Polytechnic Association. 845 



I went to England, and there I met with the same objections that 

 were raised here. They said I could not work steam expansively by 

 positive motion, because we should close the parte suddenly, and open 

 them gradually, whereas positive motion would give as rapid opening 

 as closing ; and the faster the engine is run, the worse would be the 

 effect. I found on conversing with engineers that the idea was univer- 

 sal and traditional that steam must be admitted to an engine very 

 gently indeed. The original engines were all beam engines, and the 

 strain being double the force of the steam, had broken many engines, 

 so that engineers were sore on that subject. That idea I had to com- 

 bat ; and I did it by making the engines, and running them. I found 

 on trial that the difficulty was not aggravated, but, on the contrary, 

 removed to a great extent by a moderately rapid speed. Again, 

 engineers said that high speed would be very good, provided the piston 

 could be kept in motion in one direction all the time ; but, unfortu- 

 nately, it has to be stopped and put in equally rapid motion in the 

 opposite direction. I saw that the only way was to present to their 

 senses the phenomenon that must be accounted for. When I stated, 

 in a paper that I read before the Association of Mechanical Engineers 

 in England, that the accelerating force was greatest precisely on the 

 center, the president of the association interrupted me, and said that 

 I certainly could not mean that, for there the piston was entirely pass- 

 ive, and I could not mean to dispute it. I told him I did mean to 

 dispute it ; but he shook his head very wisely, and Mr. Cowper silenced 

 me effectually by showing on the blackboard that the point of greatest 

 acceleration was some distance from the center, and that at the ends 

 and middle ol the stroke there was no acceleration. My doctrine was 

 heresy to the gospel according to Tredgold, and they would not 

 receive it. 



So I made an engine, and took it to the Paris Exposition. The 

 piston was twelve inches diameter, and it made 200 revolutions per 

 minute. It ran like a wheel, with no sign of a dead center about it, 

 and some of the greatest engineers came there and studied it for hours. 



"When that engine had run there for months, the proposition was 

 settled. 



In using the expression " centrifugal force " in relation to the 

 reciprocating parts vf a steam-engine, I have always meant the centri- 

 fugal force upon the dead centers, and at no other point. Dr. Bar- 

 nard's conclusion that the proper weight of the piston is that which 

 will be sufficient to absorb about half the initial pressure of the steam 

 is one that I arrived at as a practical result long ago ; and the larger 



