mush

"An' of course the dogs can hike along all day with that contraption behind them," affirmed a second of the men. / "Certainly," said Hal, with freezing politeness, taking hold of the gee-pole with one hand and swinging his whip from the other. "Mush!" he shouted. "Mush on there!" / The dogs sprang against the breast-bands, strained hard for a few moments, then relaxed. They were unable to move the sled.