Perhaps my readers do not know what a trestle-bridge is. To understand the story, it is necessary that they should do so. A trestle-bridge, then, such as the one before Snap, is a bridge of timber, the beams laid at right angles to the line, and each beam about two feet from its neighbour. Across the beams run the iron rails, and between the beams is nothing at all but emptiness. The whole bridge is supported on a huge scaffolding, which rises from the sides of the canyon crossed, and in some cases these bridges are as much as 150 yards from end to end, and 250 feet above the stream which generally races along below. To walk over these bridges by daylight requires a clear head and steady nerves, for, though it is easy enough to stride from beam to beam for a few yards, it becomes more difficult as you proceed: the light gleams off the water below, flickers through the open spaces and dazzles you, while the sight of the vast profound underneath, and the knowledge that one false step will send you whirling between those beams to eternity, has not a steadying effect upon you. These bridges are, most of them, very narrow, and on the one in question there was but a single line, the shunting station immediately preceding the bridge, which was not considered equal to the weight of two trains at the same time. And on this bridge the black Texan steer had elected to ramble. Clever as a goat, it stepped from beam to beam; then, as the light flickered up into its eyes, it grew nervous and stopped, afraid to come back, and afraid to go on. Again Snap heard the warning rattle of the coming train amongst the hills, a faint whistle, and then again silence. He had saved all the herd but one. Should he leave that one? 'No, I'm blowed if I will,' muttered the boy, setting his teeth and feeling just as stubborn as the steer in front of him. 'That train won't be up for another quarter of an hour--you can hear it coming for miles on a frosty night like this,' he argued, and boldly enough he started on to the bridge, stepping freely from beam to beam. The steer, seeing him coming, moved slowly on, trembling in every limb, but still determined not to be headed. 'Confound the brute,' thought Snap, 'I shouldn't wonder if he means me to follow him across the Rockies. I will head him, though!' Just then the steer made a false step. One leg went just short of the beam on to which it had intended to step. It lurched forward, and for one moment Snap thought it had gone over into the abyss. But it recovered itself somehow, and stood trembling in every limb, and bellowing piteously in its fear.