466 THE APPLICATIONS OF PHYSICAL FORCES. [BOOK iv. 



of tubular boilers is due to Mark Seguin. Owing to the enormous 

 increase of heating surface which this arrangement affords, without 

 augmenting the dimensions of the -generator, vaporization is increased 

 and the power of the engines is multiplied in proportion ; but in order 

 that so large a production of steam may take place, the activity of the 

 fire must be kept up by an energetic draught, which the small height 

 of the chimneys in a locomotive cannot give. It was therefore an 

 equally happy discovery to make use of the steam when it had acted 

 on the piston, and let it escape in the chimney itself, giving us what 

 is termed the " steam blast," that is, a rapid current at each motion of 

 the piston, which draws the air and gases of combustion through the 

 tubes, and thus forms a draught in the body of the fire. Hackworth, 

 Pelletier, and G-. Stephenson are considered to be the inventors of 

 this important improvement, which gave all its value to the tubular 

 boiler in locomotives. 



II. THE MODERN LOCOMOTIVE. 



Let us now see what the locomotive has become after forty years 

 of incessant improvements. 



Figs. 314, 315 and 316 represent a longitudinal section and 

 two transverse sections in the front and at the back of the engine, 

 and they will explain its principal arrangements. 



And first about the steam-generator. 



The boiler of a locomotive is tubular. It is composed of two 

 principal parts : one, situated behind, and of rectangular form, incloses 

 the fire, which is surrounded on all sides except the under one with 

 water ; the other, the cylindrical body, so named from the form of its 

 covering, contains two distinct chambers. In the lower half are 

 placed the tubes by which the smoke and gases of combustion pass 

 from the fire to the chimney. All the tubes, often in considerable 

 number, are surrounded by the water in the cylindrical body. The 

 upper half of the cylindrical body is the steam space, which by a 

 pipe bent forwards and backwards (pssu, Fig. 314), opens at one end 

 in the steam dome, and at the other in the steam-chest of each of the 

 two cylinders of the engine. 



The driver can open or close at will, by means of the handle r, 



