“So, that’s how he does it,” mused Kennedy, fingering the can contemplatively.  “He lets the ether evaporate in a room for a while and then causes an explosion from a safe distance with this little electric spark.  There’s where your wire comes in, McCormick.  Say, my man, you can switch on the lights from downstairs, now.”

As we waited for the watchman to turn on the lights I exclaimed, “He failed this time because the electricity was shut off.”

“Precisely, Walter,” assented Kennedy.

“But the flames which the night watchman saw, what of them?” put in McCormick, considerably mystified.”  He must have seen something.”

Just then the lights winked up.

“Oh, that was before the fellow tried to touch off the ether vapour,” explained Kennedy.  “He had to make sure of his work of destruction first—­and, judging by the charred papers about, he did it well.  See, he tore leaves from the ledgers and lighted them on the floor.  There was an object in all that.  What was it?  Hello!  Look at this mass of charred paper in the corner.”

He bent down and examined it carefully.

“Memoranda of some kind, I guess.  I’ll save this burnt paper and look it over later.  Don’t disturb it.  I’ll take it away myself.”

Search as we might, we could find no other trace of the firebug, and at last we left.  Kennedy carried the charred paper carefully in a large hat-box.

“There’ll be no more fires to-night, McCormick,” he said.  “But I’ll watch with you every night until we get this incendiary.  Meanwhile I’ll see what I can decipher, if anything, in this burnt paper.”

Next day McCormick dropped in to see us again.  This time he had another note, a disguised scrawl which read: 

Chief:  I’m not through.  Watch me get another store yet.  I won’t fall down this time. 
                       A. Spark.

Craig scowled as he read the note and handed it to me.  “The man’s writing this time—­like the second note,” was all he said.  “McCormick, since we know where the lightning is going to strike, don’t you think it would be wiser to make our headquarters in one of the engine-houses in that district?”

The fire marshal agreed, and that night saw us watching at the fire-house nearest the department-store region.

Kennedy and I were assigned to places on the hose-cart and engine, respectively, Kennedy being in the hose-cart so that he could be with McCormick.  We were taught to descend one of the four brass poles hand under elbow, from the dormitory on the second floor.  They showed us how to jump into the “turn-outs”—­a pair of trousers opened out over the high top boots.  We were given helmets which we placed in regulation fashion on our rubber coats, turned inside out with the right armhole up.  Thus it came about that Craig and I joined the Fire Department temporarily.  It was a novel experience for us both.