After the close of the fair this engine was run on several days, under a variety of loads applied by a Prony brake, in the presence of a number of engineers and others who had been invited to witness the trials. Of the diagrams taken on these trials, I find that I have preserved only the two here shown, namely, a single friction diagram from the back end of the cylinder, on a scale of 20 pounds to the inch, and a diagram showing large power, taken from the front or crank end, on a scale of 24 pounds to the inch. The former shows the trifling loss from friction in this engine. I have measured this card, and find the mean effective pressure, or difference between the areas showing the excess of the forward over the back pressure, to be 1.1 pounds on the square inch, which, assuming the opposite card to be equal with it, was the friction of the engine. The exhaust line shows the power required to reverse the direction of motion of the exhaust, which at the end of the stroke was rushing back into the cylinder. The latter is especially interesting as showing the identity of the expansion curve with the theoretical, three points on which are marked by the crosses. The sharp reaction of the indicator while the crank was passing the dead center will also be observed. After this trial I made a careful comparison of the diagrams taken under the different loads with the friction diagrams, and found the uniform results to be that the friction diagrams subtracted from the diagrams taken under the load left in each case, of six different loads, exactly the same effective work done that was shown by the brake. [Illustration: Friction Diagram from Allen Engine at Fair of American Institute, 1870.] [Illustration: Diagram from Allen Engine, Fair of American Institute, 1870, cutting off at ¹⁄₄ stroke.] From this I concluded that in these engines the use of the friction brake is unnecessary; the friction is sensibly the same under all loads, and the friction diagram only needs to be subtracted to learn from the diagram the amount of effective work done. The verdict of the judges, President Barnard of Columbia College, Thomas J. Sloane, the proposer of the gimlet-pointed wood screw, now in universal use, in place of the flat-ended screws formerly used, and inventor of the special machinery required for their manufacture, and Robert Weir, engineer in the Croton Aqueduct department, may be summed up in the single expression from their report, “The performance of this engine is without precedent.” For its success I was largely indebted, first, to the remarkable circulation and steam-generating power of the boiler, and, second, to the superheating of the steam in the second drum. CHAPTER XVIII Demonstration to the Judges of Action of Reciprocating Parts. Explanation of this Action. Mr. Williams’ Instrument for Exhibiting this Action.