414 STEPHENSON'S PATENT 



diminished the draught of the fire would be checked from the passage to the chimney 

 being too small. The heating power of the boiler would thus be injured, although 

 the amount of heating surface exposed to the water was increased, and the abstraction 

 of the heat from the hot air rendered more perfect. The exact size of tubes which 

 produce the best total effect is to be discovered only by experience, and has not yet 

 been completely decided. 



Small tubes have another disadvantage in being liable to choke up very much 

 with the particles of coke which are carried through them in great quantities by the 

 force of the draught, and in their retaining the cinders which are continually blown 

 into them, but which pass clear through larger tubes. The small tubes are often 

 contracted one third in diameter by the deposit of coke on their inner surface during 

 the day's work, and they have to be cleared out every night by passing a rod 

 through them ; the larger tubes never require clearing out. 



The tubes were at first made of copper, and some have been of wrought iron, but 

 the copper tubes were found to wear very fast, generally lasting only three or 

 four months, and were a great source of expense from the necessity of frequently 

 renewing them. Brass tubes were first tried in the locomotives on the Liverpool and 

 Manchester Railway in 1833, at the suggestion of Mr. Dixon, the resident engineer, 

 and were found to be very much superior; they are now universally used for loco- 

 motives. Brass tubes, of the dimensions mentioned above, last about two years, being 

 six or eight times as long as copper tubes of the same dimensions. This increase of 

 durability appears partly caused by their greater hardness, as it has been observed 

 that the soldered joint, which is made with harder brass, wears less than the other 

 parts ; but the whole cause has not yet been satisfactorily ascertained. The tubes are 

 much worn by the friction of the cinders that are blown through them by the force 

 of the draught; but it is very probable that their wear is principally caused by chemical 

 or thermo-electric action. The tubes in the middle and about the fourth row from 

 the bottom are worn out the first, and it is only the ends next the fire-box that are 

 destroyed. 



When the tubes become very thin they are crushed inwards by the force of the 

 steam, and the water is blown out at the ends of the tube into the fire ; when the 

 tubes are getting old, this frequently takes place whilst an engine is running, and it 

 is stopped by the accident. A plug of hard wood is driven into each end of the 

 burst tube, which is preserved from being burnt by the contact of the water inside 

 the tube, and the engine runs on again. When several of the tubes have burst and 

 been plugged up, they are taken out and replaced by new ones ; and if the engine is 

 required to be in constant use, a complete set of new tubes is soon required to avoid the 

 liability of delays from the bursting of the tubes ; they weigh about I61bs. when new, 



