174 THE SCIENTIFIC PAPERS OF 



tions of the speakers who had taken part in the discussion, one 

 result seemed to his mind to be quite certain, namely, that the 

 link-motion was doomed. He must say he did not feel the same 

 regret on that score which appeared to animate Mr. Head. The 

 link-motion had no doubt been a way out of a difficulty ; but it 

 was correct only within very narrow limits, and the moment 

 those limits were exceeded it did not produce the expansive action 

 desired. A motion had now been brought before them which 

 challenged comparison with the best expansive gear that could be 

 mentioned. There was a clean cut-off ; the steam was put on at 

 once in ample quantity, and the exhaust also was opened promptly 

 at the right time. There was also the means of adjusting the 

 action so as to make the up-stroke and the down-stroke perfectly 

 alike, which, as they all knew, was not the case with the link- 

 motion. And, as Mr. Webb had beautifully illustrated by his 

 locomotive, the new gear had the advantage of giving a large 

 useful space on the main shaft for increasing the length of the bear- 

 ings. These were very important advantages, which Mr. Joy 

 might claim for his motion. But they had also been put in pos- 

 session of two other motions, which seemed to be as perfect as 

 Mr. Joy's. Still they need not regret that result. There were 

 points of difference between the gears, though they all aimed at 

 the same result, a very perfect cut-off and a perfect mode of re- 

 versing the engine. The object was achieved by different 

 mechanical details, all of which he considered were superior, both 

 theoretically and practically, to the old link-motion. 



With the criticism offered by Mr. Boyd, in regard to a portion of 

 Mr. Joy's motion, he thoroughly agreed. The slotted disc, which 

 was turned into a different angular position, with the slide-block 

 grinding up and down in the slot, was not, he thought, a desirable 

 detail ; and he would recommend Mr. Joy to do away with it. He 

 observed it had actually been done away with in one arrangement 

 shown. All the friction was there reduced to the friction on the 

 pins, which must be preferable to friction of sliding surfaces. 

 With that exception Mr. Joy's motion appeared to him perfect, and 

 one that would no doubt receive the most earnest attention of 

 mechanical engineers. 



