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Iron rail roads are of three kinds-the Edge rail—the Tram roadand the Single rail.

In the edge rail way, the rails, as indicated by the term, are laid with the edge upwards, and the carriage is retained upon them by a flange, or projecting edge, attached to the wheels instead of the rail. Tram roads are flat rails, made usually of cast iron, with an elevated edge or flange on one side, to guide the wheels of carriages in their path. Tram rails are weaker than edge rails, and it is sometimes necessary to strengthen them with ribs underneath. In the single rail way the carriage consists of two boxes suspended on each side of the rail by an iron frame, and having two wheels placed one before the other.

Where the amount of travel is very great the rail road is double, one set of tracts being designed for carriages moving in each direction. A single road, however, is generally sufficient if it be provided with double places, called sidelings, for carriages to pass each other at convenient distances.

Horses are commonly employed for drawing loads upon rail ways; and it is estimated that a horse will draw eight times as much upon a rail way, as upon a common road. Within a few years, steam engines have been employed for propelling carriages, especially in England, by means of which the most surprising velocity is with perfect security attained.

Rail Roads in the United States have not been attempted to any considerable extent; yet several are in contemplation, and it can scarcely be doubted, but that in a few years this mode of conveyance will be extensively adopted, and in preference to canals.

In England, rude tram roads, constructed of wood, were in use nearly two centuries ago; but the present improved mode of constructing and laying the rails with iron is of very recent date. The first rail road established by act of parliament was the Stockton and Darlington, a distance of twenty five miles. The act was obtained in 1823, and the road was opened in September 1825. It consists of a single line of rails, with sidelings, every quarter of a mile, for carriages to pass each other. It is principally used for the conveyance of coals and travellers.

Still more recently the important and stupendous undertaking of constructing a rail road from Liverpool to Manchester, a distance of something more than thirty miles, has been completed. The interest excited in this rail way has arisen chiefly from the excavation of a tunnel, some account of which must be interesting to our readers.

This tunnel commences at Wapping near the Queen's Dock, and extends under the town of Liverpool, a distance of 2250 yards, or rather more than a mile and a quarter. It was constructed in seven or eight separate lengths, each communicating with the surface by means of perpendicular shafts, The opening at Wapping is by cutting 22 feet deep, and 46 wide. The whole length of the tunnel is white-washed, and lighted by gas, and the effect produced is very singular and picturesque; but the atmosphere is cold and chill, and the vapor is at times so thick, that the mere spectator of this monument of human labor and ingenuity will generally be satisfied with one visit, and not be tempted to repeat it. On the sides of the tunnel, at short distances are placed lettered boards, for the purpose of informing the visitor what

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part of the town he is then under. The distance from the roof to the surface of the ground above varies from 5 feet to 70 feet. The rails used on this road are made of forged iron, in lengths of five yards each, and weigh 35 lbs. per yard. Every three feet, the rails rest on blocks of stone, let into the ground, containing each nearly four cubic feet. Into each block, two holes six inches deep and one inch in diameter are drilled; into these are driven oak plugs, and the cast iron chairs or pedestals into which the rails are immediately fitted, are firmly spiked down to the plugs, forming a structure of great solidity and strength. On the embankments, where the road may be expected to subside, a little, the rails are laid on oak sleepers. For eighteen miles of the road the rails are placed on stone blocks, and for the other thirteen on sleepers. The double line of rails for the carriages are laid down with mathematical correctness, and consist of four equidistant rails, four feet eight inches apart, about two inches in breadth, and rising about an inch above the surface. In the formation of the railway there have been dug out of the different excavations, upwards of three millions of cubic yards of stone, clay, and soil, and the weight of the double lines of rail laid down is more than 4,000 tons.

For the purpose of giving to our readers some conception of a steam carriage, designed to be used either on a rail road, or on the common high way, our engraver has copied the model of such a carriage, not long since exhibited at London.

A, Water Cistern-B, the Boiler-C, Steering Wheel with the Conductor-D, Steel Frame which carries the Boiler-E, the Curved Steam Pipe to supply the Engines-F, Hand Pump and Pipe to fill the Boiler-G, Safety Valve-H, Notice Cocks-I, Eduction Pipe to take the steam from the Engine to the Chimney--K, the Crank-L, the Pan for the Cinders.

The subject of Rail Roads is exciting much interest at the present time in the United States. Several short ones are already constructed; one of greater length, extending from Albany to Schenectady is in

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progress, and others still are in contemplation, and will doubtless in due time be completed.

STEAM ENGINE. The steam engine, now extensively employed as an aid to locomotion on rail roads, and in steam boats, as well as to assist in the mechanic arts, is generally attributed to the Marquis of Worcester, as the inventor; but the perfecting of it belongs to James Watt, a native of Greenock, in Scotland. The improvements made in the construction of the steam engine within the last five and twenty years, are too numerous to be here described. We must content ourselves with giving our readers a brief description and a representation of a modern steam engine, which may of course be constructed of any required power, and applied to any purpose.

