Now they knew that the boat was almost abreast, as they could hear the labored breathing of the men tugging at the oars. But just then the leader of the Huns turned a flashlight in their direction, revealing the four figures. There was a shout, a sharp command in guttural tones, a dropping of oars and a click of rifles as the men raised them to their shoulders. But quick as they were, Frank was quicker. Lithe as a panther, he sprang to his feet and hurled the grenade that he had been holding in his hand full at the boat. There was a blinding flash, a terrific explosion and the air was filled with flying bodies and debris. Frank himself was thrown to the ground by the shock, but scrambled to his feet again, none the worse except for a few bruises. It was too dark to see the effects of the explosion, but the dead silence that followed told its own story. There was nothing more to be feared from that special group of enemies. His own comrades gathered around Frank, themselves too dazed and shaken by the suddenness of the whole affair to understand fully just what had happened. “What did it?” asked Billy. “That was the grenade I was telling you fellows about a few minutes ago,” replied Frank. “I thought of it again just when I dropped to the ground. I figured that I might have a chance to smash their boat with it, so I got it out and had it ready in my hand.” “Great stuff, Sheldon,” said the corporal warmly. “It did the work all right. It’s lucky we only had one boat to deal with. But now we’ve got to make tracks for the mouth of the tunnel.” “And we can’t do it any too quick either,” said Tom, as the sound of a commotion rose behind them. “Those fellows round that fire have heard that explosion and they’ll be coming on the double quick to see what’s up.” There was a growing din behind them that hastened their steps as they hurried along. Once, Billy going too near the edge nearly fell into the canal, from which he was saved by Tom’s quick grab of his arm. At intervals the corporal used his flashlight on the narrow road ahead of him but he did not dare to do it very often, for fear that the light might be seen by Huns who might be lying in wait at the tunnel’s mouth. Soon they were conscious from the increasing freshness of the air that they were nearing the entrance. They slackened their steps a little, for the noise in the rear had died down. They did not interpret this as meaning that their adversaries had given up the chase, but thought it more likely that they had halted at the scene of the explosion to try to fathom its meaning. “Now,” said the corporal, when they were within a few yards of the entrance, “I’m going to scout ahead and see whether that other boat is hanging about. If it isn’t we’ll have an easy time in swimming to the bank. If it is, we’ll have some fighting before we win out.” He stole away like an Indian, but almost before the boys realized that he had gone he was back again.