"Then you don't know what coal is, and where it comes from?" asked the Doctor. "Let me explain. There was a period in the world's remote history when the earth was much warmer than it is now--almost hot in fact. The atmosphere was filled with the gases of carbon, and the rains were an almost continuous cataclysm. Human life was impossible in these conditions. No man could have breathed such an atmosphere and lived. But the conditions were peculiarly favorable to abundant vegetable life.

There were forests such as we do not dream of now even in tropical swamps. Ferns grew to the height of great trees, vines and cane and gra.s.s and air plants filled up every available inch of s.p.a.ce, and they all grew in that carbonized atmosphere with a rapidity and luxuriance quite impossible now. All this vegetation died of course and fell to the ground as all vegetation does and has done from the beginning of time.

Wherever it fell into water and was thus shielded from the air, and wherever it managed to get itself covered with earth or rock, as in that highly disturbed volcanic age often happened, it was converted into coal by pressure and by losing certain of its volatile elements, just as charcoal is made by expelling the volatile parts from wood. So, without going any further into details, you see that the coal is preserved vegetation which grew many thousands of years ago, and that the heat we get from it is simply the sunshine it stored up at a period before ever human life existed. What a pity it is that we have to waste so much of it!"

"How do you mean, Doctor?" asked Jack.

"Why you see we waste almost all the heat that coal gives us. If we could make effective use of it all, the burning of a single pound of coal would give us force enough to lift more than eleven and a half millions of pounds a foot from the earth; but the most that we actually get out of it is force enough to lift one and a half million pounds."

"What? All that from one pound of coal?" asked Jim.

"Yes, all that, and it all means so much sunshine which fell upon the earth thousands of years ago. Curious, isn't it?"

"It's simply astounding," said Jack. "But why do we burn coal so wastefully, Doctor? Why can't we utilize more of its heat? And what becomes of the waste heat?"

"Our methods are imperfect," answered the Doctor. "In a big manufacturing city thousands of tons of coal, or what is essentially the same thing, go off into the air every day in the shape of black smoke.

You see the blackness of smoke is nothing but pure carbon or in other words coal. Then again think of the heat that goes up every smoke stack and is wasted in the air. It would run hundreds of great engines if it could be turned to account. And there is all the heat that makes an engine room so horribly torrid. Every bit of that is wasted power.

Little by little, however, we are learning to save the power that coal gives us. A high pressure engine, like an ordinary locomotive, besides wasting coal, wastes greatly more than half the expansive force of its steam. It uses the steam only once and that very imperfectly, and then lets it escape into the open air and go to waste. But the big steamships and many factories have what they call triple or quadruple expansion engines which use the same steam three or four times in propelling the machinery, and then condense it into hot water and send it back into the boiler, thus saving a vast deal of the heat that would otherwise be wasted. Still even they waste most of the heat that their coal produces."

"By the way, Doctor," interrupted Tom, "how much coal does it take to drive one of the big steamers across the Atlantic?"

"From fifteen hundred to three thousand tons," answered the Doctor, "and think what a waste that is when a few hundred tons give force enough to do the work if only the force developed could all be used."

"But how do they manage to carry any freight when they must carry such an enormous load of coal?" asked Ed.

"That is another serious waste," answered the Doctor. "For every ton of coal carried means one ton less of freight. And then, too, think of the expense incurred in putting all that coal aboard. And think too of the cost of feeding and paying wages to a large company of men to handle it after it is on board! For you know besides the stokers who shovel the coal into the furnaces, there are the 'coal trimmers' as they are called, whose duty it is to keep the coal heap properly distributed in the ship. You see a ship is not stiff and rigid like a coal pocket. It would never do to begin at one end of a coal heap and use it as it comes. That would presently leave one part of the ship with no coal load at all, while thousands of tons would burden other parts. No ship that ever was built could stand that. It would twist her out of shape, warp her seams open and send her to Davy Jones in a very little time. So from the moment the stokers begin to shovel coal into the furnaces under a steamship's boilers the coal trimmers and coal carriers must busy themselves with the night and day work of so shifting the coal as to keep its weight properly distributed. But now to come back to what I was saying. Little by little we are learning to save some small part of the enormous waste in the burning of coal. One example will ill.u.s.trate. In smelting iron--that is melting it out of the ore and separating it from the rock stuff,--the waste twenty-five years ago was simply appalling.

The furnaces were mere pots built of fire clay brick, and filled with coal or c.o.ke beneath and iron ore on top. A blast of steam or hot air was sent into them from below to make the fire burn as hotly as possible. Sometimes this blast was strong enough to blow bushels of unburned coal or c.o.ke out at the top. That however was a mere trifle as compared with the other waste. For great flames, nearly hot enough to melt iron, poured out of every furnace top and were lost in the air.

