e longer bits, and as the design of
the new guns contemplated a barrel at least eighteen inches long, the
bits had to be longer, in proportion, and the making of these consumed
nearly as much time as the actual drilling out of the barrels.

George and the Professor put in a great deal of time with the new team.
Their knowledge of training, in view of the former experience with these
animals, was such that within a week they could drive the yaks without
much difficulty, although the new team was not by any manner of means as
efficient as the lost one.

When the question of the kind of material for the guns came up, Harry
was much concerned, as in making the barrels that length would
necessarily greatly increase the weight.

"I think it would be better to make an alloy for your purposes," said
the Professor, as they were discussing the matter.

"What is an alloy?"

"It is the combination of two or more metals."

"In what way does the alloy make it better than the hardest steel?"

"It is not hardness you want, but toughness. Metals have several
properties, which are utilized for various purposes in the arts.
Surprising as it may seem, wood has greater resisting power than
diamond, and yet the precious stone is the hardest of all substances."

"But if we unite two metals are we not then making a new metal?"

"Not necessarily so. In the case of brass it is true. This is made by
uniting two parts of copper and one of zinc. Both copper and zinc in
themselves are very soft, and copper cannot well be polished in its pure
state. Brass, however, is not only much harder, but is susceptible of a
very fine polish."

"Are the alloys of all metals harder than the metals of which they are
made?"

"This seems to be a universal law in the compounding of metals. Very few
metals are used alone in the various arts and manufactures. For every
purpose some combination has been found which makes the product better.
Even coins are so alloyed. Silver and gold in the form of money would be
entirely too soft, unless alloyed with some hardening metal. Some
substances, like arsenic, antimony and bismuth, are too brittle to be
used alone. The only metals which can be used alone are aluminum, zinc,
iron, tin, copper, lead, mercury, silver, gold and platinum."

"What is bronze, of which all the ancient guns were made?"

"That is a combination of copper and tin. This product was known fully
seven hundred years before the Christian era, and