“Oh, no, it was not!  I had thoroughly examined the tower on my previous visit, and what I found there had puzzled me badly.  In fact it was not until your admirable withdrawal from Friar’s Park to-night that the horrible explanation dawned upon me ...and I realized that the object of inviting Sir Marcus to Upper Crossleys was to ‘remove’ him!  The first plan failed, of course; he never came.  He went back again on duty to Russia, I believe—­for a time.  But when he returned—­a second was adopted, at the Red House.  However—­the murder-machine erected in accordance with the earlier plan was still there—­”

“Where?” I cried in bewilderment.

“On the tower of Friar’s Park!  It was the appearance of Damar Greefe on the platform of the tower, armed with binoculars, which awakened me to the ghastly truth.  The device, never used in the case of Sir Marcus, was not to be wasted, but was to be employed to remove a dangerous obstacle from the conspirator’s path!  I had left the car near Crossleys, you see, and never in my life have I run as I ran after you to-night!”

“But, Gatton, what did you find on the tower—­and what connection exists between the tower and the explosion which occurred here to-night?”

“This:  a sort of small howitzer—­I think of Krupp’s manufacture, but you would be better able to judge than I—­is mounted on the platform of the tower!  I examined it, Mr. Addison, last night, and like a fool concluded that it had been used at some time for a local celebration and never dismounted!  It was trained—­as I remembered nearly too late—­and laid at a certain elevation in such a way that it was evidently never meant to be moved.  Yet at the time the significance of this did not strike me.  How the range was found so exactly we shall probably never know; but the truth suddenly burst upon me as you made off through the bushes and as Dr. Damar Greefe came out and began to peer through his glasses—­that it was mechanically set in such a manner that it could drop a projectile into the window above the porch of the Abbey Inn!”

“Good God!  It’s hardly credible!”

“It isn’t, I admit.  But weather conditions favored him; there wasn’t a breath of wind.  And that he succeeded is proved by the fact that at the present moment your room below is probably still full of poison gas!  Of course, it may not have been a gas-shell; he may have relied, as well he might do, on the burst!  But I’m taking no chances.  You can well imagine that failing a knowledge of the arrangement on the tower, no explanation of the mystery would ever have been found!  A thunder-bolt would be the popular theory, and if any fragments of shell were found who would ever know from where it had been fired?”

“Gatton,” I said, “I owe you my life.  But why did this fiend try to murder me?”

Gatton smiled.

“I have a theory, Mr. Addison,” he replied, “and it is this:  I believe he thought that the indiscretion of a certain mysterious lady would bring about his ruin.  If I am not mistaken, she has already gone far to put his neck in a halter; and he was determined to nip this latest adventure in the bud by removing the object of her—­”