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."

CHAPTER XI

A TERRIBLE VOYAGE AND THE SHIPWRECK

The shadows of night were now upon them. How vividly it recalled to
their minds the horrors of the five days and nights during which they
were tossed about in the little lifeboat a year before. Then they were
helpless, and now strong. At that time everything was dark and gloomy,
without a ray of hope. Contrast the situation at this time.

What a gratification it must have been to look back during the past
twe