In the following representation, A represents a wrought iron boiler, about three parts filled with water; the bottom is considerably, and the sides a little, concave, that it may receive more fully the force of the flame circulating around it. Boilers are usually of an oblong form, and are furnished with a part that takes off, in order that a person may get in to clean them when needful; they have also a valve, called the safety-valve, opening upwards, which is loaded so that the steam escapes when it is stronger than the engine requires, and, if retained, would hazard the bursting of the boiler. It is not uncommon to have two boilers, one of which is a reserve, that the engine may not be stopped, when the other requires repair.

B, is an apparatus for regulating the fire, and giving action to a bell, which regulates the quantity of coals and time of firing.

C, the steam-pipe from the boiler A to the valve I.

D, the steam-cylinder, generally called only "the cylinder;" it is connected at the top and bottom with the valve I.

E, the piston, which, by its connecting rod e, gives motion to the beam F, the other end of which, by another connecting rod, gives motion to the heavy fly-wheel G, by means of a crank. Thus, after the engine has begun to work, its power is accumulated in the fly-wheel, and may be disposed of at the pleasure of the mechanist.

H, an eccentric circle on the axle of the fly-wheel G; it gives motion by its levers, to the valve I.

I, a coffer-slide valve, which requires no packing to make it steamtight, as there is always a vacuum under it: it answers the purpose of the four valves used in double-power engines, and from the simplicity of its construction, when well made at first, is not liable to get out of order.

K, the steam-admission valve and lever, connected with a governor, which regulates the speed of the engine.

L, the cylinder of the discharging pump, for extracting the water and uncondensed vapor from the condenser M.

N, a small cistern, filled with water. Into this cistern enters a pipe from the condenser M, the top of which pipe is covered with a valve, which is called the blow-valve, sometimes the shifting valve. Through this valve the air contained in the cylinder D, and passages from it, is discharged, previously to the engine being set in motion.

O, the eduction pipe, which conducts the steam from the valve I to the condenser M.

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P, the pump which supplies with water the cistern SS, in which the condenser and discharging pump stand.

QQ, iron columns, of which the engine has four, although only two are shown; they stand upon one entire plate seen edgeway, on which the principal parts of the engine are fixed; by this means the beam and its accompaniments are supported without being connected with any part of the building, except the recess below the floor on which they stand.

RR, the recess below the floor, for containing the cistern of the discharging pump, condenser, &c. This arrangement enables those engines to be fixed up and tried at the manufactory before they are sent off, which renders the refixing easy and certain.

Before the engine is set to work, the cylinder D, the condenser M, and the passages between them, are filled with common air, which it is necessary to extract. Te effect this, by opening the valves, a communication is made between the steam-pipe C, the space below the piston in the cylinder D, the eduction-pipe O, and the condenser M. The steam will not at first enter the cylinder D, or will only enter it a little way, because it is resisted by the air; but the air in the eduction-pipe O, and the condenser M, it forcibly drives before it, and this part of the air makes its exit through the valve and water in the cistern N. The steam-admission valve is now closed, and the steam already admitted is converted into water, partly by the coldness of the condenser M, but principally by a jet of cold water which enters it through a cock opening into it from the well SS, in which the condenser is immersed.— When this steam is condensed, all the space it occupied would be a vacuum, did not the air in the cylinder D expand, and fill all the space the original quantity of it filled; but by the repetition of the means for extracting a part of the air, the remainder is blown out, and the cylinder becomes filled with steam alone. Suppose then the cylinder beneath the piston to be filled with steam, and the further admission of steam to that part of it be cut off, while the communication between it and the condenser remains open, it is obvious that there will soon be a vacuum in the cylinder, because as fast as the steam reaches the condenser, it is converted into water by the coldness of that vessel and the jet playing within it. At this moment, therefore, the steam is admitted above the piston, which it immediately presses down. As soon as the piston reaches to the bottom of the cylinder, the steam is admitted to the under side of it; and as the communication from the upper side of the piston to the condenser is opened, while the further admission of steam to that side during the upper stroke, is prevented, the steam which had pressed the piston down passes into the condenser, and is converted into water.

The motion of the piston E, by this alternate admission and extraction of the steam on each side of it, is thus necessarily continued, and the distance of its upward and downward range is called the length of its stroke. It communicates its reciprocating motion, by the connecting rod e, to the great beam F, and thence, by another connecting-rod and a crank, to the fly-wheel G.

To explain the rapid accumulation of power with an increase of the size of the engine, it must be observed, that the force of the steam generally used, is somewhat greater than the pressure of the atmosphere;