"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 r.e.t.a.r.ding 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?"
"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 sh.o.r.e 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 sh.o.r.e 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 pa.s.sing 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 twelve months and mentally calculate what they had accomplished. They had delved in many of the hidden mysteries of nature and learned the secrets. Such knowledge had been put to use. They had discovered many things that gave them pleasure, but in doing so found others that startled and grieved them. Things inexplainable and impossible to fathom had crossed their paths on almost every side.
But they were now doing the work of men. The Professor knew how they had developed, and grown brave and strong. He knew it better than the boys could realize themselves. What a source of pleasure it must have been to the kindly faced, gray-haired Professor, as he looked at his charges in admiration and love. Could anything be more inspiring than the contemplation of the work he had done?
And now the inevitable charting board was brought out, and the plan adopted which would enable them to trace the coast line. It was explained that all sailing was by the points of the compa.s.s, and for this purpose the compa.s.s was made to correspond with the regulation instrument. This is shown in Fig. 23.
[Ill.u.s.tration: _Fig. 23. MARINER"S COMPa.s.s_]
The four cardinal points are north, south, east and west. Exactly midway between each of the four points are the divisions designated northeast, southeast, southwest and northwest. Then, again, intermediate, the last divisions and the cardinal points are other markings which show that the angles are nearer one of the cardinal points than the other, so that a course may be marked off, by the compa.s.s, which, if followed for a certain time, and the speed of that period determined, can be traced and thus marked out on paper so that the outline of the coast can thereby be laid out.
[Ill.u.s.tration: _The Northern Sh.o.r.e OF WONDER ISLAND. Chart showing Voyage in "No. 3." Fig. 24._]
The Professor had the charting board before him. "I have marked our starting point, which is designated as A. It will be remembered that we marked a course due west, pa.s.sing the headland three miles from Cataract River. This is line 1. When we saw the land ahead of us last night, we changed our course by the compa.s.s to northwest, thus making a new line of travel, which you see is designated as 2. B was the point where the turn was made."
[Ill.u.s.tration: _Fig. 25. THE CHARTING BOARD_]
"But in what manner did you know how to transfer it to the board?"
"The board has two lines crossing each other at right angles to correspond with the two lines on the compa.s.s. The compa.s.s was put on the middle of the board, and the upper board turned so that the needle was on a line with the N----S line."
The boys now noticed for the first time that there were two boards, one above the other, and that the lower one was a little larger, and was attached to the boat. "Why do you have the lower board attached to the boat and larger than the upper one?"
"The lower board has on it a section of the compa.s.s, and the upper end a pointer, as you notice, and the line T represents the boat"s direction, so that when the compa.s.s was placed on the upper board, the latter was turned so that it corresponded with the points of the compa.s.s. The little pointer then accurately pointed to northwest, on the lower board, and by this means the changing of the upper board, so as to keep it due east and west at all times, will enable us to keep on our course."
It was a long and weary night. One of them slept while the others were on duty. The boys knew the time on shipboard, where the day begins at midnight, and is divided into watches of four hours each, thus making three watches for the night and three for the day. A bell is struck every half hour, so that each watch is noted by the eight strokes of the bell. What is called the dog-watches occur between 4 and 8 P.M., this period being divided in the first dog-watch between 4 and 6, and the second dog-watch between 6 and 8.
"Why should such peculiar times be taken, or the periods be divided up in that way?"
"For the very reason that we discussed the arranging of our time of watch last night; namely, so that one person would not have the same watch every night. It was agreed by us that one should have three hours"
uninterrupted sleep, while the others were on duty, so that each would in turn get three hours" work. Our arrangements are somewhat different from shipboard time, on account of our number, but the principle is the same."
During the night the wind changed to the north, so that progress was slow and required considerable tacking, and when the Professor came on duty he found the course still to the north, and on questioning Harry, found that the wind had been rather regular during his watch. Within a half hour of the time he took charge the western sh.o.r.e faded away, and the course was directed to NWW, in which direction they continued until well along in the forenoon. Then, as the land receded again, another tack was made, WbS, which means West by South.
