The boys were still at the construction site-Bob had kept an eye on the road to be sure of that. His leg gave a twinge as he mounted his bicycle. He realized with amusement that the doctor had forgotten the injury entirely. He wished he could himself. The ride did not take long, and he noted with satisfaction that there was already a fair-sized stack of loot at the point where the other machines had been left. He parked his own at the same point and sought his friends.

The four boys, it turned out, had called a temporary halt in the search for materials. They were on the hillside at the top of the wall which Bob had seen poured. The concrete had set and the forms were now being placed in preparation for the side walls; the boys were leaning over and looking down the smooth expanse of concrete. Bob, joining them, saw that the attraction was a crew of men busy with some peculiar apparatus at the bottom. They all wore breathing masks, but the man in charge was recognizable as Malmstrom's father. They seemed to have a pressure pump connected by flexible tubing to a drum of some liquid, which, in turn, was fed to a nozzle. One of the men was spraying the liquid on the concrete and the others were following with blowtorches. The boys had a rough idea of what was going on—many of the bacteria used in the tanks produced extremely corrosive substances, either as intermediate or final waste products. The glaze being applied to the wall was meant as protection from these. It consisted, actually, of one of the fluorine-bearing "plastics" developed a few years previously as a by-product of uranium isotope separation research; it was stored in the drum with one of the standard inhibitors and polymerized into a glassy varnish almost at once when this was boiled out. The fumes of the inhibitor were rather unhealthful, which was why the men were masked.

The boys, thirty feet above the scene of action, got an occasional whiff of the fumes. Not even the Hunter recognized the danger, but others did, and did something about it.

"First a sunburn that nearly toasts you alive and now this. You don't care much what happens to you any more, do you?" The group turned and looked up in surprise, to see the tall form of Bob's father looming over them. They had last noticed him well out on the floor of the tank, apparently busy, and none of them had seen him head their way. "Why do you suppose Mr. Malmstrom and his crew are wearing masks? You'd better come along with me. You may be safe enough at this distance, but there's no sense taking a chance on it" He turned and led the way along the wall, and the boys followed silently.

At the end of the finished section Mr. Kinnaird waved a hand at the far section of mold. "I'll meet you down there in a couple of minutes. I have to drive home to pick up something, and if you'd care to load up your loot in the jeep I'll drive it down to the creek with you." He watched the youngsters head downhill at top speed and descended to the floor himself via one of the diagonal braces.

He picked up the T-shirt he had removed for comfort • and stowed by one of the power saws, donned it, and walked down to the point he had indicated, where the jeep was parked. Only his son awaited him there, the others had gone on to the pile of material they had collected. Mr. Kinnaird sent the jeep along their trail, coasting most of the way.

The loading was quickly accomplished; the boys already had their hands full of the smaller scraps, and Mr. Kinnaird himself managed the longer stuff in a single armload. Then he headed on down the road, the five bicycles following. The boys, of course, made a race of it; the distance being short, they were not very far apart at the end, and even the jeep had not had to wait long for them.

Mr. Kinnaird, seeing the boys doff shoes and roll up their trouser legs, followed suit, and, with the same load of lumber under his arm, splashed behind them down the creek to the scene of operations. He looked over the skeleton of the boat, made a few constructive suggestions, and returned the way he had come, slapping as he went. "I think you kids keep trained bugs around, to discourage company," he said. The boys answered in the same vein, and finally got to work.

They paused to swim more than occasionally, and it was during one of these swims that the Hunter learned why human beings avoided jellyfish. Bob failed to do so at one point, and his guest became intimately acquainted with the nettle cells of the Coelenterata. He blocked the spread of their poison, not because he felt that his host should be encouraged to ignore the creature, but in a half-sentimental recollection of the mistakes of his first day on earth. He felt that he was paying for knowledge.

In spite of sundry interruptions a good deal of work was accomplished in the first hour or so. Then another boat made its appearance, and Charles Teroa was with them, to the intense interest of the detective and his host.

"Hi, sleepy!" Rice greeted the newcomer boisterously, waving a saw in welcome. "Having a last look around?"

Teroa eyed him in none too friendly a fashion. "It's a pity that built-in danger signal of yours can't be seen by your tongue," he remarked. "You fellows having boat trouble again? Seems to me you got it fixed once." Four eager pairs of lungs vied in giving him the story, Rice suddenly fading into silence. The visitor simply looked at him when they had finished, and the expression on his brown face changed from annoyance to amusement. Nothing he, could have said would have conveyed his thoughts more clearly, or made Rice feel sillier. Relations were a little strained between the two for the half-hour Teroa remained.

There was much talk and little work during that half-hour-Teroa enlarging on his future in great detail, with Hay and occasionally Colby contributing remarks in between times. Bob, whose knowledge of the doctor's intentions made him rather uncomfortable, said little; he spent most of the time reminding himself that it was for Teroa's own good. Rice had had his batteries silenced in the first exchange, and even Malmstrom was less talkative than usual. Bob put it down to the fact that he had always been on closer terms with Teroa than the others, and didn't like the idea of his friend's leaving. Sure enough, when Teroa returned to his boat, Malmstrom went with him, asking Colby to take home the bicycle he had left where the creek and road met.

"Charlie says we're going to meet the barge and get a tow around to the fields. He wants to see the fellows on the barge and then come back over the hill by the new tank and see the folks there. Ill go with him and walk home. I may be late, but what the heck."

Colby nodded, and the two departed, pulling strongly out into the lagoon to intercept the scavenger barge, which was making one of its periodic trips around the tanks. The others watched silently until contact was made.

"It's fun to ride him, but I'm sorry to see him go," remarked Rice at length. "Still, he'll be back every so often. Shall we get back to the boat?"

There was muttered agreement, but enthusiasm for the work seemed to have died out for the time being. They pottered around for a while, went swimming again, sawed a couple of boards to length, and eventually startled their parents by appearing at home well in advance of the eve-rung meal.

Bob, instead of settling down to schoolwork after supper, went out again. To his mother's casual question he replied that he was going "down to the village." It was true enough, and he had no intention of worrying his parents by telling them he wanted to see Dr. Seever. The doctor had said that the next drug would not be tried until tomorrow, and Bob himself had nothing specific to tell or ask; but he was uneasy about something, and could not himself decide just what. The Hunter was a good and trustworthy friend, no doubt, but he was not even at the best of times an easy being to converse with; and Bob simply had to talk.