"No, not the lamp, but what is in the lamp."
"The oil? Where can we get anything to mark with in the oil?"
"We have been making a marking material all the time we have been in the cave, and you are just as well acquainted with it as anything you know. It is the soot from the burning oil."
The boys laughed, not at the information, but at their stupidity.
"Yes; the soot is carbon, and the best soot is made by imperfectly burning oil, or fat, or any other fuel which has a large amount of carbon."
The boys found several pieces of flat chalk, one of them a little over a foot long. This was held above the flame of the candle until covered with soot.
"The first thing to do is to establish a base line. This we will mark A on the drawing. Now, starting from the point here where we lost the light, you may measure along the line to the west, we will say, until you get to the other end of the chamber. Twenty-five measures of the spear? That makes the chamber 125 feet long, and it is about 90 feet the other way. Let us roughly outline the floor plan. Now go out toward the mouth of our cave, and measure off 50 feet. Stop there. You see, I have marked the line A and have laid down the slate slab so that this line is exactly on a line with the one you have measured along the chamber. The point B, 50 feet from here, which you have just measured, makes an angle, C. I will now take the slab to the point B, and you may measure off 50 feet more, and we will call that D. That gives us another angle line, E. You see, at every point we establish a new base line. C is the base line for the line E, and so on all through the cave."
