4G8 THE APPLICATIONS OF PHYSICAL FORCES. [BOOK iv. 



of the fire could not suffice for so considerable a production of steam, 

 and without which consequently the tubular boiler in the locomotive 

 would lose its principal advantage. In locomotive engines, says M. 

 Perdonnet, each square metre of heating surface produces from two to 

 three times as much steam 'as in the boilers of fixed engines. 



Locomotives are high -pressure engines without condensation. 

 This is a necessary consequence of the following circumstances. The 

 steam must escape into the air, therefore, it cannot be a low- 

 pressure engine ; in escaping it must produce the blast, therefore it 



FIG. 315. Locomotive : transverse section across 

 tlie fire-box. 



FIG. 316. Locomotive : trarsverse section, 

 across the smoke-box. 



cannot be condensed. It is generally employed at a pressure of eight 

 or nine atmospheres. 



But it works with expansion, and a peculiar mechanism, the link- 

 motion of Stephenson, allows the expansion to be varied, and at the 

 same time renders possible a change in the direction of the motion. 

 A locomotive, like a steamboat (and the necessity of such an 

 arrangement is obvious), can be made to go backwards as well as 

 forwards. 



