But about noon, when they had pushed their little shaft about a dozen feet into the hill, the Doctor ordered a cessation of the digging.
"We must put in some supports for our roof," he said, "or we shall presently be caught in a cave in."
"How are we to do it?" asked Jack.
"Well, I am not a mining engineer," answered the Doctor, "but I've seen enough of the work to know how to protect a little shaft like this, anyhow. The engineers, when they come, will of course tear out all that we do, because they must drive a big shaft into the hill, while all we want to do is to push a little gallery three or four feet wide far enough in to find the best of the coal. But even in doing that we must securely support the roof of our mine. So we'll cut some timber and put it in place. Jack, I wish you would choose the trees to be cut."
"All right!" said Jack. "What dimensions are required?"
"First of all," answered the Doctor, "we want from six to ten pieces of oak, say four feet six inches long and fully twelve inches in diameter. They will serve for roof timbers, and will be enough to carry us thirty or forty feet further. Then for perpendicular supports—one at each end of each timber—we shall need just twice as many perfectly straight oaken sticks eight or nine inches in diameter."
"But why do you want big sticks to go crossways and comparatively little ones for the perpendicular supports?" asked Ed. "The perpendicular timbers must after all bear the weight."
"Oh, that's simple enough," said Tom, whose perceptive faculties were always alert. "You see a stick set up on end, if it is perfectly straight and set true, will bear vastly more weight than a stick of twice or three times its thickness, if laid crossways. In fact a straight eight-inch stick nine feet long, if set on end will support nearly as much as another stick nine feet thick—if there were any sticks that thick—laid lengthwise."
"That's it," said the Doctor. "We want heavy timbers across the top, supported by stout eight- or nine-inch sticks set endwise under them. Now, Jack, select the best trees and we'll all get to work as soon as dinner is over. We'll get the dinner ready while you choose the timber to be cut."
The cutting of the timber was a small task to expert young wood choppers; but it was a very difficult task for the six boys to bring the timbers to the mine and set them in place. True, only two frames had to be set up for the present, but the cross pieces, short as they were, were enormously heavy, and it required all the ingenuity as well as all the strength the boys could command, to get these two frames up, each consisting of one cross piece under the roof and two uprights supporting it.