The winds, however, were perverse during the second day. After a calm the wind veered to the west, and when in the afternoon the course was changed to SSW they had to sail close to the wind, and made slow progress.
Let us see what they found on the day"s journey along the northern point which they rounded the second day of the journey. Cliffs, like their own, were distinctly visible in the evening and during the morning of the second day, but when morning broke on the third day they saw a beautiful sh.o.r.e line, and beyond the mountain range which was seen by them on their land trip to West River. It was now certain that the mouth of that river had been pa.s.sed during the night and all regretted this.
During the whole of the third day they were forced to sail in the teeth of the wind, which necessitated frequent tacking. Not a sign of human habitation was seen on this day, but during the night, when the boys were on duty, they declared that they had seen lights to the south. The interest was most intense. Were they really rounding the island?
The course for the opening morning of the fourth day was SWbW, and early in the day they were not more than a mile from the sh.o.r.e, and then the sh.o.r.e gradually receded, but the course was not changed. The wind began to blow with greater force, and came from the southwest. As night approached it increased, but they continued tacking, hoping that they might reach the western extremity, and thus be able to run to the east before the wind.
Before eight o"clock lights were distinctly visible. They had reached the vicinity of human habitations. The boys were too excited to think of the watches which had thus far been observed. Aside from that, the wind had now reached such a violent stage that it was impossible to make any headway against it.
A consultation was had. "We must either turn to the north or go to sh.o.r.e. I fear this sudden change in the direction of the wind," was the Professor"s opinion of the situation. It would not be wise to risk the sh.o.r.e. All knew that and sadly they turned the craft to the north. It was well that they did so. Every moment, it seemed, some new impetus would be given the wind. It howled on every side of them; the waves drifted across the little ship, until everything was dripping with moisture, and the only dry spot was within the little housed enclosure which had been well protected on the open rear side, thanks to the watchful suggestions of the Professor.
To add to their discomfort of mind, the land was not in sight. There was nothing to steer by except the compa.s.s and the chart which had been laid out. They were now going north over the course that had been traversed for the past two days--the west coast of the island.
It was a long, long night. No one slept, because fear and anxiety was ever with them. They remembered now with vividness the days spent on the ocean when they were wrecked. It was a terrible succession of hours, with the wind and the lightning and the rain one continuous orgy. The Professor sat at the tiller. The sails had been taken down long before.
The impact of the driving storm against the housed structure was sufficient to drive it forward, so that the vessel could be guided.
It seemed that every blow against the boat would wrench it to atoms, but if any part had given way it was not apparent. Harry, who sat nearest the housed structure, suddenly sprang up, and pointing down, cried out: "It is leaking; look how it boils!" It was forethought on the part of the Professor to put in a supply of the oak.u.m used for caulking purposes. Harry sprang for it, and George grasped the bailing pan. After a struggle a sufficient amount was driven into interstices to keep out at least a portion of the seepage. This knowledge was most oppressive.
When a boat of this kind once springs a leak, due to a severe wrench of the sh.e.l.l itself, it is a difficult matter to remedy it, without structurally strengthening it.
Morning was now appearing, and still no land appeared in sight. More leaks appeared, and the boys were now constantly bailing and repairing.
The Professor had held the tiller for more than six hours, but he did not appear to be exhausted. At every attempt of the boys to relieve him, he only said that they had more important work in bailing and caulking.
He finally changed the course due east, and it was more by luck than exact calculation that they made out the northern end of the island which was pa.s.sed the first night out. In one night they had traveled a distance coming back that required two days and nights to traverse in the other direction. As it was they were headed for the cliffs at the point of land, and it must be avoided.
Harry saw the danger, and went back to consult the Professor. He had the tiller firmly in his grasp, and his body bent over it to keep it steady; but when Harry reached him, and touched him, there was no response.
Almost frantic, he cried to George: "Come here, quickly; something is the matter!"