"Why do you think we should have made better time?" asked the Professor.
"Because we should then have been going with the wind."
"You are entirely wrong in your assumption. Sailing ships travel faster when tacking than when sailing with the wind."
The boys looked at the Professor in astonishment.
"It does not seem possible," replied George, "that any movement of the wind pushing sidewise could be more effective than a pressure straight ahead. Can you explain the reason for the statement?"
"When the wind blows straight against a sail, certain eddies are produced which cause a convolute stream around its edges. These currents are counter to the forward movement of the vessel. Assuming that this normal pressure of the wind is 1,000 pounds, it is estimated that fully half is lost in effectiveness. On the other hand, if the ship is moving forward at right angles to the direction of the wind, and the sail is set at forty-five degrees, that is what is called a tack; while it has only about six-sevenths the surface that it had when going with the wind, the sail is constantly going into new wind and, therefore, the pressure is a constant one and most efficiently applied to the surface."
"Do you mean by this that if I hold up a sail so that the wind blows flat against it, the pressure will not be as great as if I held it at an angle?"
