"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 represents the pressure of air on each square foot of surface, which, multiplied by the sail area, 100 square feet, makes 200 pounds."
"If that is the push when it is normal, what will it be at 45 degrees?"
"Each angle of incidence has its own figure, or coefficient, or for your better understanding, value, and the value at 45 degrees is .666. So that by multiplying 200 by this value, we get a total pressure of 133.2 pounds."
"These figures are used a great deal in flying machines; are they not?"
"Yes; and that is a subject which we might pursue, but there are some things right ahead that may for the present interest us more."
Through the haze which had now settled down, a faint outline of land was made out in the distance. The course was altered to the northeast, and after a quarter-hour sail, land was again espied ahead, so that to avoid the shore the course was taken due north. This was evidence that the land projected northwardly, and the Professor suggested that the effort should be made to chart as accurately as possible the shore line. This could be done mentally.
"I had forgotten to take any note," said Harry. "What is the proper thing to observe in making these calculations?"
"Two things must always be uppermost in the mind of the explorer on the sea: First the time, and second the speed. Time can always be accurately determined, but the question of speed can come by experience only. A good sailor can very accurately determine speed by an examination of the passing water, where the sea is comparatively calm. I have known where the distances have been thus estimated within a hundred feet in each mile in a ten-mile course, and where the speeds were varied along the route. Then, a good observer must have the gift of direction. If he has sailed one hour at a certain speed in a given direction his mental chart may be of the greatest service to him. In our case it would be invaluable. It is a quality well worth our effort to acquire."