g due
west. Would not the difference in the speed of the wind make a
difference in the speed of the boat?"

"It was for that reason I stated if our rate of travel was the same we
would have made that distance. The wind has been variable at different
points along the coast, so that our average may have been four miles per
hour."

"At what speed has the wind been during the day; I mean the average
speed?"

"Less than eight miles an hour?"

"If the wind had been coming from the east we could have made much
better time, and we might then have been near the mouth of the West
River," was Harry's conclusion.

"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?"

[Illustration: _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
sa