namgating the Hudson River, 9 



Watt's engines was not sufficient to free the condenser from 

 steam, and maintain a proper vacuum. In order to be pro- 

 vided for such a case, Mr. Stevens makes, as 1 have stated, his 

 air-pump of greater power. In the North America, however, 

 it has been unnecessary to employ it, for in two passages I 

 have made in her, (one of them the most rapid she ever per- 

 formed,) the steam-gauge never rose above eight inches. Nor 

 do I consider that she has ever yet been brought up to her 

 greatest speed, as even with this comparatively low pressure 

 the steam was cut off at the half-stroke, and permitted to act 

 by its expansive force. Mr. Stevens, I believe, is of opinion, 

 that the boilers are sufficient to supply steam of 12 or 14 

 inches during the entire stroke of the engine, while the air- 

 pump has power to maintain, at the same time, a vacuum in 

 the condenser. If this be so, the speed may be enhanced, as 

 the wave, that is at present raised in front of the boat, is even 

 less than I have noticed it in front of others of not more than 

 half the speed. 



The boilers in all the boats of Stevens, as in those of Fulton, 

 are of copper ; and I do not apprehend that, with the highest 

 pressure that can be given them, any danger is to be feared by 

 the passengers. I do not consider that this is the case with 

 any high-pressure boilers. The truth is, that both are liable 

 to burst, from the natural imperfection of materials and work- 

 manship. In the case of a boiler, where the material, suppos- 

 ing the safety-valve to be fastened down, will not bear an 

 internal pressure of more than 8 or 10 pounds to the square 

 inch (marked by from 16 to 20 inches of the steam-gauge), 

 a small vent will discharge the steam, whose expansive force is 

 far from excessive, while the temperature of the water is not 

 such as to augment the volume of steam in any great degree. 

 But when the pressure amounts to 60 or 70 pounds on the inch, 

 and the boiler is proved to bear lOOlbs., as in our high-pres- 

 sure engines, no sooner is a vent given than the whole of the 

 contained water is converted into steam, whicR expands itself 

 with explosive violence. a'*^ 



I have to note another variation in the engines of the North 

 America from those of Watt : it consists in the suppres 



