il (B) is put at an angle of forty-five degrees from the direction of
the wind. The sail is still the same size vertically, but it is somewhat
smaller horizontally across the line (C), this diminution in size being
about one-seventh of the entire area. The darts (D) in both cases
represent the movement of the boat, and the darts (2) in the last sketch
show the wind striking the sail at an angle."

"In the first sketch the darts (1) strike the sail normally, as you say,
in what way do the darts (2) in the next figure strike the sail?"

"At an angle of incidence. If you will notice the behavior of the wind
in the first view it will be seen that the wind curves around the edges
of the sail, and strikes against the back of it, and thus produces the
retarding effect I referred to. On the other hand, by examining the
second sketch, the darts (2) plainly show their course across the sail
diverted from their straight source, and behind the bulging sail the air
does not press against the sail, but tries to continue in a straight
line. As a result a partial vacuum is formed along the region designated
by E, and this produces a most effective pull, since the sail constantly
tries to move forward and fill this vacuum. Is this made clear to you?"

"I can plainly see now what the action of the air is, but does the air
push just as hard against each square foot when it is at an angle as
when it is blowing against it straight?"

"That is a good observation, and one that might ordinarily be
overlooked. No, it does not, but the difference can be readily
calculated."

"Then supposing the sail to be 10 feet square, and the wind is blowing
against it straight, as in the first sketch, at the rate of twenty miles
an hour; what pressure would there be against the entire sail?"

"At that speed of wind the pressure on each square foot of surface is 2
pounds, and this multiplied by 100 equals 200 pounds."

"When it is at forty-five degrees, what is the pressure on each square
foot?"

"This is determined in the following manner: Square the speed of the
wind, which means multiplying 20 by 20, and this produces the square,
400. In mathematics, as in many of the sciences, a constant is employed.
A constant is a figure which never varies. In this case the constant is
designated by the decimal .005. That means 5/1000th, or reduced to its
lowest denomination, 1/200th. If, now, we divide 400 by 1/200, the
result will be 2 pounds. This figure thus