Fig. 21. Fig. 22. Illustrating Wind Pressure
"No; I had reference to a moving object. I can better explain the phenomenon by illustrating the two conditions: In the drawing (Fig. 21), let A represent a sail with 100 square feet of surface. The darts (1) represent the wind blowing dead against it. This is called the normal position. You will see the darts representing the direction of the movement of the wind. Now look at the next sketch (Fig. 22). Here the sail (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?"