"Of course," the Forecaster continued, "a really progressive fruit-grower ought to make himself partly independent of the Weather Bureau. He can put up frost-alarm thermometers."

"What are they, sir?" asked Anton.

"They're thermometers with an electrical attachment, something on the principle of the thermostat, which you see nowadays in big buildings. A thermostat is electrically connected with a tiny lever, and when the air of a room gets to a certain heat, the increasing temperature operates a lever and closes the steam pipe which brings the heat. When the temperature falls below a certain point, the lever is released and the steam rises again. The same principle is used as a fire alarm. When the air inside a building rises to a point hotter than it could naturally do, it operates a lever which rings an alarm bell. The frost thermometer acts exactly on the same principle. When the temperature of the air, near a fruit orchard, falls to within three or four degrees of the point at which the fruit will be harmed, the fall of the mercury breaks an electric circuit which starts an alarm bell ringing in the owner's house, perhaps a half mile away."

"I've been wondering," began Anton in his meditative way, "whether it wouldn't cost more to heat all the out-of-doors than it would be to lose some of the fruit."

"You haven't got the idea of it at all," the weather expert said briskly. "It's got nothing to do with heating the whole of out-of-doors."

"Then what are the fires for?"

"Just to heat a very small section of the air on the ground. Don't forget, boys, that a fruit tree ten feet high may have all the fruit on its lower branches, up to five or six feet, absolutely killed off, while the top branches are unharmed."

"How's that?" queried Ross in surprise. "I thought frost came down from on top, and that the higher up you went the colder it would be."

"Not at all," the weather expert answered. "Frost comes from down below.

When the air is still and clear, the earth loses heat by radiation. The heat goes up and up and through the air to higher levels, the cold earth cooling the air below. Therefore, on a frosty night, in a region where frosts are rare, or at a time of year when frosts are few, a still clear night will cause a belt of cold air perhaps only a few inches in depth, perhaps ten or twenty feet in height, this belt being several degrees colder than the air overhead.

"Now, Ross, you can see that to light huge fires, with the intention of warming up all the air, would be foolish and unnecessary. All that is needful is to heat this lower cold belt of air, a few feet in depth, and only to heat it the three or four degrees necessary to bring it to the warmth of the air above."

"But suppose a wind comes up and blows the heat away?" asked Anton.

The Forecaster smiled at the question.

"If a wind comes up," he answered, "you wouldn't need to use any heat at all, because the wind would mix the warmer air overhead with the cooler air below and there couldn't be any killing frost."

"But doesn't it cost an awful lot?"

"It costs less than to lose your crop," the weather expert replied.

"Usually you can figure that a frosty night will take a gallon of oil per tree, or from twenty to twenty-five cents. In a fruit growing section a grower is unlikely to have more than four or five still, freezing nights a year when his crop may be ruined by frost, so that he will spend a dollar or so per tree in protecting his orchard. As there are few fruit trees which bring in a profit of less than ten dollars during the season, and some a great deal more--according to the nature of the crop--the proportionate expense of heating is small compared with the amount of fruit saved."

"Then you think that heating an orchard will save the fruit?"

"Absolutely without any question," the weather expert answered. "And, if the fruit-grower will keep in close touch with the Weather Bureau, he will know when precautions are necessary. Of course, boys, it's especially important for this work that there are a number of co-operative observers, because frost is not a widespread general phenomenon. You could have a fearful killing frost down in the hollow where Anton's house is, or in the low ground near your house, Ross, and still Tom's place, on that little hill, would be quite safe. One of the things that the League of the Weather ought to be able to do this winter and spring is to see that frost is fought. Even when your fathers haven't got regular oil-pots, boys, a few smudges with heavy smoke, drifting over the orchards or the truck fields, if started early enough in the evening may check a freeze."

"Why, sir?" asked Ross. "Smoke isn't hot."

"No, my boy. But you remember that I told you that the cold was caused by the radiation of heat from the earth escaping into the air and through it. If there's a steady layer of smoke, like a blanket, floating across the land, the heat radiating from the earth will not have a chance to escape to the upper air. It will stay in the lower layer of the air and thus keep it from dropping to the killing temperatures of a true freeze. That's what the Indians of the pueblos used to do."

