118 MOTIVE POWER OF HEAT. 



If the expansion of the steam is mainly limited 

 by the dimensions of the vessels in which the dila- 

 tation must take place, the degree of condensation 

 at which it is possible to use it at first is limited 

 only by the resistance of the vessels in which it is 

 produced, that is, of the boilers. 



In this respect we have by no means attained 

 the best possible results. The arrangement of the 

 boilers generally in use is entirely faulty, although 

 the tension of the steam rarely exceeds from four 

 to six atmospheres. They often burst and cause 

 severe accidents. It will undoubtedly be possible 

 to avoid such accidents, and meantime to raise the 

 steam to much greater pressures than is usually 

 done. 



Besides the high-pressure double-cylinder en- 

 gines of which we have spoken, there are also high- 

 pressure engines of one cylinder. The greater part 

 of these latter have been constructed by two in- 

 genious English engineers, Messrs. Trevithick and 

 Vivian. They employ the steam under a very high 

 pressure, sometimes eight to ten atmospheres, but 

 they have no condenser. The steam, after it has 

 been introduced into the cylinder, undergoes 

 therein a certain increase of volume, but preserves 

 always a pressure higher than atmospheric. When 

 it has fulfilled its office it is thrown out into the 



