Dr. Renwick states that he made, in a boat called the "New Philadelphia," one of the most remarkable passages ever performed. He left New York at five in the afternoon, with the first of the flood, and landed at Catskill, distant 95·8 geographical miles from New York, at a quarter before twelve. Passengers were landed and taken in at seven intermediate points: the rate, including stoppages, was therefore 14·2 miles per hour; and if half an hour be allowed for stoppages, the actual average rate of motion would be fifteen miles and three quarters an hour. As the current, which in this case was with the course of the vessel, did not exceed three miles and a half an hour, the absolute velocity through the water would have been somewhat under twelve miles an hour. This speed is nearly the same as the speed obtained from taking the average time of the voyages between New York and Albany at ten hours and a half; it would therefore appear that the great speed attained in this trip must have been chiefly, if not altogether, owing to the effect of the current.
(235.)
The severe storms and formidable sea encountered on the lakes render necessary for the navigation, vessels in all respects as strong and powerful as those which navigate the open ocean.
(236.)
(237.)
(238.)
(239.)
The pressure of steam used in these machines is such as is never used in European engines, even when worked on railways. A pressure of one hundred pounds per inch is here considered extremely moderate. The captain of one of these boats, plying between Pittsburg and St. Louis, told Mr. Stevenson that "under ordinary circumstances his safety valves were loaded with a pressure equal to one hundred and thirty-eight pounds per square inch, but that the steam was occasionally raised as high as one hundred and fifty pounds to enable the vessel to pass parts of the river in which there is a strong current;" and he added, by way of consolation, that "this pressure was never exceeded except on extraordinary occasions!"
The dimensions and power of the Mississippi steamers may be collected from those of the St. Louis, a boat which was plying on that river in 1837. That vessel measured two hundred and fifty feet on deck, and had twenty-eight feet breadth of beam. Her draught of water was eight feet, and her measured capacity one thousand tons. She was propelled by two engines with thirty-inch cylinders, and ten feet stroke; the safety valve being loaded at one hundred pounds per square inch.
The paddle wheels of these vessels are attached to the paddle shaft, in such a manner as to be thrown into and out of gear, at discretion, by the engineer, so that the paddle shaft may revolve without driving the wheels: by this expedient the power of the engine is used to feed the boilers while the vessel stops at the several stations. The vessel is therefore stopped, not, as is usually the case, by stopping the engines, but by throwing the wheels out of connection with the paddle shaft. The engines continue to work, but their [Pg503] power is expended in forcing water into the boiler. By this expedient the activity of the engines may, within practical limits, be varied with the resistance the vessel has to encounter. In working against a strong current, the feed may be cut off from the boilers, and the production of steam, and consequently the power of the engines, thereby stimulated, while this suspension of the feed may be compensated at the next station.