CHAP. XIII. THE STEAM-BLAST. 265 



the subject of numerous experiments. 1 When the I 

 engine was first tried, it was thought that the blast 

 in the chimney was not sufficiently strong for the pur- 

 pose of keeping up the intensity of the fire in the furnace, 

 so as to produce high-pressure steam with the required 

 velocity. The expedient was therefore adopted of 

 hammering the copper tubes at the point at which 

 they entered the chimney, whereby the blast was con- 

 siderably sharpened ; and on a further trial it was found 

 that the draught was increased to such an extent as to 

 enable abundance of steam to be raised. The rationale 

 of the blast may be simply explained by referring to the 

 effect of contracting the pipe of a water-hose, by which 

 the force of the jet of water is proportionately increased. 

 Widen the nozzle of the pipe, and the force is in like 

 manner diminished. So is it with the steam-blast in 

 the chimney of the locomotive. 



Doubts were, however, expressed whether the greater 

 draught secured by the contraction of the blast-pipe was 

 not counterbalanced in some degree by the negative 

 pressure upon the piston. Hence a series of experiments 

 was .made with pipes of different diameters ; and their 

 efficiency was tested by the amount of vacuum that 

 was produced in the smoke-box. The degree of rare- 

 faction was determined by a glass tube fixed to the 

 bottom of the smoke-box, and descending into a bucket 

 of water, the tube being open at both ends. As the 

 rarefaction took place, the water would of course rise in 

 the tube ; and the height to which it rose above the sur- 

 face of the water in the bucket was made the measure of 

 the amount of rarefaction. These experiments proved 

 that a considerable increase of draught was obtained by 

 the contraction of the orifice; accordingly, the two blast- 

 pipes opening from the cylinders into either side of the 



1 For further details as to the steam-blast, see Robert Stephenson's Account, 

 given in the Appendix to this volume. 



