like water waves, and the shattering effect comes
only at the crests, so that all the windows might be broken in a house,
say, half a mile from an explosion, and no windows be broken in a house
two hundred yards nearer. The first house would have been smitten by a
destructive wave crest, the second passed over by a harmless wave
trough. And, by the way, when windows are broken by these blasts of
concussion, it appears that they are usually broken _outward_, not
inward, and that the fragments are found on the ground outside the
house, not on the floors inside. The reason of this is that the
concussion waves leave behind them a partial vacuum, and windows are
broken by the air _inside_ houses rushing out.

"How about thunder-storms?" I asked.

"There is always danger," said the expert, "and all hands hurry out of
the works as soon as the lightning begins to play. If a bolt struck a
lot of dynamite it would set it off."

Then he explained that the policy of dynamite manufacturers is to handle
explosives in small quantities, say a ton at a time, each lot being
finished and hauled away in wagons before another lot is started. This
is possible because of the short time occupied in making dynamite. He
assured me, for instance, that if there were only raw materials at the
works on a certain morning when the seven-o'clock whistle blew, it would
be perfectly possible to have a ton of dynamite-cartridges finished,
packed in boxes, and loaded on freight-cars by nine o'clock.

After this some one told of a thrilling happening in the mixing-house,
by the great vat, wherein nitroglycerin is mixed with porous earth,
called dope, and becomes dynamite. Over this vat four men work
continually, two with rakes, two with hoes, kneading half a ton or more
of explosive dough to the proper consistency.

One day a powder-car loaded with heavy stone got loose on its track a
quarter of a mile up the slope, and started down the steep grade. The
tracks ran straight into the mixing-house. The switch was open, and the
first thing these men knew, there was an angry clang at the switch, and
then a swift, heavy car was plunging toward the open door, with every
chance that it would set off twelve hundred pounds of dynamite there.
Workmen outside shouted, and then stared in horror. Not a man in the
mixing-house moved. All four kept their places around the vat, held
tight to their rakes and hoes, while the car, just missing the dynamite,
hurled its ma