of plant food. I'd like to know about the supply. How much is there
in the air and how much do the crops require?"

"As you know," said Percy, "the atmospheric pressure is about
fifteen pounds to the square inch."

"Yes, I've heard that, I know."

"Well, that means, of course, that there are fifteen pounds of air
resting on every square inch of the earth's surface; in other words,
that a column of air one inch square and as high as the air goes,
perhaps fifty miles or more, weighs fifteen pounds."

"Yes, that is very clear."

"There is only one pound of carbon in ten thousand pounds of
ordinary country air. Now, there are one hundred and sixty square
rods in an acre, and since there are twelve inches in a foot and
sixteen and one-half feet in a rod, it is easy to compute that there
are nearly a hundred million pounds of air on an acre, and that the
carbon in this amounts to only five tons. A three-ton crop of corn
or hay contains one and one-fourth tons of the element carbon; so
that the total amount of the carbon in the air over an acre of land
is sufficient for only four such crops; while a single crop of corn
yielding a hundred bushels to the acre, such as we often raise in
Illinois on old feed-lots or other pieces of well treated land would
require half of the total supply of carbon contained in the air over
an acre. However, the largest crop of corn ever grown, of which
there is an established authentic record, was not raised in
Illinois, but in the state of South Carolina, in the county of