THE GROWTH OF THE STEAM-ENGINE. 



541 



Messrs. Stevens. The hull of the Phoenix had hollow water-lines 

 sixty-five years ago, and this important characteristic of modern ves- 

 sels is, therefore, an American improvement. 







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2 



The North America (Fig. 64) was built in 1827. The hull was 

 stiffened by the " hog-frame," now as distinctive a characteristic of 

 the vessel as are the gallows-frame and the skeleton-beam of the 



engine. 



This engine is not usually quite as economical in fuel as are the 

 screw-engines last described ; but it has the advantages which are 

 so extremely important in the shallow, flexible hulls of our river-boats 



