"It won't work with a small detector like they have, sir. With our big set here, we could send them a message, but that outfit they have might burn out. It has a limited sealed motor supply that must break down an initial current resistance on the grids before the rectifiers can convert it to audible sound. With the set operating continuously, power drainage is small, but begin changing your signal beam and the power has to break down the grid resistance several hundred times for every short signal sent. It would burn out their set in a matter of hours. "It works like a slide trombone, sort of. Run your slide way out, and you get a slowly vibrating column of air, and that is heard as a low note, only on sonar it would be a short note. Run your slide way up, and the vibrations are progessively faster and higher in pitch. The sonar set, at peak, is vibrating so rapidly that it's almost static, and the power flow is actually continuous. But, starting and stopping the set continuously, the vibrators never have a chance to reach a normal peak, and the power flow is broken at each vibration in the receiver--and a few hours later your sonar receptor is a hunk of junk." "All right, Harker. Your discussion is vague, but I get the general idea that my suggestion wasn't too hot. Well, have whoever is on duty call me if any signals come through." The Commander set down his cup, said goodnight, and moved off down the hushed corridor. Harker returned to his letter and a chewed stub of pencil, while he scowled in a fevered agony of composition. It was a letter to his girl, and it had to be good. * * * * * Night had begun to fall over the forest roof, and stole thickening down the muddy cathedral aisles of great trees, and Hague listened hopefully for the halt signal from the whippet tank, which should come soon. He was worried about Bucci who was laughing and talking volubly, and the officer decided he must have a touch of fever. The dark, muscular gunner kept talking about his young wife in what was almost a babble. Once he staggered and nearly fell, until Hurd took the pneumatic gun barrel assembly and carried it on his own shoulders. They were all listening expectantly for the tank's klaxon, when a brassy scream ripped the evening to echoing shreds and a flurry of shots broke out ahead. The scream came again, metallic and shrill as a locomotive gone amok; yells, explosive-bullet reports, and the sound of hammering blows drifted back. "Take over, Brian," Hague snapped. "Crosse, Hurd--let's go!" The three men ran at a stagger through the dragging mud around a turn in the trail, and dropped the pneumatic gun swiftly into place, Hurd at firing position, Crosse on the charger, and Hague prone in the slime snapping an ammunition belt into the loader.