These moments were fraught with peril, and not alone peril to the huge machine that Tom Swift had built, but peril to those who remained in the cab of the electric locomotive, as her forward trucks struck the open switch.

There was a mighty jerk that brought a shout from Ned Newton's lips and a grunt from Mr. Damon. Tom clung to his swivel-seat, staring ahead.

The pilot of the electric locomotive shot over on the siding; the forward trucks followed, then the great drivers. The whole locomotive swerved into the siding, but for several breathless seconds Tom was not at all sure that the monster would not jump the rails and head into the ditch!

Meanwhile his gaze measured the speed of that flying figure in the Mackinaw as it scuttled up the slope through the open grove of hard wood and pine. He could not at first see Koku, but he knew the giant was headed for the fugitive, whether the latter proved to be Andy O'Malley or not.

Tom's gaze flashed to what lay ahead of the electric locomotive. As it seemed to joggle back into balance, gain its uprightness, as it were, the inventor saw the great, log-braced bumper between the two rails at the end of the siding. With what force would the locomotive hit that obstruction?

Until the trailers were over the switch Tom dared not give her the brakes. To lock the brake shoes upon the wheels might easily throw the locomotive off the rails. But the instant he felt the tail of the long locomotive swerve off the switch he jabbed the compressed air lever and the wild shriek of the brake shoes answered to his effort.

Then the bumper was but a few yards ahead. The electric locomotive was bound to collide with it. And under the speed at which it had been running, now scarcely reduced by half, the collision was apt to be a tragic happening!

Weeks of effort might be ruined in that moment! If the crash was serious, thousands of dollars might be lost! In truth, Tom Swift apprehended the possibility of a disaster, the complete results of which might put the test of his invention forward for weeks--perhaps for months.

Nor could he do a thing to avert the disaster. He had reversed and set the brakes immediately after the last wheel of the trailer was on the siding. Nothing more could he do as the great electric locomotive bore down upon the solid timber at the far end of this short track.

Those few seconds, as the locked wheels slid toward the end of the siding, were about as hard to bear as any experience the young inventor had ever gone through. It was not so much the peril of the accident, it was the possibility of what might happen to the locomotive.

Within those few moments, however, Tom considered more than the safety of his companions and himself, and more than the peril of wreck to his locomotive. He considered the schedule of the trains on this division of the Hendrickton & Pas Alos and remembered all those that might be within this sector at this time.

If the locomotive smashed into the bumper with force enough to wreck the structure, would some approaching train on the westbound track not be endangered?

The thought was parent to Tom's act before the collision occurred. With a single swift motion he reached for the signaling apparatus which he had established in connection with his wireless telephone.

Just the moment before the head of the locomotive rammed that seemingly immovable barrier at the end of the siding there flashed into the air from Tom's annunciator the code word agreed upon announcing a wreck, and the number of the sector on which the electric locomotive was then running.

The next moment the crash occurred.

Tom had leaped up with a shout of warning. "Hang on!" was his cry. But when the locomotive had struck and rebounded Ned, from far down the aisle of the locomotive, wanted to know in a very peevish tone what he should have hung on to?

"My elbows!" he groaned. "I've skinned 'em, and my back has got a twist in it like the Irishman thought he had when he put on his overalls hind-side to. What's happened?"

"Bless my radiolite!" growled Mr. Damon. "My watch crystal is broken all to finders, if you want to know. Bless my shock- absorbers! you won't do this locomotive a bit of good, Tom Swift, if you stop it so abruptly."

"And that's the surest word you ever said" responded Tom, hurrying to the door. "I don't know what's broken, but we're still on the rails. The most immediate thing to learn, is the where-abouts of the fellow who did this."

"Who opened the switch?" cried Ned.

"I believe it was Andy O'Malley. Come on, Ned! Koku is after him and I don't want him to tear O'Malley apart before I get there."

"O'Malley has got powerful interests behind him, and it might go hard with Koku if he injured the spy and some of these Westerners caught him," suggested Mr. Damon.

"They ought to thank Koku for manhandling the fellow--if he does," said Ned.

"As a matter of fact," replied Tom, "Koku will merely hold to the fellow until we get there. But my giant's strength is enormous, and he does not always know the strength of his grasp. he might hurt the fellow. Come on," and Tom leaped from the doorway of the electric locomotive.

Ned leaped down the ladder after his chum.

"Which way did they go?" he asked.

"Across the ditch and up the hill," said Tom. "Mr. Damon!" he called back to that eccentric man, "will you please remain there and watch the locomotive?"

"I certainly will. And I'm armed, too," shouted Mr. Damon. "Don't fear for this locomotive, Tom. I am right on the job."

Tom waved his hand in reply, leaped the ditch, and started up through the wood. Ned was close behind him, and the two young men ran as hard as they could in the direction Tom had seen Andy O'Malley, followed by the giant, running.

In places the earth was slippery with pine needles, and the ground was elsewhere rough. Therefore the chums did not make much speed in running after the giant and his quarry. But Tom was sure of the direction in which the two had disappeared, and he and Ned kept doggedly on.

They went over the crest of the hill and lost sight of the siding and the locomotive. Here was a sharp descent into a gulch, and some rods away, in the bottom of this gully, the young fellows obtained their first sight of Koku. He was still running with mighty strides and was evidently within sight of the man he had set out after in such haste.

"Hey! Koku!" shouted Tom Swift.

The giant's hearing was of the keenest. He glanced back and raised his arm in greeting. But he did not slacken his pace.

"He must see O'Malley, Tom," cried Ned Newton.

"I am sure he does. And I want to get there about as soon as Koku grabs the fellow," panted Tom.

"He'll maul O'Malley unmercifully," said Ned.

"I don't want Koku to injure him," admitted Tom, and he increased his own stride as he plunged down into the gully.

The young inventor distanced his chum within the next few moments. Tom ran like a deer. He reached the bottom of the gully and kept on after Koku's crashing footsteps. At every jump, too, he began to shout to the giant:

"Koku! Hold him!"

The giant's voice boomed back through the heavy timber: "I catch him! I hold him for Master! I break all um bones! Wait till Koku catch him!"

"Hold him, Koku!" yelled Tom again. "Be careful and don't hurt him till I get there!"

He could not see what the giant was doing. The timber was thicker down here. It might be that the giant would seize the man roughly. His zeal in Tom's cause was great, and, of course, his strength was enormous.

Yet Tom did not want to call the giant off the trail. Andy O'Malley must be captured at this time. He had done enough, too much, indeed, in attempting the ruin of Tom's plans. Before the matter went any further the young inventor was determined that Montagne Lewis' spy should be put where he would be able to do no more harm.

But he did not want the man permanently injured. He knew now that Koku was so wildly excited that he might set upon O'Malley as he would upon an enemy in his own country.

"Koku! Stop! Wait for me!" Tom finally shouted.

Now the young inventor got no reply from the giant. Had the latter got so far ahead that he no longer heard his master's command?

Tom pounded on, working his legs like pistons, putting every last ounce of energy he possessed into his effort. This was indeed a desperate chase.