear. He ran after us, but such a chase was hopeless. He stopped, in despair.

With a gesture of vexation he took a step or two mechanically off the road.

Elaine and I were coming fast to the bridge now.

In their hiding-place, Del Mar's men were watching breathlessly. The leader was just about to press the plunger when all of a sudden a branch in the thicket beside him crackled. There stood the farmer and his dog!

Instantly the farmer seemed to take in the situation. With a cry he threw himself at the man who had the plunger. Another man leaped at the farmer. The dog settled him. The others piled in and a terrific struggle followed. It was all so rapid that, to all, seconds seemed like hours.

We were just starting to cross the bridge.

One of the men broke away and crawled toward the plunger box. Our car was now in the middle of the bridge.

Over and over rolled the men, the dog doing his best to help his master. The man who had broken away reached toward the plunger.

With a shout he pushed it down.

Our car had just cleared the bridge when we were startled by a terrific roar behind us. It was as though a thousand tires had blown out at once. Elaine shut off the engine automatically and we looked back.

The whole bridge had been blown up. A second before we had been in the middle of it.

As the explosion came, the men who had been struggling in the thicket, paused, startled, and stared out. At that instant the old farmer saw his chance. It was all over and he bolted, calling the dog.

Along the road to the bridge he ran, two of the men after him.

"Come back," growled the leader. "Let him go. Do you want us all to get caught?"

As the farmer ran up to the bridge, he saw it in ruins. But down the road he could see Elaine and myself, sitting in the car, staring back at the peril which we had so narrowly escaped. His face lighted up in as great joy as a few moments before it had showed despair.

"What can that have been?" asked Elaine, starting to get out of the car. "What caused it?"

"I don't know," I returned, taking her arm firmly. "But enough has happened to-day. If it was intended for us, we'd better not stop. Some one might take a shot at us. Come. We have the car. We can get out before any one does anything more. Let's do it. Things are going on about us of which we know nothing. The safest thing is to get away."

Elaine looked at the bridge in ruins and shuddered. It was the closest we could have been to death and have escaped. Then she turned to the wheel quickly and the little car fairly jumped ahead.

"Oh, if Craig were only here," she murmured. "He would know what to do."

As we disappeared over the crest of the next hill, safe, the old farmer and his dog looked hard at us.

The silence after the explosion was ominous.

He glanced about. No one was pursuing him. That seemed ominous, too.

But if they did pursue he was prepared to elude them. They must never recognize the old farmer.

As he turned, he deliberately pulled off his beard, then plunged again into the woods and was lost.

CHAPTER IX

THE SUBMARINE HARBOR

It was not long after the almost miraculous escape of Elaine and myself from the blowing up of the bridge on the sh.o.r.e road that Del Mar returned from his mysterious mission which had, apparently, taken him actually down to the bottom of the sea.

The panel in the wall of his library opened and in the still dripping submarine suit, holding under his arm the weird helmet, Del Mar entered. No sooner had he begun to remove his wet diving-suit than the man who had signalled with the heliograph that we had found Del Mar's message from "below," whatever that might mean, entered the house and was announced by the valet.

"Let him come in immediately," ordered Del Mar, placing his suit in a closet. Then to the man, as he entered, he said, "Well, what's new?"

"Quite a bit," returned the man, frowning still over Elaine's accidental discovery of the under-water communication. "The Dodge girl happened to pick up one of the tubes with a message just after you went down. I tried to get her by blowing up the bridge, but it didn't work, somehow."

"We'll have to silence her," remarked Del Mar angrily with a sinister frown. "You stay here and wait for orders."

A moment later he made his way down to a private dock on his grounds and jumped aboard a trim little speed boat moored there. He started the motor and off the boat feathered in a cloud of spray.

It was only a moment by water before he reached the Dodge dock. There he tied his boat and hurried up the dock.

Elaine and I arrived home without any further experiences after our hairbreadth escape from the explosion at the bridge.

We were in doubt at first, however, just what to do about the mysterious message which we had picked up in the harbor.

"Really, Walter," remarked Elaine, after we had considered the matter for some time, "I think we ought to send that message to the government at Washington."

Already she had seated herself at her desk and began to write, while I examined the metal tube and the note again.

"There," she said at length, handing me the note she had written. "How does that sound?"

I read it while she addressed the envelope. "Very good," I replied, handing it back.

She folded it and shoved it into the envelope on which she had written:

Chief, Secret Service, Washington, D. C.

I was studying the address, wondering whether this was just the thing to do, when Elaine decided the matter by energetically ringing the bell for Jennings.

"Post that, Jennings, please," she directed.

The butler bowed just as the door-bell rang. He turned to go.

"Just a minute," I interrupted. "I think perhaps I'd better mail it myself, after all."

He handed me the letter and went out.