h and rock, sometimes hundreds of feet in thickness. There is
always danger that the roof will cave in, and so the rooms are not
made large, and great pillars of coal are left to hold up the roof.

Not many years ago the miner used to do all the work with his muscles;
now machines do most of it. The miner then had to lie down on his side
near the wall of coal in his "room" and cut into it, close to the
floor, as far as his pickaxe would reach. Then he bored a hole into
the top of the coal, pushed in a cartridge, thrust in a slender squib,
lighted it, and ran for his life. The cartridge exploded, and perhaps
a ton or two of coal fell. The miner's helper shoveled this into a car
and pushed it out of the room to join the long string of cars.

[Illustration: HOW A COAL MINE LOOKS ABOVEGROUND

All that shows on the surface is the machinery shed where the various
engines work to keep the air fresh, and bring up the miners and the
coal.]

That is the way mining used to be done. In these days a man with a
small machine for cutting coal comes first. He puts his cutter on the
floor against the wall of coal and turns on the electricity. _Chip,
chip_, grinds the machine, eating its way swiftly into the coal, and
soon there is a deep cut all along the side of the room. The man and
his machine go elsewhere, and the first room is left for its next
visitors. They come in the evening and bore holes for the blasting.
Once these holes were bored by hand, but now they are made with
powerful drills that work by compressed air. A little later other men
come and set off cartridges. In the morning when the dust has settled
and the smoke has blown away, the loaders appear with their shovels
and load the coal into the cars. Then it is raised to the surface and
made ready for market.

Did you ever notice that some pieces of coal are dull and smutty,
while others are hard and bright? The dull coal is called bituminous,
because it contains more bitumen or mineral pitch. This is often sold
as "run-of-mine" coal,--that is, just as it comes from the mine,
whether in big pieces or in little ones; but sometimes it is passed
over screens, and in this process the dust and smaller bits drop out.

The second kind of coal, the sort that is hard and bright, is
anthracite. Its name is connected with a Greek word meaning ruby. It
burns with a glow, but does not blaze. Most of the anthracite coal is
used in houses, and householders will not buy it unless t