“Where’s the artillery?” asked Jimmie, flashing his light about the aeroplanes. “I thought I heard cannonading.” As briefly as possible, Ben explained what had taken place, and the three walked over to the spot where the missile had struck and exploded. There was a great hole in the ground, and tiny fragments of a tin can lay scattered about, lying at some distance from the hole. “Nitroglycerine!” exclaimed Ben, picking up one of the fragments. “That only goes to show,” Jimmie answered, wrinkling his freckled nose, “that this trip of ours is not at all like a Sunday School picnic. I wish we had caught him before he mounted his machine,” he went on. “I’d like to fill him so full of holes that he could go away and play that he was a Swiss cheese.” There was very little sleep in the camp that night. The boys were away at daylight, and a couple of hours later saw the machines snugly tucked away in a hangar not far from the aviation field near Forest Park. They waited about the post-office, taking turns watching at the general delivery window, until nearly noon but, as the reader well understands, Havens did not make his appearance. Their vigil during the afternoon produced no better results. Toward evening they tried to reach Havens by wire in New York, but their dispatches met with no response for a long time. At last a message came from the millionaire’s private office at the hangar in Westchester county. It was very brief, and gave only the information that Havens had taken a stateroom for St. Louis the previous evening, and that he had mysteriously disappeared before the train had left the city. “That’s a knock-out!” exclaimed Jimmie. “And now,” asked Ben with a puzzled look, “shall we go back to New York and help find Havens, or shall we cross the continent in quest of the burglars?” CHAPTER VII. THE SIGNAL FIRE. “I’ll tell you what it is,” Jimmie said, as the boys sat in a little restaurant on Fourth street, discussing the situation, “if we turn back to New York now, we’ll be off the beat. Havens told us to go out to Monterey, didn’t he?” “He certainly did!” answered Carl. “Well then,” continued Jimmie, “we ought to go on to Monterey. Look here, kids,” he went on, “we don’t know what took place in New York after we left. We don’t know that Havens didn’t disappear from that stateroom for the sole purpose of getting out of the way of the fellows who tried to burn his hangar. What do you think of that idea?” “It appears to me to be a sound one,” Ben responded. “Mr. Havens may have met with members of the gang we are fighting. In that case it would be nothing strange if he managed a mysterious disappearance for his own protection. Would it, now?”