606 TRANSACTIONS OF THE 



great a deficiency as they made out, required to be accounted for; 

 and, for a time, the resistance of the blast-pipe was deemed a 

 sufficient explanation. But there were some who did not believe 

 that more than half the power could be expended in tlie blast. 

 They got up another theory, which was stated by Mr. Vigrolles, 

 in a lecture, something in this way : ' The speed we now give to 

 the pistons, is four or five times greater than that heretofore 

 given in stationary and marine engines, and, consequently, the 

 steam cannot follow with sufficient rapidity, the piston runs away 

 from it; the steam that gets in has to be rammed in, as it were, 

 by the superior pressure in the boiler ; and the pressure in the 

 cylinder, as the eifect proves, is scarcely half what is shown by 

 the gauge upon the boiler.' To remedy this evil, they naturally 

 proposed to enlarge the openings through which the steam had 

 to pass on its way to the cylinder ; and to accomplish this, they 

 advanced the eccentric, so as to open the port a little earlier 

 than had been the custom. There was a perceptible improve- 

 ment ; it was agreed that the steam got in with less ramming, 

 and the piston started under a good head of steam. The lead 

 was increased and experimented upon, until it was found by trial 

 that three-eighths gave the best results on passenger engines. 

 And this practice continued from '34 to '39; the valve worked 

 with a lead, but had only one-sixteenth lap. 



At this time, '39, it began to be suspected that the back 

 pressure was in some measure lessened by the lead ; and, to test 

 this theory, a valve was made with three-eighths lap, so as to 

 prevent the admission of steam before the commencement of the 

 stroke, and yet to release it at the usual time. It also cut off the 

 steam a little before the end of the stroke, and this gave some ad- 

 vantage from expansion. The result was highly satisfactory, and 

 the lap was increased to one-half, five-eights, and three-fourths, 

 and finally to one inch, and each successive increase showed a 

 marked improvement ; and in consequence of these trials, they 

 settled, for some years, upon the lap and travel which cut off the 

 steam at 0.78 of the stroke, and released it 0.95. 



Of course it was well known tliat stationary engines, especially 

 tlie Cornish pumping engines, expanded steam to a much greater 

 extent, and derived great advantage from it ; but the apparatus 

 by which they did it, was deemed too complicated and loose for 

 locomotives. But new devices were tried with some success ; se- 

 parate cut off valves were got up, but never were mucli adopted 

 in England, owing to the taste for simplicity which prevailed, 



