closed as tightly as the rest, projected a bit. On its face was a
little typewritten card bearing the inscription: "E-M GUN."

He pulled the drawer open and glanced over the data in it.

"Just what is an electro-magnetic gun?" I asked, interpreting the
initials on the drawer.

"Well," he explained as he turned over the notes and sketches,
"the primary principle involved in the construction of such a gun
consists in impelling the projectile by the magnetic action of a
solenoid, the sectional coils or helices of which are supplied
with current through devices actuated by the projectile itself. In
other words, the sections of helices of the solenoid produce an
accelerated motion of the projectile by acting successively on it,
after a principle involved in the construction of electro-magnetic
rock drills and dispatch tubes.

"All projectiles used in this gun of Fortescue's evidently must
have magnetic properties and projectiles of iron or containing
large portions of iron are necessary. You see, many coils are
wound around the barrel of the gun. As the projectile starts it
does so under the attraction of those coils ahead which the
current makes temporary magnets. It automatically cuts off the
current from those coils that it passes, allowing those further on
only to attract it, and preventing those behind from pulling it
back."

He paused to study the scraps of plans. "Fortescue had evidently
also worked out a way of changing the poles of the coils as the