Every bit of that heat represented power that was literally cast to the winds. All that has been greatly improved since. The flames and heat that escape from the blast furnaces are now very generally harnessed and made to do further work. They are used to heat great steam boilers and thus create the power that operates rolling mills and gigantic forges, and vast machine shops. But we still waste very much more than half the heat that coal gives us--often more than nine-tenths of it."

"But, Doctor," said Tom, "If we go on wasting our coal at such a rate, won't we use it all up presently? And will not civilization have to stop then?"

"There are three answers to that," replied the Doctor: "1st. That we shall more and more learn to economize in this matter of heat wasting;

"2nd. That our coal supply in this country seems to be sufficient to last for millions of years yet; and

"3rd. That long before it is exhausted the ingenuity of man will probably discover means of securing power from some other source than coal."

"What, for example?"

"Well, perhaps we shall learn how to utilize terrestrial magnetism directly. You know this earth of ours is a gigantic magnet, and magnetism is the raw material of electricity, if I may so express it. At present we get all the electricity we use out of the earth, but we have to do it by burning coal to run dynamos. Perhaps we shall find ways to save that expense by drawing the electricity directly from the earth. We have already done something closely resembling that, with the result of a great saving."

"How was that?"

"Why when the telegraph was first invented it was necessary to double the wire lines, putting up two wires every time by way of completing the circuit. You know electrical energy will not manifest itself, or as we say, the electric current will not flow, unless there is a circuit established. Well at first they established the circuit by running two parallel wires, one to carry the current one way and the other to bring it back. That's a clumsy way to put it, but it will answer my purpose in explanation. After a while somebody found out that the earth is a better conductor of electricity than any wire could be, and so the circuit was established simply by running each end of a single wire into the ground, making the earth do the work formerly done by the other wire. That simple discovery saved exactly one half the expense of telegraph companies for wires."

By this time it was growing late and as the boys had a hard morrow's work before them the Doctor ceased talking and all went to their bunks.

CHAPTER XLII

_In the Service of the King_

Very early the next morning the boys, who had caught the Doctor's enthusiasm, began again their task of digging through the "out crop"

coal, which began now to grow softer and more workable, while the coal itself grew steadily better in quality.

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.

When night came only one of the two frames was in place, and it was obvious, as Jack said, that "another half day must be wasted on such work" before they could begin mining again. But that evening the Doctor dug two bushels of coal out of the farthest end of the shaft, built a special fire, placed the coal on it, and carefully covered it with earth.

"What are you doing, Doctor?" asked his crony, Tom.

"I'm making a c.o.ke oven, Tom," he replied. "I want to see how our coal will c.o.ke."

"But I don't understand about c.o.ke," answered Tom. "Why is it that when you burn most of the substance out of coal it will make a hotter fire than with all its combustible materials in it?"

"That isn't quite the case, Tom," answered the Doctor. "What we do in making c.o.ke is chiefly to expel the gas from the coal and to roast out the sulphur, which seriously interferes with the making of sufficient heat to smelt iron. Some coal gets burnt up in the process; some makes an indifferent and nearly worthless c.o.ke; while some makes a c.o.ke that would melt the heart of a miser. Now, as I told you the other night, I am convinced as a geologist, that a little further in our mine we shall come to coal so free from sulphur that we can smelt iron with it without making c.o.ke of it at all. But it is always preferable to make c.o.ke of it, and so I'm trying to see what sort of c.o.ke our coal will make. Of course we haven't come to the real coal yet, but I can tell a good deal by what we have now. We'll let my little c.o.ke oven roast all night and in the morning I'll know a great deal more than I do now. But if you have any question in your mind as to the gas making capacity of this coal, I'll remove it at once."

With that he went to the camp fire, seized a blazing brand and applied it to the little mound of earth under which he had buried his coal.

Instantly the whole outside of the mound was aflame.

"That's the gas," said the Doctor. "You see there's plenty of it, even in the imperfect coal that we've reached. It will burn out presently and meantime its heat will help roast my coal into c.o.ke."

After supper the boys again plied the Doctor with questions concerning coal. Tom began it by saying:

"You told us the other evening, Doctor, that the value of a bed of coal depends upon many things besides its location and its accessibility to market. What are those things?"

"Thickness, for one thing," answered the Doctor, "and that is a point in which our mine excels. You see coal seams are of every thickness, from that of a knife blade to beds 100 feet through. Those last are very rare, however. In this country the seams vary from knife blade thickness to about nine or ten feet. Now, in working a coal mine the men, of course, must have room to stand up in the shaft, so that wherever the vein is less than six feet thick a good deal of rock or earth must be removed so as to give sufficient height to the mine. It costs as much to remove the rock or earth as to handle a like amount of coal, and the stuff is worthless. So you see it is greatly more profitable to work a thick than a thin vein. Indeed there are very few veins under three or four feet thick that it pays to work at all. Our deposit here appears to be about nine feet thick, and that means much to us.