McCormick passed his hand over his forehead to brush off the perspiration.  “I guess this thing has got on my nerves,” he muttered hoarsely.  “Everywhere I go they talk about nothing else.  If I drop into the restaurant for lunch, my waiter talks of it.  If I meet a newspaper man, he talks of it.  My barber talks of it—­ everybody.  Sometimes I dream of it; other times I lie awake thinking about it.  I tell you, gentlemen, I’ve sweated blood over this problem.”

“But,” insisted Kennedy, “I still can’t see why you link all these fires as due to one firebug.  I admit there is an epidemic of fires.  But what makes you so positive that it is all the work of one man?”

“I was coming to that.  For one thing, he isn’t like the usual firebug at all.  Ordinarily they start their fires with excelsior and petroleum, or they smear the wood with paraffin or they use gasoline, benzine, or something of that sort.  This fellow apparently scorns such crude methods.  I can’t say how he starts his fires, but in every case I have mentioned we have found the remains of a wire.  It has something to do with electricity—­but what, I don’t know.  That’s one reason why I think these fires are all connected.  Here’s another.”

McCormick pulled a dirty note out of his pocket and laid it on the table.  We read it eagerly: 

Hello, Chief!  Haven’t found the firebug yet, have you?  You will know who he is only when I am dead and the fires stop.  I don’t suppose you even realise that the firebug talks with you almost every day about catching the firebug.  That’s me.  I am the real firebug, that is writing this letter.  I am going to tell you why I am starting these fires.  There’s money in it—­an easy living.  They never caught me in Chicago or anywhere, so you might as well quit looking for me and take your medicine. 
                       A. Spark.

“Humph!” ejaculated Kennedy, “he has a sense of humour, anyhow—­A.  Spark!”

“Queer sense of humour,” growled McCormick, gritting his teeth.  “Here’s another I got to-day: 

Say, Chief:  We are going to get busy again and fire a big

department-store next.  How does that suit Your Majesty?  Wait till the fun begins when the firebug gets to work again. 
                       A. Spark.

“Well, sir, when I got that letter,” cried McCormick, “I was almost ready to ring in a double-nine alarm at once—­they have me that bluffed out.  But I said to myself, ’There’s only one thing to do—­see this man Kennedy.’  So here I am.  You see what I am driving at?  I believe that firebug is an artist at the thing, does it for the mere fun of it and the ready money in it.  But more than that, there must be some one back of him.  Who is the man higher up—­we must catch him.  See?”

“A big department-store,” mused Kennedy.  “That’s definite—­there are only a score or so of them, and the Stacey interests control several.  Mac, I’ll tell you what I’ll do.  Let me sit up with you to-night at headquarters until we get an alarm.  By George, I’ll see this case through to a finish!”