CHAP. IX.] 



THE LOCOMOTIVE. 



407 



the valves of a diaphragm, q, which gives passage to the steam, stops 

 it or introduces it, in greater or less quantity into the pistons ; this 

 is called the regulator, and on account of its form, the butterfly- valve. 



On the convex surface of the cylindrical body are seen the 

 accessory apparatus safety-valves, pressure-gauge, water-gauge, and 

 steam-whistle. 



What is the distinctive characteristic of the boiler of a locomotive ? 

 It is, undoubtedly, the enormous extent of the heating surface rela- 

 tively to the whole capacity. To show in. what proportion this 



FIG- 314. Locomotive : longitudinal section. 



element is increased by the adoption of the tubes, we may quote 

 some numbers. In a Crampton's locomotive the coverings of the 

 fire, that is, the surface for heating by radiation, is but 8'65 square 

 metres, the surface for heating by contact, that is to say that of the 

 tubes that take up the gases of combustion, is 88'92 square metres, 

 that is, more than ten times as great. In an English goods traffic 

 engine the numbers are respectively 970 m. and 18070 m. ; the tubes 

 augment the heating surface in the ratio of 1 : 1S'6. Whence, we 

 repeat, the importance of the steam blast, without which the activity 



H H '2 



