miles; the Tennessee, navigable for two hundred and fifty miles; the Cumberland, navigable for four hundred and forty miles; the Kentucky, navigable for one hundred and thirty miles; and the Green River, navigable for one hundred and fifty miles. The total length of the Ohio and its tributaries is estimated at above seven thousand miles. (237.) Steam-boats were introduced on the Mississippi about the year 1812, the period of their first introduction in Europe; and their increase has been rapid beyond all precedent. In the year 1831 there were one hundred and ninetyeight steamers plying on its waters; and the number in 1837 amounted to nearly four hundred. These vessels are built chiefly on the banks of the Ohio, at the towns of Pittsburg and Cincinnati, at distances of about two thousand miles from the mouth of the river they are intended to navigate. (238.) These steamers, which are decidedly inferior to those which navigate the eastern waters, are generally of a heavy build, fitted to carry goods as well as passengers, and vary from one hundred to seven hundred tons burthen. Their draught of water is also greater than that of the eastern river steamers-varying from six to eight feet. The hull, at about five feet from the water line, is covered with a deck, under which is the hold, in which the heavy part of the cargo is stowed. About the middle of this deck the engines are placed, the boilers and furnaces occupying a space nearer to the bow, near which two chimneys are placed. The firedoors of the furnaces are presented towards the bow, and exposed so as to increase the draught. That part of the first deck which extends from the machinery to the stern is the place allotted to the crew and the deck passengers, and is described as being filthy and inconvenient in the extreme. A second deck is constructed, which extends from the chimneys near the bow to the stern of the vessel. On this is formed the great cabin or saloon, which extends from the chimneys to within about thirty feet of the stern, where it is divided by a partition from the ladies' cabin, which occupies the remaining space. These principal cabins are surrounded by a gallery about three feet in width, from which, at convenient places, an ascent is supplied by stairs to the highest deck, called the hurricane or promenade deck. (239.) The engines by which these boats are propelled are totally different from the machinery already described as used in the eastern steamers. They are invariably non-condensing engines, worked by steam of extremely high pressure; the boilers are therefore tubular, and the cylinders small in diameter, but generally having a long stroke. 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 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. The stoppages to take in goods and passengers, and for relays of fuel, are frequent. "The liberty which they take with their vessels on these occasions," says Mr. Stevenson, "is somewhat amusing: I had a good example of this on board a large vessel, called the Ontario. She was steered close in shore amongst stones and stumps of trees, where she lay for some hours to take in goods: the additional weight increased her draught of water, and caused her to heel a good deal; and when her engines were put in motion, she actually crawled into the deep water on her paddle wheels: the steam had been got up to an enormous pressure to enable her to get off, and the volume of steam discharged from the escapement pipe at every half stroke of the piston made a sharp sound almost like the discharge of fire-arms, while every timber in the vessel seemed to tremble, and the whole structure actually groaned under the shocks." Besides the steamers used for the navigation of the Mississippi, innumerable steam tugs are constantly employed in towing vessels between the port of New Orleans and the open sea of the Gulf of Mexico. Before the invention of steam navigation, this southern capital of the United States laboured under the disadvantage of possessing almost the only bad and inconvenient harbour in the vast range of coast by which the country is bounded. New Orleans lies at a distance of about one hundred miles from the Gulf of Mexico. The force of the stream, the frequency of shoals, and the winding course of the channel rendered it scarcely possible for a sailing vessel to pass between the port and the sea with the same wind. The anchorage was every where bad, and great difficulty and risk attended the mooring of large vessels to the banks. The steam engine has, however, overcome all these difficulties, and rendered the most objectionable harbour of the Union a safe and good seaport, perfectly easy of approach and of egress at all times; a small steam tug will take in tow several large ships, and carry them with safety and expedition to the offing, where it will dismiss them on their voyage, and take back vessels which may have arrived. APPENDIX. On the Relation between the Temperature, Pressure, and Density of Common Steam. THERE is a fixed relation between the temperature and pressure of common steam, which has not yet been ascertained by theory. Various empirical formulæ have been proposed to express it, derived from tables of temperatures and corresponding pressures which have been founded on experiments and completed by interpolation. The following formula, proposed by M. Biot, represents with great accuracy the relation between the temperature and pressure of common steam, throughout all that part of the thermometric scale to which experiments have been extended. Let a = 5.96131330259 log. a2 = 0.74110951837 The relation between the temperature t with reference to the centesimal thermometer, and the pressure p in millimètres of mercury at the temperature of melting ice, will then be expressed by the following formula:: Formulæ have, however, been proposed, which, though not applicable to the whole scale of temperatures, are more manageable in their practical application than the preceding. For pressures less than an atmosphere, Southern proposed the following formula, where the pressure is intended to be expressed |