In the mild winters and early springs of Issaquena County, there seemed little reason for the boys of the League to trouble themselves with frost warnings, but, at the Forecaster's urgency, the boys kept wide awake for it. It happened, though, that the lads had talked so much about their frost protection plans that several of the farmers decided to get some oil-burning fire-pots for use that spring, in the event of a freeze. Jed Tighe, however, one of the few people of the neighborhood who had shown but a perfunctory interest in the League, laughed to scorn the idea of buying the fire-pots, as Fred had suggested in a recent issue of the _Review_. Even Jed Tighe read the little sheet every week, in spite of his alleged scornfulness.

One afternoon, when Ross was over at the club-house, where he spent so much of his spare time, Anton pointed out that the conditions were ripe for a killing frost.

"The hottest to-day was sixty-two degrees," he said, "and you remember Mr. Levin told us that one wasn't ever safe unless the maximum was sixty-four. There's not a cloud in the sky anywhere and there's practically no wind, and what there is, Tom told me over Bob's wireless, is from the northwest, and that's the worst quarter. I was just going to take the dew-point when you came in."

"Let's do it now, Anton," said Ross. "Got the cup?"

For answer the crippled lad took down from the shelf a small tin mug. It was already bright and shining, but he polished it until it looked like silver.

"I've got the jug of ice-water ready," he said.

Pouring some tap water into the cup, and filling it about one third full, he began to stir it round and round with a thermometer. The mercury in the tube quickly dropped, until it read 50, showing the temperature of the water.

"Now, Ross," said Anton, "pour in the ice-water slowly."

Ross picked up the pitcher and began to let the water trickle in a tiny stream into the bright tin cup. Anton went on stirring.

Steadily the mercury descended in the tube as the water in the cup grew colder and colder. Ross poured in more and more slowly. Then suddenly, quite suddenly, while both boys were watching, the brightness of the tin cup clouded over, as though with a sudden fog. Anton drew out the thermometer and looked at it.

"The dew-point's only thirty-four," he cried, "and as we've got to figure frost at three or four degrees lower, it'll be so cold that there won't be any fog to stop a freeze. Ross, it's just the night for a killing frost. What do you think we'd better do?"

The older lad hesitated.

"If you don't mind, Anton," he said, "I'll stay to supper, and we'll see what your night observations say."

By evening the threats of a frost were even more definite and the two boys consulted what had best be done.

"I can easily get Father to start his fire-pots," said Ross, "we got them all fixed up this winter. Bob's dad has got some fruit, and we can warn him by wireless, and we could get a lot of the fellows together. I don't want to make a mistake, though. If we suggest that the fire-pots ought to be started and then it doesn't freeze, we'll hurt the League a lot more than we'll help it."

"I wish we could talk it over with Mr. Levin," said Anton, "but he's down with one of his sick spells and we oughtn't to disturb him.

Whatever we do, we've got to do it on our own."

"Let's get Bob here," suggested Ross, "he's got a steady head."

"And Fred," Anton added, "he's read all the Weather Bureau stuff on Frosts, I know. He's been writing his articles for the _Review_ from them."

"All right," said Ross, "I'll slip over and call for Fred and you get Bob on the wireless and ask him to come over here."

An hour later, the four boys were poring over the weather maps, comparing notes and observations and trying to decide whether they ought to do anything. Fred, always ready to take up something new, was for plunging ahead, on the chance that there might be frost, but doubted whether a frost was likely. Ross, as head of the League, was a little timid and afraid to make a serious mistake. Anton was firmly convinced that a killing frost would come before morning. Bob settled it.

"Better for the League to be laughed at than chance having the crops ruined," he said.

This turned the scale, and from a discussion of the advisability of frost warning, the question turned to the best way of letting people know. It was decided that Bob should return to his wireless, get as many of his connected operators in touch as possible and get them to warn their districts. Fred, who had persuaded his father to install a 'phone, was to get in touch with the few farmers in the district who had telephones and ask them to spread the warning. Anton was to borrow his father's buggy and drive to points not reached in any other way, and Ross was to go on his pony. By this means, the county would be fairly well covered. The boys were just separating, when Bob stopped.

"Jed Tighe!" he said.

"Oh, let the old skinflint go," said Fred, "there isn't any way of reaching him, any way."

"That doesn't seem quite fair," said Ross, dubiously, "he's got more fruit than anybody else."

"It isn't fair," said Bob.

"I've been wondering," said Anton, "if we oughtn't to notify Jed Tighe somehow."

"We've got to," said Bob.

"And only get rowed at for our pains," declared Fred.

This was so likely that all the boys felt the truth of the remark and there was a moment's silence.