APPENDIX. NICHOLAS WOOD'S ACCOUNT. 499 



positive there was no blast-pipe." But Mr. O. D. Hedley says 

 there was one ; and he gives a representation of the engine from 

 the first edition of Mr. Nicholas Wood's ' Practical Treatise on 

 Railroads/ published in 1825, in proof of what the engine actually 

 was. The illustration, however, entirely confutes the assertion 

 that the Wylam engine contained any blast at all. In fact, it 

 embodied a contrivance for the express purpose of preventing 

 a blast. Mr. Wood explains clearly enough how this object was 

 secured, contrasting it with Stephenson 's Killingworth engine 

 to the disadvantage of the latter, which had a blast, whilst the 

 other had none. For it is a curious fact that Mr. Wood at 

 that time did not approve of the steam-blast, and he referred to 

 the Wylam engine in illustration of how it might be avoided. 



The evidence contained in Mr. Wood's book, published as it 

 was in 1825, is especially valuable as showing the express pur- 

 pose for which George Stephenson invented and adopted 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 upwards, accelerates 

 the velocity of the current of heated air accordingly " (p. 147). 

 And again, at another part of the book, he says: "There is 

 another great objection urged against locomotives, which is, the 

 noise that the steam makes in escaping into the chimney ; this 

 objection is very singular, as it is not the result of any inherent 

 form in the organisation of such engines, but an accidental 

 circumstance. When the engines were first made, the steam 

 escaped into the atmosphere, and made comparatively little 

 noise; it was found difficult then to produce steam in sufficient 

 quantity to keep the engine constantly working, or rather to obtain 

 an adequate rapidity of current in the chimney to give sufficient 

 intensity to the fire. To effect a greater rabidity, or to increase 

 the draught of the chimney, Mr. Stephenson thought that by 

 causing the steam to escape into the chimney through a pipe with 

 its end turned upwards, tJie velocity of the current would be ac- 

 celerated, and such was the effect ; but, in remedying one evil 

 another has been produced, which, though objectionable in some 



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