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 projectile passed, causing them then to repel the projectile, which must have added to its velocity.  He seems to have overcome the practical difficulty that in order to obtain service velocities with service projectiles an enormous number of windings and a tremendously long barrel are necessary as well as an abnormally heavy current beyond the safe carrying capacity of the solenoid which would raise the temperature to a point that would destroy the coils.”

He continued turning over the prints and notes in the drawer.  When he finished, he looked up at us with an expression that indicated that he had merely satisfied himself of something he had already suspected.

“You were right, Burke,” he said.  “The final plans are gone.”

Burke, who, in the meantime, had been telephoning about the city in a vain effort to locate Baron Kreiger, both at such banking offices in Wall Street as he might be likely to visit and at some of the hotels most frequented by foreigners, merely nodded.  He was evidently at a loss completely how to proceed.

In fact, there seemed to be innumerable problems—­to warn Baron Kreiger, to get the list of the assassinations, to guard Miss Lowe against falling into the hands of her anarchist friends again, to find the murderer of Fortescue, to prevent the use of the electro-magnetic gun, and, if possible, to seize the anarchists before they had a chance to carry further their plans.

“There is nothing more that we can do here,” remarked Craig briskly, betraying no sign of hesitation.  “I think the best thing we can do is to go to my own laboratory.  There at least there is something I must investigate sooner or later.”