And then, suddenly as that northern light appeared, it was extinguished or hidden. Then, before any one could speak, again it flamed. Again it disappeared, and the explanation occurred to all three at the same instant. "Signalling, of course," muttered Upton. "Now get two companies into line, facing west; then we'll leave our horses with them and creep out toward the bridge." Another moment, and while Flint was noiselessly leading the foremost two into line, the major and the staff-officer had dismounted, handed their reins to silent Snipe, and out they went, crouching low, into the westward darkness, while every man breathed hard and listened. Then the southern light began to flash and disappear alternately. "We are far out to the west of Centreville," murmured Flint. "Those windows are hidden from that point. They doubtless think no one can see them here." Five minutes, and still no sound came from their venturesome scouts. They had had time to go all the way across if need be. "What d'you s'pose they signal for?" whispered a young soldier in the leading set, whereupon the sergeant turned and muttered, "Hush!" and men began to realize that it was a time to listen--not to talk. All of a sudden, low, clear, and distinct, a whistle was sounded not four hundred yards away. The first thought to strike every man was, the major! but the major had gone straight to the west; this sound came from across the wide field well to the northward of the supposed position of the bridge. Before there was time to comment the answer was given straight out ahead, soft, yet just as distinct. Then all three horses left with Snipe pricked up their ears and whirled toward the northwest, for from that quarter came the sound of hoof-beats, the low thud and rumble of horses moving at lively lope. Swift, invisible, they swooped down from the northward across the front. Then came sudden check, then silence, then the next minute the hollow sound of iron-shod hoofs upon resounding boards. First one horse, at a walk, then two, three, half a dozen together, and then silence again. Two minutes later, back from the front, running, came the major. "Forward, just as you are!" he muttered to Flint. "The bridge is safe," and, swinging into saddle and bidding Snipe come on with the lieutenant's horse, he sped swiftly away across the field. At its western limit, at the edge of a deep, black trench that stretched away southward toward Bull Run, they found the staff-officer, standing at the old wooden bridge. "They've left it intact," murmured Upton, gleefully, "and they've been scouting around our right flank for indication of any attack from this direction, and have missed us entirely. Now let 'em come back and get it if they can!"