Read it through once

Again Hal’s whip fell upon the dogs. They threw themselves against the breast-bands, dug their feet into the packed snow, got down low to it, and put forth all their strength. The sled held as though it were an anchor. After two efforts, they stood still, panting. The whip was whistling savagely, when once more Mercedes interfered. She dropped on her knees before Buck, with tears in her eyes, and put her arms around his neck.