As this invention has been the subject of considerable controversy, it becomes necessary to add a few words respecting it in this place. It has been claimed as the invention of Trevithick in 1804, of Hedley in 1814, of Goldsworthy Gurney in 1820, and of Timothy Hackworth in 1829. With respect to Trevithick, it appears that he discharged the waste steam into the chimney of his engine, but without any intention of thereby producing a blast;[39] and that he attached no value to the expedient is sufficiently obvious from the fact that in 1815 he took out a patent for urging the fire by means of fanners, similar to a winnowing machine. The claim put forward on behalf of William Hedley, that he invented the blast-pipe for the Wylam engine, is sufficiently contradicted by the fact that the Wylam engine had _no_ blast-pipe. "I remember the Wylam engine," Robert Stephenson wrote to the author in 1857, "and I am positive there was no blast-pipe." On the contrary, the Wylam engine embodied a contrivance for the express purpose of _preventing_ a blast. This is clearly shown by the drawing and description of it contained in the first edition of Nicholas Wood's "Practical Treatise on Railroads," published in 1825. This evidence is all the more valuable for our purpose as it was published long before any controversy had arisen as to the authorship of the invention, and, indeed, before it was believed that any merit whatever belonged to it. And it is the more remarkable, as Nicholas Wood himself, who published the first practical work on railways, did not at that time approve of the steam-blast, and referred to the Wylam engine in illustration of how it might be prevented. The following passage from Mr. Wood's book clearly describes the express object and purpose for which George Stephenson invented and applied the steam-blast in the Killingworth engines. Describing their action, Mr. Wood says: "The steam is admitted to the top and bottom of the piston by means of a sliding valve, which, being moved up and down alternately, opens a communication between the top and bottom of the cylinder and the pipe that is _open into the chimney and turns up within it_. The steam, after performing its office within the cylinder, is thus thrown into the chimney, and the power with which it issues will be proportionate to the degree of elasticity; and _the exit being directed upward, accelerates the velocity of the current of heated air accordingly_."[40] And again, at another part of the book, he says: