d fuel on the
night that there wasn't a frost and he wouldn't have let his crop freeze
on the nights that the temperature really did drop below the danger
point. For example, boys, if the League of the Weather had been in
existence at that time and could have given good frost warnings, all
that crop would have been saved, wouldn't it?"

"Yes, sir," said the boys, "it would."

"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 t