Almost as though some giant hand had dropped an immense cloak over the fire in the barn, so did the blaze die down instantly after Tom Swift's extinguishing liquid had been dropped into the seething caldron of flame. For a moment there was even no smoke, but as the embers remained hot and glowing for a time, though the flames themselves were quenched, a rolling vapor cloud began to ascend shortly after the first cessation of the fire. But this only lasted a little while.

"You've turned the trick, Tom!" cried Ned, leaning far over to look at what was left of the barn and its contents.

"Bless my insurance policy, I should say so!" exclaimed Mr. Damon. "It was certainly neat work, Tom!"

"It does look as if I'd struck the right combination," admitted Tom, and he felt justifiable pride in his achievement.

"Look so! Why, hang it all, man, it is so!" declared Ned. "That fire went out as if sent for by a special delivery telegram to give a hurry-up performance in another locality. Look, there's hardly any smoke even!"

This was so, as the three occupants of the rapidly moving airship could see when Tom circled back to pass again over the almost destroyed structure. He had waited until it was almost consumed before dropping his chemicals, as he wished to make the test hard and conclusive. Now the fire was out except for a few small spots spouting up here and there, away from the center of the blaze.

"Yes, I guess she doesn't need a second dose," observed Tom, when he saw how effective had been his treatment of the fire. "I had an additional batch of chemicals on hand, in case they were needed," he added, and he tapped some unused bombs at his feet.

"I call this a pretty satisfactory test," declared Ned. "If you want to form a stock company, Tom, and put your aerial fire- fighting apparatus on the market, I'll guarantee to underwrite the securities."

"Hardly that yet," said Tom, with a laugh. "Now that I have my chemical combination perfected, or practically so, I've got to rig up an airship that will be especially adapted for fighting fires in sky-scrapers."

"What more do you want than this?" asked Ned, as his chum prepared to descend in the speedy machine.

"I want a little better bomb-releasing device, for one thing. This worked all right. But I want one that is more nearly automatic. Then I am going to put on a searchlight, so I can see where I am heading at night."

"Not your great big one!" cried Ned, recalling the immense electric lantern that had so aided in capturing the Canadian smugglers.

"No. But one patterned after that." Tom answered.

"Bless my candlestick!" exclaimed Mr. Damon, "what do you want with a searchlight at a fire, Tom? Isn't there light enough at a blaze, anyhow?"

"No," answered the young inventor, as he made his usual skillful landing. "You know all the big city fire departments have searchlights now for night work and where there is thick smoke. It may be that some day, in fighting a sky-scraper blaze from the clouds at night, I'll have need of more illumination than comes from the flames themselves."

"Well, you ought to know. You've made a study of it," said Mr. Damon, as he and Ned alighted with Tom, the latter receiving congratulations from a number of his friends, including members of the Shopton fire department who were present to witness the test.

"Mighty clever piece of work, Tom Swift!" declared a deputy chief. "Of course we won't have much use for any such apparatus here in Shopton, as we haven't any big buildings. But in New York, Chicago, Pittsburgh and other cities--why, it will be just what they need, to my way of thinking."

"And he needn't go so far from home," said Mr. Damon. "There is one tall building over in Newmarket--the Landmark. I happen to own a little stock in the corporation that put that up, along with other buildings, and I'm going to have them adopt Tom Swift's aerial fire-fighting apparatus."

"Thank you. But you don't need to go to that trouble," asserted Tom. "My idea isn't to have every sky-scraper equipped with an airship extinguisher."

"No? What then?" asked Mr. Damon.

"Well, I think there ought to be one, or perhaps two, in a big city like New York," Tom answered. "Perhaps one outfit would be enough, for it isn't likely that there would be two big fires in the tall building section at the same time, and an airship could easily cover the distance between two widely separated blazes. But if I can perfect this machine so it will be available for fires out of the reach of apparatus on the ground, I'll be satisfied."

"You'll do it, Tom, don't worry about that!" declared the deputy chief. "I never saw a slicker piece of work than this!"

