Sterry's anxiety was really more on her account than on his own.

He knew there was little danger of himself being struck by the bullets of the rustlers, who, as I have shown, had no possible chance of taking any sort of aim, but she was a conspicuous target, which it would seem they ought to hit with little difficulty.
Often must a person in the situation of Sterry leave everything to his horse.

He did not seek to guide Queenie, but sat, or rather lay, in the saddle and on her neck, as she skimmed like a swallow over the undulating prairie.
Strange imaginings were in the brain of the young man during those few minutes.

He listened to each shot of the Winchesters, and then, instead of feeling any apprehension for himself, waited for the dreaded evidence that his horse had been struck.
The skilful railway engineer, sitting in his cab, with his hand on the throttle, can discover, on the instant, the slightest disarrangement in the mass of intricate mechanism over which he holds control.

His highly trained senses enable him to feel it like a flash.