One of those who refused permission were Gwynne & Co., the principal partner a nephew of my centrifugal-force friend of earlier days. They exhibited a centrifugal pump supplying a waterfall. They employed Mr. Zerah Colburn, then editor of _The Engineer_, to investigate their pair of non-condensing engines and find out why they used so much steam. He borrowed my indicator to make a private test. Of course, I never saw the diagrams, but Mr. Colburn informed me that by making some changes he had reduced the back pressure to 7 pounds above the atmosphere, which he claimed to be as good as could be expected. No material improvement in the engines was to be observed, however. Some of the diagrams taken on these tests exhibited almost incredible faults. The only really good ones were from a pair of engines made by Easton, Amos & Sons, also to drive a large centrifugal pump, built for drainage purposes in Demerara, and sustaining another waterfall. These showed the steam cut off sharply at one third of the stroke by separately driven valves on the back of the main slides. A mortifying feature of this work for myself was that on testing the indicator Mr. Clark found that the area of the piston, which was represented to be one quarter of a square inch, was really considerably less than this, showing lamentable inaccuracy on the part of the makers, as well as my own neglect to discover it. This rendered the instrument valueless for measuring power, but it showed the character of the diagrams all right. The finest mechanical drawing I ever saw--or any one else, I think--was shown in this exhibition. It was a drawing of the steamship “Persia,” then the pride of the Cunard fleet, and was the only mechanical drawing ever admitted to the walls of the National Gallery, where it had appeared the year before. It represented side and end elevations and plan, as well as longitudinal and cross-sections, was painted and shaded in water-colors, and involved an almost incredible amount of work. It was made by Mr. Kirkaldy, then a draftsman in the employ of the Napiers, of Glasgow, the builders of the vessel. I am tempted to refer to this, as it forms a prominent datum point from which to measure the development of steam navigation in the brief space of forty years. The vessel did not possess a single feature, large or small, that now exists. It was of only about 3000 tons burden. It was an iron ship built in the days of the rapid transition from wood to steel. It was propelled by paddle-wheels. These were driven by a pair of side-lever engines. The engines had each a single cylinder. The steam pressure carried was nominally 25 pounds above the atmosphere, but practically only from 15 to 20 pounds. Full pressure was not pretended to be maintained. They had jet condensers. All forged work was of iron. The vessel was steered by hand. The rigging, standing as well as running, was of hemp. It was full bark-rigged. [Illustration: FREDERICK E. SICKELS]