how does irving link the legend to history to describe "the apparation"

Here is a quote by Washington Irving on this subject,

"The dominant spirit, however, that haunts this enchanted region, and seems to be commander-in-chief of all the powers of the air, is the apparition of a figure on horseback, without a head. It is said by some to be the ghost of a Hessian trooper, whose head had been carried away by a cannon-ball, in some nameless battle during the Revolutionary War, and who is ever and anon seen by the country folk hurrying along in the gloom of night, as if on the wings of the wind. His haunts are not confined to the valley, but extend at times to the adjacent roads, and especially to the vicinity of a church at no great distance ... the body of the trooper having been buried in the churchyard, the ghost rides forth to the scene of battle in nightly quest of his head, and that the rushing speed with which he sometimes passes along the Hollow, like a midnight blast, is owing to his being belated, and in a hurry to get back to the churchyard before daybreak."