"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 tri[p. 77]angulation correctly, and do this in our subsequent measurements, unless it should be necessary to make the measurements at sea. The plan followed on shipboard 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 shore, 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 ship. 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 assume 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."