o 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