Part 9 (2/2)
DISCOVERY OF THE ISLAND AND EXPLORATIONS
Weary, they dragged themselves over the cliffs and down to the waiting natives. Muro was surprised to learn that they had found no signs of people, so they rowed back to the s.h.i.+p, and after a hearty meal, retired for the night.
In the morning when they asked John for his opinion, he said, ”The only course is to sail south, and circ.u.mnavigate the island. In doing so we shall effect a landing every ten miles or so, and then go into the interior. This will thus enable us to learn all about the land. At the same time we must survey the island, so as to learn its extent, as well as its general shape and outline.”
”But how can we survey it without the instruments?”
”That is readily done, by observing the headlands, or some special coast line marks, and then taking the angles from those points.”
”Well, that will be interesting, at least. How shall we start?”
”Do you see that point to the south which may be five or ten miles away?”
”Yes.”
”Now, examine the compa.s.s, and turn it so that the cardinal points are directly north and south. Now sight across the face of the compa.s.s so that you get the exact line between this point and yonder object. What do you make it to be?”
”Why I make it out to be S. E.”
[Ill.u.s.tration: _Fig. 5. Measuring by Triangulation._]
”That is correct. The line 1 is south by east.”
”But how can we find out how long line 1 is?” asked Harry.
”Why by triangulation,” said George, quickly.
”I know that, but how can we do it on sea?”
”It can be done on sea, as well as on land, but we had better go and make the first measurement by triangulation correctly, and do this in our subsequent measurements, unless it should be necessary to make the measurements at sea. The plan followed on s.h.i.+pboard will be found similar to the plan followed on land.”
The boat was manned and the boys with a crew of the men and John made for the sh.o.r.e, and together they went inland to a point marked B (Fig.
5), and sighted across to the same object C that was noted of the s.h.i.+p.
This, then, gave three lines, 1, 2 and 3, forming a triangle.
”If these angles are placed on a paper the distance from A to C can be determined on the principles of proportion,” remarked John.
”How is that done?”
”We will a.s.sume that the lines 1, 2, are at right angles to each other.
This is not necessary, but it happens to be so in this case. Let us first measure the distance along the line 2, which may be any number of inches, or feet. Suppose we call the line one inch long. Then draw the line 1, so that it will be sufficiently long to be sure and meet the line 3.”
”Yes; I now see how it is done,” remarked George, with enthusiasm. ”If the line 3 is drawn at the angle we got, when we looked at C, from B, the line will cross line 1 at C.”
”That is correct. Now, if it is one inch from A to B, we may use that inch as a measurement to get the distance along the line.”
<script>