And that was the verdict of all who had witnessed the performance.

With the successful completion of this exacting test and the knowledge that he had perfected the major part of his aerial fire-extinguisher--the chemical combination--Tom Swift was now able to devote his attention to the "frills" as Ned called them. That is, he could work out a scheme for attaching a searchlight to his airship and make better arrangements for a one-man control in releasing the chemical containers into the heart of a big blaze.

Tom Swift owned several airships, and he finally selected one of not too great size, but very powerful, that would hold three and, if necessary, four persons. This was rebuilt to enable a considerable quantity of the fire-extinguishing liquid to be stored in the under part of the somewhat limited cockpit.

This much done, and while his men were making up a quantity of the extinguisher, using the secret formula, and storing it in suitable containers, Tom began attaching a searchlight to his "cloud fire-engine," as Koku called it.

The giant was aching to be with Tom and help in the new work, but Koku was faithful to the blinded Eradicate, and remained almost constantly with the old colored man.

It was touching to see the two together, the giant trying, in his kind, but imperfect way, to anticipate the wishes of the other, with whom he had so often disputed and quarreled in days past. Now all that was forgotten, and Koku gave up being with Tom to wait on Eradicate.

While the colored man was, in fact, unable to see, following the accident when Tom was experimenting with the fire extinguisher, it was hoped that sight might be restored to one eye after an operation. This operation had to be postponed until the eyes and wounds in the face were sufficiently healed.

Meanwhile Rad suffered as patiently as possible, and Koku shared his loneliness in the sick room. Tom came to see Rad as often as he could, and did everything possible to make his aged servant's lot happier. But Rad wanted to be up and about, and it was pathetic to hear him ask about the little tasks he had been wont to perform in the past.

Rad was delighted to hear of Tom's success with the new apparatus, after having been told how quickly the barn fire was put out.

"Yo'--yo' jest wait twell I gits up, Massa Tom," said Rad. "Den Ah'll help make all de contraptions on de airship."

"All right, Rad, there'll be plenty for you to do when the time comes," said the inventor. And he could not help a feeling of sadness as he left the colored man's room.

"I wonder if he is doomed to be blind the rest of his life," thought Tom. "I hope not, for if he does it will be my fault for letting him try to mix those chemicals."

But, hoping for the best, Tom plunged into the work ahead of him. He did not want to offer his aerial fire extinguisher to any large city until he had perfected it, and he was now laboring to that end.

One day, in midsummer, after weary days of toil, Tom took Ned out for a ride in the machine which had been fitted up to carry a large supply of the chemical mixture, a small but powerful searchlight, and other new "wrinkles" as Tom called them, not going into details.

"Any special object in view?" asked Ned, as Tom headed across country. "Are you going to put out any more tree fires?"

"No, I haven't that in mind," was the answer. "Though of course if we come across a blaze, except a brush fire, I may put it out. I have the bombs here," and Tom indicated the releasing lever.

"What I want to try now is the stability of this with all I have on board," he resumed. "If she is able to travel along, and behave as well as she did before I made the changes, I'll know she is going to be all right. I don't expect to put out any fires this trip."

In testing the ship of the air Tom sent her up to a good height, heading out over the open country and toward a lake on the shores of which were a number of summer resorts. It was now the middle of the season, and many campers, cottagers and hotel folk were scattered about the wooded shore of the pretty and attractive body of water.

Tom and Ned had a glimpse of the lake, dotted with many motor boats and other craft, as the airship ascended until it was above the clouds. Then, for a time, nothing could be seen by the occupants but masses of feathery vapor.

"She's working all right," decided Tom, when he found that he could perform his usual aerial feats with his craft, laden as she was with apparatus, as well as he had been able to do before she was so burdened. "Guess we might as well go down, Ned. There isn't much more to do, as far as I can see."

Down out of the heights they swept at a rapid pace. A few moments later they had burst through the film of clouds and once more the lake was below them in clear view.

Suddenly Ned pointed to something on the water and cried:

"Look, Tom! Look! A motor boat in some kind of trouble! She's sinking!"