Shepard had made an extraordinary impression upon him and he did not believe that the man would be coming at such a pace unless he came with great news.
He saw Shepard stop, give the pass word to the pickets, then gallop on, ford the river and come straight toward the heart of the army.

Dick ran forward and met him.
"What is it ?" he cried.
"General Pope's tent! Where it is! I can't wait a minute." Dick pointed toward a big marquee, standing in an open space, and Shepard leaping from his horse and abandoning it entirely, ran toward the marquee.

A word or two to the sentinels, and he disappeared inside.
Dick, devoured with curiosity and anxiety, went to Colonel Winchester with the story of what he had seen.
"I know of Shepard," said the colonel.

"He is the best and most daring spy in the whole service of the North.

I think you're right in inferring that he rides so fast for good cause." Shepard remained with the commander-in-chief a quarter of an hour.